Green Science 9 Final PDF (2076)

12. Critical angle for a certain medium can be defined as the angle of incidence in a denser medium when the corresponding angle of refraction in rarer medium becomes 90°. 13. When the angle of incidence in a denser medium is more than the critical angle, the ray of light reflects in the same medium. This process is called total internal reflection of light. 14. Mirage is an optical illusion which can be observed as if there is a pond on hot desert or coal-tarred road. 15. A mirage is formed when an inverted image of the distant object is formed on the desert or coal-tarred road. 16. When the sunlight passes through a prism, it splits into seven colours. This process is called dispersion of light. 17. The major cause of dispersion of light is the difference in speed of light of different colours in the same medium. The speed of different colours in glass medium is different. 18. The wave which is not affected by electric field and magnetic field is called electromagnetic wave. 19. Electromagnetic waves can propagate through vacuum or without medium. Sunlight is an example of electromagnetic wave. 20. X-rays are harmful rays for human beings having wavelength from 0.01nm to 10 nm. They cannot penetrate through bones but can easily penetrate through skin and muscle. 21. X-rays are used to know about fracture and cracks in bones. They are used in radiotherapy. 22. Ultraviolet rays are commonly known as UV-rays. They are harmful rays having wave length between 10nm to 400 nm. Sequential General Exercise 1 1. Choose the best answer from the given alternatives. a. The critical angle for glass medium is .................. 49° 24° 42° 50° b. When sunlight passes through a glass prism, it splits into .............. colours. five seven three two c. Which of the rays has the maximum wave length? gamma rays X-rays microwave radio wave d. The waves which are used for transmission of television are .............. radio waves microwaves infrared rays X-rays GREEN Science (Physics) Book-9 101

2. Answer the following questions. a. What is light? b. What are the sources of light? Give any three examples. c. Define refraction of light. Draw a figure showing this process. d. Define the following: i. Incident ray ii. Refracted ray iii. Denser medium iv. Rarer medium e. State the laws of refraction of light. f. Write down the cause of refraction of light. g. What is angle of incidence? h. What is angle of refraction? i. What is refractive index? j. Write down the formula to calculate refractive index. k. What is critical angle? l. Write down the value of critical angle for glass, water and ice. m. What is total internal reflection of light? n. What is mirage? How is it formed? o. What is dispersion of light? Draw a neat figure showing this process. p. Write down the cause of dispersion of light. q. What are electromagnetic waves? r. Write down the uses of X-rays and UV-rays. 3. Give reason. a. A pencil appears bent when it is half-immersed in water. b. Diamond sparkles in dim light. c. Light bends when it passes form one medium to another. d. The depth of a pond appears less than its real depth. e. X-rays are harmful for human health. 4. Differentiate between. a. Incident ray and refracted ray b. Denser medium and rarer medium c. X-rays and UV-rays 102 GREEN Science (Physics) Book-9

5. Complete the given figures. a. b. c. d. 49° Water 42° Glass e. f. 90° 90° 6. Draw a neat and labelled figure showing a. Total internal reflection of light b. Dispersion of light 7. A rainbow is seen in a rainy day. Describe its reason. 8. Numerical Problems a. The velocity of light in air is 3 × 108 m/s, calculate the velocity of light when it enters water. The refractive index of water is 1.33. [Ans: 2.25 × 108 m/s] b. The refraction index of water is 4/3. If the apparent depth of water in pond is 4m, calculate the real depth. [Ans: 5.32 m] c. The refractive index of glass is 1.5. Calculate the value of critical angle. [Ans: 42°] d. The real depth of a pond is 5.32 m and its apparent depth is 4m. Calculate the refractive index. [Ans: 1.33] e. A ray of light is passing through a medium making an angle of 45°. If the refractive index of the medium is 1.4, calculate the angle of refraction. [Ans: 30°] GREEN Science (Physics) Book-9 103

Grid-based Exercise 2 Group ‘A’ (Knowledge Type Questions) (1 Mark Each) 1. What is refraction of light? 2. Define emergent ray and angle of reflection. 3. State Snell’s law. 4. What is the refractive index of glass medium? 5. What is meant by denser medium? Write. 6. What is total internal reflection of light? 7. What is optical fibre? 8. What is mirage? 9. What is critical angle? 10. What is dispersion of light? 11. What is the velocity of light in air and glass medium? 12. What is endoscope? Why is it used? 13. How is light travelled in a curved pipe? For Group ’B’ (Understanding Type Questions) (2 Marks Each) 14. The apparent depth of water in a pond appears less than the real depth. Why? 15. Light bends away from normal when it passes from the denser medium to the rarer medium. Why? 16. Why is the refractive index of glass more than that of water? Give reason. 17. Mirage is seen in pitched road or desert during hot summer season. Why? 18. X-ray is called an electromagnetic wave. Why ? Diamond has extraordinary brilliance, why? 19. The red ray lies at the top and violet ray lies at the bottom during dispersion of light. What is the reason behind this fact? 20. Mention the differences between X-ray and visible ray on the basis of source and nature. For Group ‘C’ (Application Type Questions) (3 Marks Each) 21. Write any three uses of X-ray. 22. What is endoscope? Why is it used? What is the relationship among refractive index, real depth and apparent depth? 104 GREEN Science (Physics) Book-9

23. Complete the given ray diagrams. a. N b. air c. 45° B air 50° 90° water 90° AM ice 24. Write down the importance of Ultraviolet ray and X-ray in the field of medical science. 25. Write down the importance of electromagnetic waves. Draw a figure showing dispersion of light through a prism. For Group ‘D’ (Higher Abilities Type Questions) I (4 Marks Each) 26. Study the given diagram and answer the following N questions: i O Air a. Which phenomenon is shown in the given r Glass diagram? M b. Name the angle of incidence, angle of refraction and emergent angle in the given diagram. R c. Which medium is denser out of air and glass? Explain the reason. 27. Answer the following questions on the basis of given ray Light A diagram. Prism B a. Name the rays A and B. b. Which ray has more speed (A or B) in the glass medium? c. Which phenomenon is shown in the given diagram? d. Ray B is bending more than the ray A. What is the reason behind it? 28. Write short notes on: b. Ultraviolet ray a. X-ray 29. The speed of light in air is 3 × 105 km/s and the refractive index of paraffin is 1.44. Calculate the speed of light in the paraffin. Differentiate between angle of incidence and critical angle in two points. 30. A ray of light is passing through a medium by making an angle of 45°. If the refractive index of the medium is 1.4, calculate the magnitude of angle of refraction. The apparent depth of water in a pond appears less than the real depth. Why? GREEN Science (Physics) Book-9 105

UNIT Sound 6 Weighting Distribution Theory : 10 Practical: 2 Before You Begin In our surroundings, we see different things that produce sound. When materials vibrate, sound is produced. So, sound is the form of energy which is produced due to vibration of a material medium. Sound produces sensation of hearing. The substances that produce sound are called sources of sound. Temple bell, horn of vehicles, loudspeaker, guitar, television, etc. are some sources of sound. Sound waves are produced when a material vibrates. Sound propagates through these waves. Learning Objectives Syllabus After completing the study of this unit, students will be able to: i. introduce some terms related to sound, i.e. speed, • Introduction to sound and sound wave frequency and wavelength. • Wave length ii. introduce echo and reverberation and differentiate • Frequency between them. • Speed of sound • Reflection of sound iii. describe the effect of echo and reverberation. • Echo • Reverberation iv. solve simple numerical problems related to sound. • Simple numerical problems related to sound Glossary: A dictionary of scientific/technical terms sound : the form of energy which is produced due to vibration of a body longitudinal wave : a wave that vibrates in the direction that it is moving frequency : the number of complete cycles made in one second echo : the repetition of sound caused by reflection reverberation : the prolongation of the original sound 106 GREEN Science (Physics) Book-9

Sound There are different forms of energy. Sound energy is one of them. Sound produces the sensation of hearing in our ears. It is produced when a body vibrates. So sound is the form of energy produced due to vibration of a material medium. A vibrating body is a source of sound. Radio, temple bell, horn of vehicles, guitar, etc. are some examples of sources of sound. Sound wave The wave produced due to vibration of a material medium is called a sound wave. It is produced due to vibration of solid, liquid and gas. Sound wave needs a medium (solid, liquid or gas) for propagation. It cannot propagate through vacuum. Fig. R CR C R CR C 8.1 C = Compression R = Rarefaction Sound wave When sound waves coming from a source Do You Know propagate through air, and reach our ears, we hear the sound. Sound propagates in the A compression is that part of a form of longitudinal waves. A longitudinal longitudinal wave in which the particles wave consists of alternately arranged of the medium are closer to one another. compressions and rarefactions. When a sound wave passes through air, the particles Rarefaction is the part of a longitudinal of air vibrate back and forth parallel to wave in which the particles are farther the direction of propagation forming apart. compressions and rarefactions. Activity 1 Take a rubber pad and a tuning fork. Hit the rubber pad with the tuning fork. Do you hear vibrations. Now, touch the tuning fork. What do you feel? Write down the conclusion of this activity. Activity 2 Take a small stone and go to a nearby pond. Throw the stone in the pond. Be careful while throwing the stone. Do you see ripples in the pond? Now, throw a leaf on the surface of the ripples. What do you observe? GREEN Science (Physics) Book-9 107

Characteristics of Sound Wave 1. Frequency of Sound Frequency of sound is defined as the number of complete cycles produced in one second. It is denoted by 'f'. In SI system, frequency is measured in hertz(Hz). The larger units of frequency are kilohertz(kHz), Megahertz (MHz), etc. The relation between hertz, kilohertz and megahertz is given below: 1000 Hz = 1 kHz Do You Know 1000 kHz = 1MHz 1 MHz = 1000000 Hz The frequency of a sound wave is 40 Hz means that the sound wave produces 40 completes cycles or vibrations in one second. 2. Time period The total time taken to from a complete wave is called time period. It is denoted by T. In the SI system, the unit of time period is second (s). Time period is calculated by the given formula: Time period (T) = 1 In short, frequency (f) 1 T= f 3. Wave length The total distance covered by a sound wave in one vibration is called wave length. It is denoted by the Greek symbol Lambda (λ). In the SI system, wave length is measured in meter (m). 4. Amplitude Amplitude of a sound wave is defined as the maximum displacement of vibrating particle from its mean position. It is denoted by 'a'. Its SI unit is metre (m). 5. Wave velocity Wave velocity can be defined as the total distance covered by a sound wave per unit time. Its SI unit is meter per second (m/s). The mathematical relation between wave velocity, frequency and wavelength is called wave equation. Wave velocity (v) = frequency of sound (f) × wavelength (λ) In short, v = f × λ 108 GREEN Science (Physics) Book-9

Solved Numerical: 1 The frequency of a sound wave is 60 Hz and its wave length is 85 m. Calculate the speed of sound in the medium. Given, Frequency (f) = 60 Hz Wavelength (λ) = 85 m Speed of sound (v) = ? We know, v = f × λ = 60 × 85 = 5100 m/s \\ The speed of sound (v) = 5100 m/s Solved Numerical: 2 The speed of sound in water is 1500 m/s. If the wave length is 12.5 m, calculate the frequency of the sound. Given, Speed of sound (v) = 1500 m/s Wave length (λ) = 12.5 m Frequency (f) = ? We know, v = f × λ or, 1500 = f × 12.5 or, f = 1500 12.5 = 120 Hz \\ The frequency of sound is 120 Hz. Relation between velocity of sound and medium Sound wave can propagate through solids, liquids and gases. Sound travels with different speed in different media. The speed of sound is maximum in solids and minimum in gases. The molecules in solids are closely packed together and hence carry more vibrations. As a result, the speed of sound is maximum in solids. GREEN Science (Physics) Book-9 109

The speed of sound in some common media is given below: Medium Speed of sound 1. Iron (20 °C) 5130 m/s 2. Glass (25 °C) 5500 m/s 3. Aluminium (25 °C) 5100 m/s 4. Water (20 °C) 1498 m/s 5. Alcohol (25 °C) 1210 m/s 6. Air (0 °C) 332 m/s 7. Hydrogen (0 °C) 1284 m/s Speed of sound in Air or Gas medium The speed of sound in air or gas medium depends on the following factors. 1. Density of air The speed of sound is more in the object having less density. Similarly, the speed of sound is less in the object having more density. In fact, the speed of sound is directly proportional to the square root of the density of air. 2. Temperature of air When the temperature of air increases, the speed of sound in air increases and vice-versa. 3. Humidity of air The speed of sound increases when the humidity of air increases and the speed of sound decreases when the humidity of air decreases. Humidity is the amount of water vapour present in air. 4. Motion of air molecules The speed of sound increases when the direction of movement of sound and air molecules is the same. But the sped of sound decreases if the direction of the sound is opposite to that of the direction of air molecules. Types of Sound wave There are three types of sound waves on the basis of their frequency. They are as follows: (i) Ultrasound (ii) Audible sound (iii) Infrasound (i) Ultra sound Ultra sound is the sound having frequency more than 20,000 Hz. Ultra sound is widely used in medical sector to examine the inner body organs. Ultrasound cannot be heard by human ears. However, bat, insects, rats, birds can hear and produce ultrasound. This 110 GREEN Science (Physics) Book-9

Fig.sound carries more energy due to its high frequency and hence has a great penetrating power. Application of ultrasound Ultrasound is the sound wave having frequency more than 20kHz. So the wave length of ultrasound is very short. Ultrasound can travel from one place to another without bending. 1. Doctors use ultrasound to i. determine the sex of the child in the mother's womb. ii. observe the growth of the embryo. iii. kill bacteria. iv. investigate the diseases in the inner parts of the body. v. locate and observe tumour. vi. do bloodless operation, etc. 2. Ultrasound is used in SONAR to measure the depth of the sea, lake, rive, etc. SONAR stands for Sound Navigation and Ranging. A SONAR consists of a vibrator with high frequency, a source and a detector or receiver. Ultrasound is sent towards the bottom of the sea from its surface. The receiver gets the reflected ray from the bottom of the sea. It also records the time taken by the sound to return the surface. Then the depth of the sea is calculated by the given formula. speed of sound (v) × time (t) Depth (d) = 2 or, v×t d= 2 8.2 SONAR The process of finding the depth of the sea or distance with the help of ultrasound is called echolocation. GREEN Science (Physics) Book-9 111

Solved Numerical: 3 A man shouts in front of a cliff and hears the echo after 0.4 seconds. If the speed of sound in air is 330 m/s, calculate the distance between the man and the cliff. Given, Time (t) = 0.4 s Speed of sound (v) = 330 m/s Distance (s) = ? We know that, s = v × t 2 or, s = 330 × 0.4 m = 66 m 2 \\ The distance between the man and the cliff (s) = 66 m (ii) Audible sound The sound which can be heard by human Do You Know beings is called audible sound. The frequency of audible sound ranges from 20 Rhinoceros can produce infra sound Hz to 20,000 Hz. All sounds that we hear are of frequency 6 Hz. called audible sound. The frequency of sound wave ranges from 1 Hz to 108 Hz. (iii) Infrasound The frequency of sound of man is about 6.5 KHz and that of woman is Infrasound is the sound having frequency 8.5 KHz. less than 20Hz.We cannot hear infrasound. It is produced during earthquake and volcano eruption. Some animals like rhinoceros, elephant, whale, etc. produce infrasound. Activity 3 Take a sensitive stop watch and go to a nearby cliff or in front of a tall building along with your science teacher. Shout in front of the cliff or wall and record the time taken to hear the echo. Now calculate the distance between you and the cliff or wall using the formula. Reflection of sound When a sound wave travelling in a medium strikes a surface, it returns to the same medium. This process is called reflection of sound. So reflection of sound is defined as returning of sound in the same medium when it strikes a hard surface. When we shout in front of a cliff or wall or a hill, we can hear a reflected sound. The reflected sound is called echo. Reflection of sound obeys the laws of reflection of light. We utilize the reflection of sound in the working of megaphone, sound boards, etc. 112 GREEN Science (Physics) Book-9

Activity 4 Take a cardboard, a watch, a mirror and two plastic pipes of equal diameter. Arrange these materials as shown in the figure. Mirror Plastic pipe B Plastic pipe A Fig. Cardboard 8.3 Human Watch Place a watch at the end of plastic pipe A and hear the sound from the end of pipe B. Now, move the plastic pipe B at different angles and listen to the sound. What can you conclude from this activity? Echo When a sound wave strikes any hard surface, it gets reflected, which is called echo. So, echo can be defined as the repetition of sound caused by the reflection of sound. The conditions required for formation of echo are as follows: 1. The minimum distance between the source of sound and the reflecting surface should be at least 17.2 metres. 2. The size of the reflector should be large 3. The loudness of the sound should be sufficient. Let us consider the distance between the sources of sound an the reflecting surface is x meter and the speed of sound in air is 332 m/s. Therefore, the distance covered by sound wave in 0.1 second is 2x meter. Given, Speed of sound in air (v) = 332 m/s Distance covered by sound (s) = 2xm Time (t) = 0.1s The distance covered by sound wave (2x) = Speed of sound × time or, 2x = 332 × 0.1 GREEN Science (Physics) Book-9 113

332 × 0.1 or, x = 2 = 16.6 m \\ The distance covered by sound wave (s) = 16.6 m Reverberation If we speak in a large unoccupied hall, we can hear a number of echoes of original sound. This phenomenon is called reverberation. So reverberation can be defined as the prolongation of original sound due to reflection. Reverberation is the process of intermixing of original sound with a reflected sound. A number of echoes are heard during reverberation. Reverberation occurs when the distance between the source of sound and the reflecting surface is less than 17.2 m. We hear reverberation in an unfurnished empty room and a newly built room. Reverberation does not occur in an occupied room or furnished room due to the presence of sound absorbing materials. Activity 5 Go in a empty room or an unfurnished room and produce sound \"Hello\". Can you hear reverberation? Now, go in an occupied or furnished room of the same size and repeat the above activity. Can you hear reverberation ? What is the reasons behind this fact? Sound absorbing materials are kept in the walls of cinema hall to prevent reverberation so that the audience can hear clear sound. But the sound of singer is prolonged to add melody. Differences between echo and reverberation Echo Reverberation 1. It is the repetition of sound caused by 1. It is the prolongation of original sound reflection. due to reflection. 2. The minimum distance required to 2. The distance required to take place take place an echo is 17.2 m. reverberation should be less than 17.2 3. The intensity of echo is less than that m. of original sound. 3. The intensity of reverberation is more than that of original sound. Application of reflection of sound 1. Reflection of sound is used to investigate diseases like kidney stone, tumour, etc. 2. It is used to investigate the minerals. 3. It is used in SONAR to find out the depth of the sea, lake, river, etc. 4. It is used to observe the growth of the embryo inside the mother's womb. 5. It is used by soldiers to identify weapons. 114 GREEN Science (Physics) Book-9

Refraction of Sound The bending of sound when it passes from one medium to another is called refraction of sound. When sound passes from a denser medium to a rarer medium, it bends away from the normal. Similarly, when sound passes from a rarer medium to a denser medium it bends towards the normal. It shows that sound also obeys the laws of refraction of light. Different layer of air have different temperature. Due to variation in temperature one layer may act as a denser medium and another layer acts as a rarer medium. Sound is heard more clear at night than that in a day due to total internal reflection of sound waves. Sound is heard more clear at night than in day time, why? The surface of the land gets heated due to heat of the sun during day time. Due to this, the temperature of land surface becomes maximum and it decreases gradually on moving upwards. As a result, layers of air behave as denser medium while moving upwards. So, the sound waves bend towards normal and diverge upwards from the source. Due to this reason, less sound waves reach the listener and the sound is not heard clearly. Cold air Hot air Sound wave Sound wave Fig. Hot air Cold air At night 8.4 At day On the other hand, the land surface gradually cools down at night due to absence of solar heat and the layers of air near the land surface behave as denser layer than the upper layers. So the sound waves coming from the source get refracted away from the normal while moving upwards. These sound waves suffer total internal reflection and reach the listener on the surface of the land. As a result, sound is heard more clear at night than in the day time. Intensity of sound Sound waves carry energy with them. The amount of energy that a sound wave carries in one second is called intensity of sound. In simple language, the loudness of a sound is called intensity of the sound. It is measured in decibel (dB). The frequency of some sound /activities is given blow: Activities/sound Intensity (in decibel) 1. Whispering 0 – 20 dB 2. Environment of library 20 – 40 dB GREEN Science (Physics) Book-9 115

3. Sound of printing press 70 – 80 dB 4. Conversation 40 – 60 dB 5. Sound of motor car 110 – 120 dB 6. Sound due to mechanical failure 140 – 160 dB 7. Railway station 85 – 110 dB 8. Heavy street traffic 60 – 70 dB 9. Maximum hearing limit of pain 120 – 140 dB When sound propagates, energy gets transmitted along with the sound. Every wave carries energy from the source. When a wave carries more energy, its amplitude increases. A sound is heard large if it has more intensity and vice-versa. The sound of more intensity affects our ears. The sound with intensity more than 120 dB harms our ears. Deafness is the condition of not hearing the sound of intensity upto 80 dB. The sound ranging from 60 dB to 140 dB is hard to hear. Factors affecting the intensity of sound 1. Amplitude The intensity of sound is directly proportional to the amplitude. 2. Density of the medium The intensity of sound is directly proportional to the density of gas medium. The intensity of sound in cold air is more than that of hot air because the density of cold air is more than that of hot air. 3. Frequency of sound The intensity of sound is directly proportional to the frequency of sound. 4. Distance from the source of sound The intensity of a sound is more near to its. Source and the intensity gradually decreases while moving away from the source. It is because when sound propagates away from the source, some energy is absorbed by air molecules and some energy gets reflected and refracted. 5. Area of vibrating body The intensity of sound increases when the area of the vibrating increases. For example, the intensity of the sound produced by a large temple bell is more than that produced by a smaller one. Pitch of sound We hear sound of different shrillness. The shrillness of a sound is called the pitch of the sound. The sound of a girl is more shrill as compared to that of a boy. Pitch of a sound cannot be measured but it can be felt. The pitch of the sound helps to differentiate between 116 GREEN Science (Physics) Book-9

Fig.a flat sound and a thin or shrill sound. The pitch of a sound depends on the frequency. It means that when the frequency of sound increases, its pitch also increases and vice-versa. Following examples help us to get the concept of pitch of sound. i. The pitch of the sound produced by thin string of guitar is more than the sound produced by its thick string. ii. The pitch of the sound of a child is more than the sound of an adult. iii. The pitch of the sound produced by a long flute is more than that produced by a short flute. Noise pollution The production of unwanted loud sound is called noise pollution. Vehicles, loudspeakers, crowd, musical instruments, machineries, etc. are the sources of noise. Causes of noise pollution i. Automobiles like, trucks, buses, cars, 8.5 motorcycles, aeroplanes, etc. causes noise pollution. ii. Various machines in industries and construction sites cause noise pollution. iii. Use of loudspeakers, tape recorders, etc. causes noise pollution. iv. Playing musical instruments in high volume and blowing pressure horns in vehicles also cause noise pollution. Effects of noise pollution i. Noise pollution weakens hearing capacity. ii. It increases irritability and loss of concentration. iii. It increases blood pressure and mental tension. iv. It causes insomnia, migraine and headache. v. It may rupture the ear drum and cause deafness. vi. Noise pollution results in psychological disorders. Ways to reduce noise pollution i. Avoiding the use of pressure horns in vehicles. ii. Establishing airports away from the human settlement. iii. Using silencers in vehicles. iv. Using earmuffs while working in factories. v. Plantation around industries states. GREEN Science (Physics) Book-9 117

Key Concepts 1. Sound produces the sensation of hearing in our ears. It is produced when a body vibrates. 2. The wave produced due to vibration of a material medium is called a sound wave. It is produced due to vibration of solid, liquid and gas. 3. Frequency of sound is defined as the number of complete cycles produced in one second. It is denoted by ‘f’. In SI system, frequency is measured in hertz(Hz). 4. The total distance covered by a sound wave in one vibration is called wave length. It is denoted by the Greek symbol Lambda (λ). 5. Wave velocity can be defined as the total distance covered by a sound wave per unit time. 6. The mathematical relation between wave velocity, frequency and wavelength is called wave equation. 7. Sound travels with different speed in different media. The speed of sound is maximum in solids and minimum in gases. 8. The speed of sound increases when the direction of movement of sound and air molecules is the same. But the sped of sound decreases if the direction of the sound is opposite to that of the direction of air molecules. 9. Ultrasound is the sound having frequency more than 20,000 Hz. Ultrasound is widely used in medical sector to examine the inner body organs. 10. The process of finding the depth of the sea or distance with the help of ultrasound is called echolocation. 11. Reflection of sound is defined as returning of sound in the same medium when it strikes a hard surface. 12. Echo can be defined as the repetition of sound caused by the reflection of sound. 13. Reverberation can be defined as the prolongation of original sound due to reflection. Reverberation is the process of intermixing of original sound with a reflected sound. 14. Infrasound is the sound having frequency less than 20Hz.We cannot hear infrasound. It is produced during earthquake and volcano eruption. 15. Reflection of sound is used to investigate diseases like kidney stone, tumour, etc. 16. The bending of sound when it passes from one medium to another is called refraction of sound. 17. The amount of energy that a sound wave carries in one second is called intensity of sound. In simple language, the loudness of a sound is called intensity of the sound. It is measured in decibel (dB). 18. The shrillness of a sound is called the pitch of the sound. The sound of a girl is more shrill as compared to that of a boy. Pitch of a sound cannot be measured but it can be felt. 19. The production of unwanted loud sound is called noise pollution. Vehicles, loudspeakers, crowd, musical instruments, machineries, etc. are the sources of noise. Sequential General Exercise 1 1. Choose the best answer from the given alternatives. a. Frequency of a sound is the number of waves formed in ........... second. 1 2 3 5 118 GREEN Science (Physics) Book-9

b. Which of the following is ultrasound? f = 25000 Hz f = 20Hz f = 2 Hz f = 19000 Hz c. Which of the following is the intensity of normal conversation? 20 dB 60 – 70 dB 40 – 60 dB 70 – 80 dB d. The full from of SONAR is ..................... Sound Navigation and Ranging Sound Navigator and Range Sound Navigation and Range Sound Neutralization and Ranging 2. Answer the following questions. a. What is sound ? b. What are sources of sound? Give any two examples. c. What is longitudinal wave? Give one example. d. Write any three characteristics of longitudinal wave. e. What is time period? Write its formula. f. Define wavelength and amplitude. g. What is wave velocity? Write down its formula. h. Write down the relationship between speed of sound and medium. i. Name the factors that affect the speed of sound in air. j. What is ultrasound? Write its practical applications. k. What is audible sound? l. What is infrasound? When is it produced? m. Define reflection of sound. Draw a figure to show this process. n. What is echo? Write down the conditions required to hear an echo. o. What is reflection of sound? Write its applications. 3. What is meant by intensity of sound ? Write down the factors affecting it. 4. Define pitch of sound with examples. 5. What is noise pollution? Write its causes and preventive measures. 6. Differentiate between: a. Echo and Reverberation b. Intensity and Pitch of sound c. Frequency and Wave length 7. Give reason: a. Sound wave is called a longitudinal wave. GREEN Science (Physics) Book-9 119

b. Sound is heard more clear at night. c. SONAR is useful for human beings. d. Sound absorbing materials are kept in walls of cinema hall. 8. Numerical problems a. The sound of wave length 0.022 m has a frequency of 15 kilohertz. Calculate the speed of the wave. [Ans: 330 m/s] b. If the speed of a sound wave in a medium is 5100 m/s and the frequency is 200 Hz, calculate the wave length. [Ans: 25.5 m] c. The speed of sound in a medium is 1200 m/s and the wave length is 15.5 m. Calculate the frequency of the sound wave. [Ans: 77.41 Hz] d. A sound wave is sent at the bottom of a sea. If the echo is heard after 2 second, calculate the depth of the sea. The speed of sound in water is 1500 m/s. [Ans: 2250 m] e. If the sound of thunderstorm in the sky is heard after 20 seconds, calculate the height where thunder storm occurred. The speed of sound in air is 332 m/s. [Ans: 3320 m] f. A ship transmits a sound to the sea of depth 2250m. If the echo is received after 3 seconds, calculated the speed of sound in water. [Ans: 1500m/s] g. The depth of a sea is 6200m and the speed of sound in water is 1500m/s. Calculate the time at which th echo is heard after sending the sound waves. [Ans: 8.26s] Grid-based Exercise 2 Group ‘A’ (Knowledge Type Questions) (1 Mark Each) 1. What is source of sound? 2. What is wavelength? 3. What is frequency? Write down its SI unit. 4. Define mechanical wave with one example. 5. Define longitudinal wave with one example. 6. What is meant by the sound spectrum? 7. Which type of sound can be heard by human ear? 8. What is echolocation? Write. 9. What is echo? 10. What is fathometer? 11. Write down the intensity of the sound that makes us deaf and can rupture the ear drum. 12. What is the frequency of the sound of a man and woman? 13. What is intensity of sound? In which unit is it measured? 120 GREEN Science (Physics) Book-9

For Group ’B’ (Understanding Type Questions) (2 Marks Each) 14. Sound wave cannot propagate in a vacuum. Why? 15. Write any two differences between echo and reverberation. 16. Why the speed of sound in hot air is more than that in the cold air. 17. Why is the wavelength of girls voice is shorter than that of the boys? Give reason. 18. Why is it harmful to settle near the airport? Give reason. 19. Write any two differences between infrasound and ultrasound. 20. Sound absorbing materials are kept in the walls of cinema hall. Give reason. For Group ‘C’ (Application Type Questions) (3 Marks Each) 21. Write any three effects noise pollution. 22. Write any three control measures of sound pollution. 23. Describe in brief the effects of sound pollution in human life. 24. Write down utility of SONAR. How is the depth of sea measured by using sound? Write in brief. 25. If you are kept blind folded in a room, how can you find out whether the room is occupied or blank by producing sound? For Group ‘D’ (Higher Abilities Type Questions) (4 Marks Each) 26. The velocity of sound in three media carbon dioxide, steel (Medium) (Speed) and water is given in the table. Answer the following A 5200 m/s questions on this basis. B 258 m/s C 1498 m/s Name the media ‘A’, ‘B’ and ‘C. Write down the reason for the fact that the speed of sound in^medium ‘A’ is the highest. What happens to the speed of sound when medium ‘B’ is heated? Explain the reason. 27. Differentiate between echo and reverberation. The speed of sound in a medium is 1200m/s. If the wavelength of the sound is 15.5m, calculate the frequency of the sound. Is the sound audible to human ears? 28. A ship transmits a sound to the sea of depth 2250 m. If the echo is received after 3 seconds, calculate the speed of sound in water. Differentiate between Echolocation and Reverberation. 29. A sound is sent at the bottom of a pond. If the echo is heard after 6 seconds, calculate the depth of the pond. The speed of sound in water is 1498m/s. Differentiate between audible sound and infrasound. 30. How is the density of a sea measured by using sound? Explain. GREEN Science (Physics) Book-9 121

UNIT Current Electricity and 7 Magnetism Weighting Distribution Theory : 10 Practical: 2 Before You Begin When a cell is connected to a bulb with copper wires, the bulb glows due to continuous flow of electrons through the wire. Such flow of electrons through a conductor is called current electricity. So, the form of energy which is produced due to continuous flow of electrons is called current electricity. Cell, battery, dynamo and generator are the sources of current electricity. Current electricity is a very useful form of energy. It can be converted into heat energy, light energy, sound energy, magnetic energy. It is used to light bulb, to operate heater, television, computer, radio, fan, telephone, etc. A magnet is a substance that attracts magnetic substances like iron, nickel, etc. and rests in north-south direction when suspended freely The lodestone is a naturally occurring magnet. The Chinese found that the lodestone, when suspended freely, always pointed north-south direction. These days magnets are made artificially from iron, nickel, etc. in various shapes and sizes depending on their uses. These magnets are called artificial magnets. A magnet has two poles at the two ends, a north pole and a south pole. When a bar magnet is suspended by a string freely, one end of the magnet points towards north and the other end towards south. The end which points towards north is called north pole and another is called south pole. The substances which are attracted by a magnet are called magnetic substances. The phenomenon related to attractive and directive properties of magnets is called magnetism. Learning Objectives Syllabus After completing the study of this unit, students will be able to: • Ohm’s law • Resistance i. their measurement. • Conductivity ii. state Ohm’s law and establish the relation among V, • Magnetic field • Magnetic lines of force R and I. • Terrestrial magnetism iii. measure the conductivity of different substances. iv. describe magnetic field and magnetic lines of force and demonstrate them. v. explain the elements of earth’s magnetism (angle of dip and angle of declination). Glossary: A dictionary of scientific/technical terms current electricity : the form of energy which is produced due to continuous flow of electrons through a conductor resistance : the property of a conductor to oppose the flow of electricity through it magnetism : the phenomenon of attracting magnetic substances terrestrial : connected or related with the earth 122 GREEN Science (Physics) Book-9

Current Electricity When a cell is connected to a bulb with copper wires, the bulb glows due to continuous flow of electrons through the wire. Such flow of electrons through a conductor is called current electricity. So, the form of energy which is produced due to continuous flow of electrons is called current electricity. Cell, battery, dynamo and generator are the sources of current electricity. Current electricity is a very useful form of energy. It can be converted into heat energy, light energy, sound energy, magnetic energy. It is used to light bulb, to operate heater, television, computer, radio, fan, telephone, etc. Open Electric Circuit Fig. Switch Fig. (OFF) The electric circuit in which a load does not work is called open circuit. In an open circuit, Open circuit electric current does not flow continuously. So the load (bulb) does not work. A circuit may be open (i) if the switch is turned 'OFF', (ii) wire has broken or (iii)load (bulb) is damaged. 7.1 Closed Electric Circuit The electric circuit in which a load works Switch (ON) continuously is called closed electric circuit. In this condition, current flows continuously through the circuit. The switch is turned 'ON' in a closed circuit. 7.2 Symbols Used in Circuit Diagrams Closed circuit While drawing circuit diagrams, electrical equipment and components of electric circuit are represented by their symbols. The main components and electrical equipments with their symbols and functions are as follows: S. No. Components/Devices Symbols Function 1. Wire To make circuit 2. Wires (not connected) To make electric path 3. Wires (connected) To open and close 4. Switch circuit GREEN Science (Physics) Book-9 123

5. Electric cell Source of electricity 6. Battery Source of electricity 7. Resistor/Load 8. Bulb To convert electrical or energy into another form To convert electricity or into light 9. Ammeter A To measure the current 10. Voltmeter V To measure the voltage 11. Fuse or To break the circuit in case of overloading 12. Galvanometer G To detect electric current An electric circuit showing various components in proper order of combination is given below: AV Fig. 7.3 Electric circuit Electric current The rate of flow of charge in an electric circuit is known as electric current. It is denoted by I. In SI system, electric current is measured in ampere (A). 124 GREEN Science (Physics) Book-9

Electric current flowing through an electric circuit is calculated by the given formula: Electric current (I) = Flow of charge (Q) Q Time (t) t or, I = The SI unit of flow of charge is coulomb (C) and that of time is second. So, the SI unit of electric current is coulomb/second or ampere (A). One ampere current One ampere current (1A) can be defined as the rate of flow of one coulomb charge in one second. We know, I = Q = 1C = 1A t 1s One coulomb charge consists of 6 × 1018­ electrons. So 1A current is also called the rate of flow of 6 × 1018 electrons through a conductor in 1 second. Ammeter and Galvanometer Ammeter is an electric equipment which is used to measure the amount of electric current flowing through a conductor. In order to measure the electric current flowing through an electric circuit, the positive terminal of the cell or battery is connected to the positive terminal of the ammeter and the negative terminal of the cell or battery is connected to the negative terminal of the ammeter. Fig. 7.4 Galvanometer Ammeter Ammeter is a device having low resistance. It is always connected in series in an electric current so that all current flowing through the circuit passes through ammeter. The device which is used to detect the presence of electric current is called galvanometer. It is also used to measure very small amount of electric current. In an electric circuit, galvanometer is always connected in series. GREEN Science (Physics) Book-9 125

Voltmeter The electric equipment which is used to measure the potential difference (p.d.) between any two points of a closed electric circuit is called voltmeter. In an electric circuit, voltmeter is always Fig. connected in parallel to a device, or a source of electricity in order to find the difference in 7.5 potential between any two points of the source or a device. Voltmeter has a high resistance. So Voltmeter it is not connected in series. Differences between Ammeter and Voltmeter Ammeter Voltmeter 1. Ammeter is used to measure the 1. Voltmeter is used to measure the magnitude of electric current flowing potential difference between any two through a circuit. points of an electric circuit or source. 2. It is always connected in series with 2. It is always connected in parallel with the circuit. a resistance. Electromotive force Metals have free electrons so they are called good conductor of electricity. But the free electrons present in metals do not move on their own without supplying energy from an external source. When a source (cell or a battery) is connected to a conductor, it supplies energy necessary for flow of electrons through the conductor and during this process some amount of work is done. So, electromotive force can be defined as the amount of energy supplied to move unit charge (i.e. 1 coulomb charge) throughout the whole circuit. Potential difference Electromotive force creates potential difference across the two ends of a conductor. As a result, an electric current flows from the positive terminal of the source to the negative terminal. During this process, a certain amount of work is done to bring unit charge from one point to another which is called potential difference (p.d.). So, the amount of work done in bringing unit charge or one coulomb charge from one point to another point of a conductor is called potential difference. It is denoted by V. In SI system, potential difference is measured in volt (V). Potential difference is measured in closed circuit. One volt potential difference We know, Potential difference (p.d.) = Charge Work done to another moved form one point \\ 1 volt p.d. = 1 1 joule coulomb 126 GREEN Science (Physics) Book-9

When 1 joule work is done move one coulomb charge from one point to another point of a conductor, it is called one volt potential difference (1 V p.d). Differences between potential difference and electromotive force. Potential difference Electromotive force 1. It is the amount of work done in 1. It is the amount of energy supplied to bringing one coulomb charge from carry one coulomb charge throughout one point to another. the whole circuit. 2. It is less than electromotive force. 2. It is more than potential difference. 3. It is measured in closed circuit. 3. It is measured in open circuit. Ohm's law The relation between electricity flowing through a circuit (I), resistance in the circuit (R) and the potential difference or voltage (V) was discovered by the famous German Physicist George Simon Ohm. This relation is popularly known as Ohm’s law. Ohm's law states that,\"When the temperature and other physical conditions remain constant, the electric current flowing through a conductor is directly proportional to the potential difference across its two ends.” or, I ∝ V or, V ∝ I or, V = RI \\ V = IR ............... (i) Where 'R' is a constant. It is the resistance of the conductor. Form equation (1), I = V R Or, R = V .................... (2) I From above relation, it becomes clear that the resistance of a conductor is the ratio of the potential difference across two ends of a conductor and the current following through it. In SI system, resistance (R) is measured in ohm (Ω). Similarly, electric current (I) is measured in ampere (A) and potential difference is measured in volt (V). Experimental verification of Ohm's law Fig. A 7.6 V ● Take four dry cells of 1.5 V each, a resistor, an ammeter a voltmeter and a switch. GREEN Science (Physics) Book-9 127 ● Now, prepare an electric circuit by connecting them as shown in the figure. ● Measure the voltage and current by connecting four cells in the circuit and fill it in the table.

● Remove one cell from the circuit and measure the voltage and current. ● Now, measure the voltage and current by combining two cells in the circuit and then by connecting one cell in the circuit. ● Calculate the resistance in each case and fill in the given table. Observation Table: S.No. Voltage (V) Current (A) Resistance (R) Conclusion 1. 2. 3. 4. ● Now, plot a graph putting the voltage (V) along the X-axis and current (I) along the Y-axis. The graph is obtained as shown in the given figure. Y Current Resistance (R) (I) Fig. 7.7 Voltage (V) X This graph shows the voltage (V) is directly proportional to the current (I). This activity proves Ohm's law. Solved Numerical: 1 A load of 6 Ω is connected to a source of 12 V potential difference. Calculate the amount of current. Solution: Resistance (R) = 6 Ω Voltage (V) = 12 V Current (T) = ? We know, V = IR or, I = V R 128 GREEN Science (Physics) Book-9

or, I = 12 6 = 2A \\ The amount of current that flows through a circuit = 2A. Conductors Insulators and Semi-conductors The substances through which electric current flows very easily are called good conductors of electricity. Examples : copper, silver, aluminium, etc. The resistors that are sold commercially have very high resistance. They are used to control the amount of current flowing through a circuit. Substances like rubber, dry wood, brick, plastic, paper, etc. do not have free electrons so they do not allow electricity to flow through them. The substances which do not allow electric current to pass through them are called bad conductor or insulators. Some substances like silicon, germanium, etc. contain a few free electrons. So they allow less amount of current to flow them. They are called semi-conductors. The substances which allow less amount of current flow through them are called semi- conductors. More current flows through semi-conductors when their temperature increased. Semi-conductors pass more current than insulators and less current than conductors. Resistance When electric current flows through a conductor, the conductor opposes the flow of current partially. However, the resistance of a conductor depends on various factors like type and nature of conductor, length of conductor, thickness of conductor and so on. Therefore, resistance vary according to nature of the conductor. The property of a conductor due to which it opposes the flow of current through it is called resistance. It is denoted by R. In SI system, resistance is measured in ohm (Ω). The resistance of copper wire is very less but the resistance of nichrome wire is very high. When one volt potential difference is produced while flowing 1A current, the resistance of the conductor is called one ohm (1Ω). The larger units of resistance are kiloohm (KΩ)and megaohm (MΩ) . Relation between ohm, kiloohm and megaohm 1000 Ω = 1K Ω 1000 k Ω = 1M Ω 1M Ω = 1000000 Ω Metals contain a large number of free electrons. Therefore, electric current flows easily through them. But they also have resistance. The resistance of silver wire and copper wire is very less as compared to that of nichrome wire and tungsten wire. GREEN Science (Physics) Book-9 129

Metals like copper, aluminium, etc. have a large number of free electrons. So they produce very low resistance. They have very less resistivity. Therefore, they are called good conductors of electricity. Factors affecting resistance of a conductor 1. The resistance (R) of a conductor increases with the increase in the length (l) of conductor, i.e. R∝l 2. The resistance of a conductor (R) is inversely proportional to its area (A) of cross- section, i.e. R = 1 A 3. The resistance of a conductor (R) is directly proportional to the temperature (T) of the conductor, i.e. R∝T 4. The resistance of the coiled wire is more than that of the straight wire. Combination of Resistance Generally, resistances are connected by two ways. They are: i. Series combination of resistances ii. Parallel combination of resistance i. Series combination of resistances R1 R2 R3 In this type of combination, one resistance is Fig. connected after another in series. In this type of connection, the same current flows through all resistances. In figure, three resistances, R1, R2 and R3 are +– connected in series. So, the total resistance (R) 7.8 is equal to the sum of individual resistance, i.e. (R1 + R2 + R3), Fig: Series combination of resistance or, R = R1 + R2 + R3 In series combination, total resistance increases with the increase in the number f resistance. Similarly, the current in series combination decreases with increase in the number of resistances. If one of the bulb gets fused, remaining bulbs/devices stop working as the circuit is opened. 130 GREEN Science (Physics) Book-9

ii. Parallel combination of resistances R1 R2 In this combination, each resistance is individually R3 connected to the two terminals of the electric source. In this type of combination, the total potential difference across each resistance remains the same. So, the current flowing through each resistance also remains the same. In parallel combination, total resistance is equal to Fig. Fig. the sum of reciprocal of individual resistances. 1 = 1 + 1 + 1 7.9 V R R1 R2 R3 Do You Know In parallel combination, the total resistance decreases on increasing the number of In domestic electric circuits, all resistances. In this connection, if one bulb appliances are connected in parallel gets fused, remaining bulbs/devices work so that if one appliance goes off due to continuously. any reason, the others may still keep on working. Activity 1 Prepare an electric circuit as shown in the figure by connecting two dry cells, a switch, a bulb, an ammeter with a conducting wire (copper wire). Leave a gap AB as shown in the figure. 1. Now, take two pieces of copper wire (one 5 cm long and another 50cm long) and connect A the gap AB one by one by using those pieces of wire. Measure the amount of current flowing 7.10 AB through the circuit. When the circuit is completed by using the short wire (5 cm) the ammeter shows more current flowing through the circuit than the current flown when the circuit is completed by using long wire. It shows that the resistance of a conductor increases on increasing its length. In other words, resistance (R) of a conductor is directly proportional to the length (L) of the conductor. In short, R ∝ L Were, R = Resistance of the wire L = length of the wire 2. Take two wires one thick and another thin having the same length of 10cm. Complete the electric circuit by connecting the circuit by thin and thick wire one by one. Note down the reading of the ammeter in each case. GREEN Science (Physics) Book-9 131

When the circuit is completed by thick wire, more current flows and when the circuit is completed by thin wire less current flows. It shows that the resistance of a conductor decreases if its thickness increases and vice-versa. It means that the resistance of a conductor is inversely proportional to the thickness or cross-sectional area of the conductor. In short, 1 R ∝ a Where, R = Resistance of a conductor a = Cross-sectional area of the conductor. 3. Now, complete the circuit by using a piece of copper wire and a piece of nichrome wire of equal length and equal thickness. Measure the current flowing through the circuit in each case. More amount of current flows when the circuit is completed by using copper wire and very less amount of current flows when the circuit is completed by using nichrome wire. Copper and nichrome are two different materials. Therefore, they have different resistance though they have the same length and thickness. It shows that the resistance differs according to the nature of the conductor, 4. Take a piece of nichrome wire. Complete the circuit using the nichrome wire and measure the current. Now, heat the piece of nichrome wire with the help of a Bunsen burner. Complete the circuit by using hot piece of nichrome wire. Measure the current again. What do you observe? Less current flows when the circuit is completed with hot nichrome wire than it is done with cold nichrome wire. It shows that the resistance of a conductor increases when the temperature increases. The reverse of this is also true. or, R ∝T where, R = Resistance T = Temperature From the above activities, we can concludes that the following factors affect the resistance of a conductor. 1. Length of the conductor 2. Thickness of the conductor 3. Temperature of the conductor 4. Nature of the conductor. Conductivity Most metals are good conductors of electricity and most non-metals are bad conductors or insulators. However, graphite is a non-metal which can conduct electricity. We know that the resistance of a conductor is directly proportional to the length of the conductor, i.e. R ∝ L ...................... (i) 132 GREEN Science (Physics) Book-9

The resistance of a conductor is inversely proportional to the thickness or cross-sectional area of the conductor, i.e. R ∝ 1 ......................... (ii) a Form equation (1) and (2), we get R ∝ L a or, R = δ L ................... (3) a Where δ is a constant called the resistivity of a conductor. Its SI unit is Ω m (ohm meter). So, conductivity can be defined as the reciprocal of resistivity. The conductivity of a conductor is more if its resistivity is less. Similarly, the conductivity of a conductor is less if its resistivity is more. The conductivity of tungesten, nichrome, etc. is less due to more resistivity. Similarly, the conductivity of copper, gold, aluminium, silver, etc. is more due to less reactivity. Generally, the conductivity of metals is more than that of non-metals. On the other hand, the resistivity of non-metals is more than that of metals. Similarly, the conductivity of good conductors is more than that of poor conductors. Magnetism In previous classes, you have studied about magnet and its properties. Similarly, you have studied the methods of making method artificially. A magnet is a substance which attracts the substances like iron, cobalt, nickel, steel, etc. The substances like iron, cobalt, nickel, etc. are called magnetic substances because they are attracted by a magnet. Fig. 7.11 U-shaped magnet Bar magnet The property of a magnet due to which it attracts magnetic substances kept inside the magnetic field is called magnetism. Magnetic force is developed when the molecular magnets present in a magnetic substance align parallel to each other. The magnetic force is maximum at the poles and minimum at the centre. A magnet has two poles. They are north pole (N) and south pole (S). Magnets may be natural or artificial. The natural magnets have less magnetism than the artificial magnets. GREEN Science (Physics) Book-9 133

Magnetic Field The attractive force of a magnet Fig. Fig.7.12 decreases when magnetic substances are taken away from the magnet. A magnet A magnet is attracting iron dust attracts those magnetic substance which Magnet field are kept nearby the magnet. When we increase the distance between a magnet and magnetic substances there is no influence of magnet on these substances. So, magnetic field of a magnet can be defined as the space around a magnet upto where its influence on magnetic substances can be seen. A magnet attracts only theses magnetic substances which are kept inside the magnetic field. Magnetic field is a vector quantity. The 7.13 magnetic field of a powerful magnet is large and that of less powerful magnet is small. Magnetic Lines of Force Inside the magnetic field, numerous lines of force arise form the north pole of a magnet and these lines of force move towards the south pole in the from of curves. These curves are known as magnetic lines of force. So, magnetic lines of force are the paths along which unit north pole moves in a magnetic field. We can trace magnetic lines of force with the help of a magnetic compass. N N N N EE EE S S Fig. SS 7.14 (a) (b) Magnetic lines of force The magnetic field of a magnet is stronger in the pole than in its middle. So the magnetic lines of force are crowded at the poles of a magnet. They always start form the north pole of a magnet and end at south pole of the same magnet. The magnetic lines of force are the closed and continuous curves. The north pole of a magnetic compass kept inside the magnetic field becomes parallel to the magnetic line of force towards the direction of magnetic line of force. When a compass 134 GREEN Science (Physics) Book-9

Fig.is moved form the north pole towards the path shown by it, a continuous and closed curve is obtained which is known as magnetic line of force. When magnetic lines of force are traced by pointing the north pole of a bar magnet towards the geographical north pole, the magnetic lines of force are obtained as shown in the figure 7.12 (a). Similarly, when magnetic lines of force are traced by pointing the north pole of a bar magnet towards the geographical south, the magnetic lines of force are obtained as shown in the figure 7.12 (b). The tangent drawn to a magnetic line of force at any point gives the direction of magnetic field at that point. Properties of magnetic lines of force 1. Magnetic lines of force are continuous and closed curves. 2. Magnetic lines of force always start from the north pole of a magnet and end at south pole (outside the magnet). 3. Magnetic lines of force are crowded near the poles of the magnet as magnetic field is stronger than in the middle of the magnet. Activity 2 Take a bar magnet and place it on the centre of a cardboard paper kept on a wooden board. 7.15 Draw the outline of the bar magnet with a pencil. Draw a dot at the north pole of the bar magnet. Adjust a magnetic compass in such a way that the north pole of the compass lies on the dot drawn. Draw a dot on the north pole of the compass and adjust the north pole of the compass on the second dot. Continue this process till you move to the south pole of the magnet. Now, connect these dots with a pencil. You will get a curve starting from the north pole of the magnet. This curve is called magnetic line of force. By repeating the above process, we can get numerous magnetic lines of force. While drawing magnetic lines of force, we get two points where the magnetic compass does not show any particular direction. These points are called neutral points. These points are denoted by (×) are the neutral points. GREEN Science (Physics) Book-9 135

Neutral points can be defined as the points near magnet where the magnetic field of the magnet is completely neutralized by the earth's Fig. Fig.magnetic field. Therefore, a magnetic compass cannot Neutral point show any particular direction. 7.16 At neutral points, the magnetic field of a magnet and the magnetic field of the earth's magnet is equal but opposite to each-other. Therefore, the resultant field at the poles is zero. Terrestrial magnetism The earth also exhibits magnetism. The earth itself can be taken as an extremely large magnet. So the earth has its own magnetic field. These are magnetic lines of force in the earth's magnetic field. A magnetic compass always remains parallel to the direction of magnetic lines of force of the terrestrial magnet. A bar magnet when suspended freely by tieing with a polyster or nylon thread always remains parallel to the magnetic lines of force of the terrestrial magnet. When we trace the magnetic lines of force using a magnetic compass form the magnetic north pole of the earth, we reach the magnetic south pole of the earth. When an iron rod is buried in the soil facing it north south direction it develops magnetic properties with 4 – 5 days. The magnetic north pole of the earth lies at the geographical south pole and the magnetic south pole of the earth lies at geographical north pole, The magnetic north pole is located at the corner of Antarctica. Similarly, the magnetic south pole is located at Northern Canada. Angle of declination The magnetic poles of the terrestrial magnet and the geographical poles of the earth do not lie in the same plane. So the imaginary line that joins geographical north - south direction and the imaginary line that joins magnetic north - south poles cross each other making a certain angle. This angle is called angle of declination. A q B Magnetic south Geographical F pole of the earth north pole q Angle of declination 7.17 D C Geographical E Angle of declination south pole Angle of declination can be defined as the angle between the magnetic meridian and geographical meridian at a place. The value of angle of declination varies from place to place on the earth. Angle of declination is denoted by q. It helps to find out true geographical direction. 136 GREEN Science (Physics) Book-9

The imaginary line passing through magnetic Declination north pole and magnetic south pole of the earth is known as magnetic meridian. Similarly, the imaginary line passing through geographical north pole and geographical south pole is known as geographical meridian. We should know the value of angle of declination Fig.7.18 of a certain place to know the exact geographical Fig. direction. Angle of declination is used by pilots, navigators, sailors, travellers, etc. to find out the exact direction. Angle of dip Horizontal Angle of q dip Angle of dip at a certain place is defined as the angle made by dip needle to the S horizontal line of that place. A magnetic needle which rotates freely and arranged north -south direction is called dip needle. Dip needle remains parallel to the Vertical magnetic lines of force of the earth's 7.19 magnet. In a certain place, a dip needle inclines towards the ground. It means that the magnetic lines of force of the earth's magnet also incline towards the ground. The angle of dip varies from place to place on the surface of the earth. The value of angle of dip at equator is zero. It is due to the equal effect of north pole and south pole of the terrestrial magnet. The value of angle of dip in Kathmandu is 42° means that the dip needle placed in Kathmandu makes an angle of 42° to the horizontal line of the place. The value of angle of dip increases gradually while moving towards the north or south form the equator and the value of angle of dip at poles is 90°. Sequential General Exercise 1 1. Choose the best answer from the given alternatives. a. Electricity is the form of energy which is produced due to continuous flow of ..................... Protons neutrons electron atoms b. The SI unit of current is ..................... ohm ampere volt watt GREEN Science (Physics) Book-9 137

c. The resistance of a conductor depends on ..................... length of the conductor temperature of the conductor nature of the conductor all of th above d. The device which is used to measure the potential difference is called ..................... ammeter voltmeter galvanometer dip needle e. The value of angle of dip at the equator is ..................... 0° 90° 45° 180° 2. Answer the following questions. a. What is electric current? b. Write any two sources of electric current. c. Define electric circuit. Draw a neat figure showing a closed electric circuit. d. What is meant by electromotive force? Define potential difference. e. State Ohm's law and prove V = IR. f. Define resistance of a conductor and write its three units. g. Write any three factors that affect the resistance of a conductor. h. What happens to the resistance of a conductor when i. the length of the conductor is increased? ii. thickness of the conductor is decreased? iii. temperature of the conductor is increased? i. What is meant by conductivity? How does it differ from resistivity? j. What is magnetism? k. What is magnetic field? l. What are magnetic lines of force? m. What is terrestrial magnetism ? Write any two evidences of existence of terrestrial magnetism. n. What is angle of declination? Write its advantages. o. What is angle of dip? What is its value: i. at equator? ii. at north pole? iii. at south pole? p. What is meant by the statement that the angle of dip in Kathmandu is 42°? q. What are neutral points? 3. Differentiate between: a. Potential difference and Electromotive force b. Voltmeter and Ammeter c. Angle of dip and Angle of declination 138 GREEN Science (Physics) Book-9

4. Give reason: a. Ammeter is always connected in series in an electric circuit. b. Voltmeter is always connected in parallel. c. The angle of dip at the equator is 0°. d. The angle of dip at poles is 90°. e. Angle of declination is very useful for us. 5. Draw a neat figure showing magnetic lines of force. 6. Numerical problems: a. If 360 coulomb charge flows through a conductor in 3 minutes, calculate the current flowing through a circuit. [Ans: 2V] b. A current of 5A flows when 300 coulomb charge flows through a circuit. Calculate the time taken to flow the charge. [Ans: 60 s] c. Calculate the current flowing through a circuit when a resistance of 6 Ω is connected to the potential difference of 3V. [Ans: 0.5A] d. Calculated the resistance of a conductor when a current of 6A can be drawn by connecting it to 12V potential difference with a wire. [Ans: 2 Ω] e. A current of 2A flows through a circuit when a conductor of 6 Ω is connected to a source. Calculate the potential difference or voltage. [Ans: 12V] Grid-based Exercise 2 (1 Mark Each) Group ‘A’ (Knowledge Type Questions) 1. What is current electricity? 2. Define electromotive force. 3. Define one ohm resistance. 4. What is an electric load? Write with an example. 5. What is the actual direction of current flow? Write. 6. What is ammeter? How is it connected in an electric circuit? 7. What is potential difference? 8. What is voltmeter? How is it connected in an electric circuit? 9. State Ohm’s law. 10. What is angle of declination? 11. What is magnetic field? 12. What is a neutral point? 13. What is the magnitude of angle of dip at the magnetic equator? For Group ’B’ (Understanding Type Questions) (2 Marks Each) 14. Copper is called good conductor and silicon is called semiconductor. Give reason. 15. Hydroelectricity is called a renewable source of energy, why? Why is voltmeter connected parallel with loads ? 16. Write any two differences between potential difference and electromotive force. 17. Write any two differences between Ammeter and Voltmeter. GREEN Science (Physics) Book-9 139

18. We should know angle of declination to know the actual geographical direction of a certain place, why? 19. Differentiate between angle of dip and angle of declination. 20. The value of angle of declination changes from place to place. Why? For Group ‘C’ (Application Type Questions) (3 Marks Each) 21. State Ohm’s law and prove that V = IR. 22. Define series combination and parallel combination of cells. In which condition are cells connected in series? Z 23. How is galvanometer connected in an electric circuit? Write down the utility of angle of declination. 24. Write any three evidences that prove the magnetic property X of the earth. Label ‘X’, ‘Y’ and ‘Z’ shown in the given diagram. Y 25. How is the angle of dip at a certain place measured? Describe in brief with figure. For Group ‘D’ (Higher Abilities Type Questions) (4 Marks Each) 26. Study the electric circuit shown in the given diagram and answer the following questions: a. What happens to the brightness of bulb when the number of electric bulb is reduced ? Why ? b. Write down an advantage of such type of circuit. c. Name the type in which the bulbs are connected in the circuit. Define the type of connection of bulbs. 27. A current of 0.33 A is flowing through a circuit. If the potential difference across two points of a wire is 5V, calculate the resistance. Differentiate between closed circuit and open circuit. 28. The power of the bulb in a car is 60W. Calculate the current that flows when it is connected to a battery of 6V. Also, calculate the resistance of the bulb. Differentiate between semiconductor and insulator. 29. Study the given figures and answer the following questions: a. Which type of combination of loads 6V 6V is shown in the fig. (a) and fig. (b)? 1.5 Ω 1.5 Ω 1.5 Ω b. How much current flows in the circuit (a) and (b)? 1.5 Ω 30. The voltage of an electric line is 220V. If 4.54 A current flows through the line when a heater is connected to the circuit, calculate the power of the heater. Differentiate between angle of declination and angle of dip. 140 GREEN Science (Physics) Book-9

Chemistry UNIT Classification of 8 Elements Weighting Distribution Theory : 9 Practical: 2 Before You Begin Any thing having mass and volume is called matter. Different substances around us are matter. Solid, liquid and gas are three states of matter. Some matter can be decomposed into simple substances. These substances are called compounds. A compound is a substance formed by combination of two or more elements in a fixed proportion by weight. The smallest particle of a compound is called molecule. Water (H2O), Sodium chloride (NaCl), Carbon dioxide (CO2), etc. are some examples of compounds. Some matter cannot be divided into simple substance by ordinary chemical means. These matter are called elements. The smallest particle of an element is called atom. An element is a pure substance made of atoms. Hydrogen (H), Oxygen (O), Carbon (C), Sodium (Na), etc. are some examples of elements. Scientists have discovered 118 elements so far. Among them 92 elements are found in nature and the remaining 26 elements have been synthesized in laboratory by scientists. Learning Objectives Syllabus After completing the study of this unit, students will be able to: • Element and compound i. define atom and describe the atomic structure of • Atomic structure • Electronic configuration elements and their electronic configuration. • Valence electron and valency • Chemical bond ii. introduce valency in terms of combining capacity • Molecular formula of elements and write molecular formulae of some • Radicals and their types common compounds. • Electrovalent bond and iii. introduce radicals and ions with examples. covalent bond iv. write molecular formulae of some common compounds. Glossary: A dictionary of scientific/technical terms matter : anything having mass and volume atom : the smallest particle of an element molecule : the smallest particle of a compound valency : the combining capacity of an atom with another atom to form a molecule symbol : an abbreviation of the full name of an element abbreviation : a short form of a word GREEN Science (Chemistry) Book-9 141

Elements Different types of substances are found in our surroundings. Among them, some cannot be broken down into simple substances by ordinary chemical means. These substances are called elements. An element is a pure substance which cannot be divided into two or more simpler substances by ordinary chemical means. Hydrogen (H), Carbon (C), Oxygen (O), Sodium (Na), Calcium (Ca), Copper (Cu), Silver (Ag), Gold (Au), etc. are examples of elements. Elements are pure substances formed by only one kind of atoms. Fig. Fig.8.1 Fig. Gold Iron Silver Copper Hydrogen, gold, iron, silver and copper are called elements because they are formed by only one type of atoms and they cannot be broken down into simpler substances by ordinary chemical methods. All the atoms of the same element are identical but atoms of different elements are different. Scientists have discovered 118 elements so far. Among them 92 are found in nature and remaining 26 elements have been discovered by chemists in the laboratory. At normal temperature and pressure, elements exist in all three states, viz. solid, liquid and gas. Elements like iron, copper, gold, silver, sodium, magnesium, aluminium, etc. are found in solid state. Elements like mercury, bromine, caesium and gallium are found in liquid state and elements like hydrogen, helium, nitrogen, oxygen, neon, argon, etc. are found in gaseous state. Compound Substances like water (H2O), sodium chloride (NaCl), H H carbon dioxide (CO2), calcium carbonate (CaCO3) etc. O are some examples of compounds. They are formed by combination of two or more elements in a fixed proportion by weight. So, the chemical substance formed by a combination of two or more elements in a 8.2 Water (H2O) fixed proportion by weight is called a compound. A compound is made of two or more atoms. Two or more active elements undergo chemical reaction which results in the formation of a compound. A compound does not contain the properties of its constituent elements. Water is a compound formed by combination of two 8.3 OCO atoms of hydrogen and one atom of oxygen. Carbon dioxide (CO2) Carbon dioxide is a compound formed by combination of one atom of carbon and two atoms of oxygen. Sodium chloride (NaCl) is formed by combination of one atom of sodium and one atom of chlorine. 142 GREEN Science (Chemistry) Book-9

Magnesium chloride (MgCl2) is a compound Cl Mg Cl formed by a combination of one atom of magnesium Magnesium chloride (MgCl2) and two atoms of chlorine. Similarly, Aluminium chloride is formed by combination of one atom of Aluminium and three atoms of chlorine. Fig.8.4 Fig. Differences between Elements and CompoundsFig. Elements Compounds 1. An element is a pure substance formed 1. A compound is a pure substance by only one type of atoms. formed by a combination of two or more types of atoms. 2. The smallest particle of an element is 2. The smallest particle of a compound is called atom. called a molecule. 3. An element cannot be split into simpler 3. A compound can be split into two or substances by ordinary chemical more simpler substances by ordinary means. chemical means. Atom An atom is the smallest particle of Shell or orbit an element which can take part in a chemical reaction. All atoms of Proton an element are similar but atoms of different elements are different. For 8.5 Neutron example, all atoms of gold are identical Nucleus but the atom of gold differs from that Electron of copper or silver. Scientists have discovered 118 types of atoms so far. An atom Among them, most atoms cannot exist freely in nature but atoms of helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe) and radon (Rn) exist freely in nature. Carbon (C), hydrogen (H), oxygen (O), sodium (Na),calcium (Ca), Copper, (Cu), Silver (Ag), Gold (Au), etc. are the examples of atom. Molecule A molecule is smallest particle of an element or a OO OO compound which can exist freely in nature. There are a C thousand types of molecules. The molecule of an element 8.6 Oxygen molecule Carbon dioxide molecule consists of two or more atoms of the same kind where as the molecules of a compound consists of two or more atoms of different elements. Hydrogen (H2), oxygen (O2), ozone (O3), water (H2O), calcium carbonate (CaCO3), sodium chloride (NaCl), magnesium carbonate (MgCO3), etc. are some examples of molecules. GREEN Science (Chemistry) Book-9 143

A molecule of oxygen consists of two atoms of oxygen. Similarly, a molecule of carbon dioxide consists of one atom of carbon and two atoms of oxygen. Structure of an atom Electron An atom is made of three fundamental Fig. Shell or orbit particles or sub-atomic particles. They Nucleus are proton, neutron and electron. Proton Among them, proton and neutron are located in the nucleus whereas electrons Neutron revolve around the nucleus in elliptical orbits or shell. Protons are positively charged sub- 8.7 Oxygen atom atomic particles located in the nucleus of an atom. The mass of one proton is equal to that of one hydrogen atom. The mass of one hydrogen atom is considered as one amu. (atomic mass unit). The mass of one proton is 1837 times more than that of one electron. A proton is denoted by p+. Neutrons are chargeless sub-atomic particles located in the nucleus of an atom. The mass of a neutron is equal to that of a proton. A neutron is denoted by n°. Electrons are negatively-charged sub-atomic particles that revolve around the nucleus in elliptical orbits or shells. The mass of one electron is 1837 times smaller than that of one proton. An electron is denoted by e–. Atomic mass unit (amu) The mass of an atom is very small. It cannot be expressed into milligram, gram and kilogram. So the mass of proton, neutron and electron is expressed in atomic mass unit (amu). The mass of a hydrogen atom is 1 amu. 1 gram is equal to 6 × 1023 amu. The mass of a proton is 1 amu. So the mass of 6 × 1023 protons is equal to 1 gram. The mass of one proton is equal to that of one neutron. Similarly, the mass of one neutron is 1837 times more than that of one electron. In short, 1p+ = 1n° = 1837e– = 1 amu Electric charge Protons and electrons are electrically Do You Know charged. Protons have positive charge and An atom is electrically neutral due to the electrons have negative charge but neutrons presence of equal number of protons and do not have any charge. The unit of electronic electrons having opposite charges. charge is coulomb (coul). In a neutral atom, the number of protons is equal to that of electrons. One coulomb charge contains 6.25 × 1018 electrons. The given table shows the comparative study of proton, neutron and electron. 144 GREEN Science (Chemistry) Book-9

S.N. Sub-atomic particles Symbol Charge Mass Location 1. Proton p+ +ve 1amu Nucleus 2. Neutron n° nil 1 amu Nucleus 3. Electron e– –ve Orbit or shell 1 amu 1837 Atomic number An atomic number of an atom is the total number of protons present in the nucleus of that atom. It is also defined as the total number of electrons present in a neutral atom. An atomic number is denoted by z. In short, Atomic number (z) = No. of p+ = No. of e– in a neutral atom Atomic mass or Atomic weight The sum of number of protons and neutrons present in the nucleus of an atom is called atomic mass or atomic weight. It is denoted by A. Atomic mass (A) = No. of p+ + No. of n° No. of p+ = Atomic mass (A) – No. of n° No. of n° = Atomic mass (A) – No. of p+ No. of n° = Atomic mass (A) – Atomic number (z) [ Atomic number = No. of p+] Worked out Numerical: 1 The atomic number of calcium atom is 20 and its atomic mass is 40. Calculate the number of protons, neutrons and electrons. Given, Atomic no. (z) = 20 Atomic weight (A) = 40 We know, Atomic no. (z) = No. of p+ = No. of e– = 20 = 20 \\ No. of p+ = 20 No. of e– = 20 Now, Atomic mass = No. of p+ + No. of n° GREEN Science (Chemistry) Book-9 145

or, 40 = 20 + No. of n° = 20 or, No. of n° = 40 – 20 Electronic Configuration Electronic configuration is the systematic distribution of electrons in various orbits or energy levels around the nucleus of an atom. The distribution of electrons around the nucleus was proposed by Bohr and Bury in 1921 AD. 7 6 5 4 3 2 1 p+ K L M N O P Q n° 2 8 18 32 32 18 8 Fig. 8.8 Electronic configuration in various orbits In an atom, the shells or orbits are named as K, L, M, N, O, P and Q according to the increasing distance from the nucleus. 2n2 rule determines the maximum number of electrons that are accommodated in different shells of an atom. In this rule, 'n' represents the number of shells or orbits. Orbit/shell No. of shell Maximum number of electrons K n=1 2n2 = 2 × 12 = 2 L n=2 2n2 × 2 × 22 = 8 M n=3 2n2 = 2 × 32 = 18 N n=4 2n2 = 2 × 42 = 32 The outermost shell of an atom cannot accommodate more than 8 electrons. So 2n2 rule is applicable for a few elements and shells K, L, M and N. The maximum number of electrons that can be accommodated by shells O, P and Q is 32, 18 and 8 respectively. 146 GREEN Science (Chemistry) Book-9

Atomic Name of Symbol Number of Atomic mass Electronic Valency number elements (p+ + n°) configuration 1. Hydrogen p+ n° e– K L MN 2. Helium 3. Lithium H 1 0 1 1+0=1 1 1 4. Beryllium 5. Boron He 2 2 2 2 + 2 = 4 2 0 6. Carbon 7. Nitrogen Li 3 4 3 3 + 4 = 7 21 1 8. Oxygen 9. Fluorine Be 4 5 4 4 + 5 = 9 22 2 10. Neon 11. Sodium B 5 6 5 5 + 6 = 11 23 3 12. Magnesium 13 Aluminium C 6 6 6 6 + 6 = 12 24 4 14. Silicon 15. Phosphorus N 7 7 7 7 + 7 = 14 25 3 16. Sulphur 17. Chlorine O 8 8 8 8 + 8 = 16 26 2 18. Argon 19. Potassium F 9 10 9 9 + 10 = 19 2 7 1 20. Calcium Ne 10 10 10 10 + 10 = 20 2 8 0 Na 11 12 11 11 + 12 = 23 2 8 1 1 Mg 12 12 12 12 + 12 = 24 2 8 2 2 Al 13 14 13 13 + 14 = 27 2 8 3 3 Si 14 14 14 14 + 14 = 28 2 8 4 4 P 15 16 15 15 + 16 = 31 2 8 5 3, 5 S 16 16 16 16 + 16 = 32 2 8 6 2, 6 Cl 17 18 17 17 + 18 = 35 2 8 7 1 Ar 18 22 18 18 + 22 = 40 2 8 8 0 K 19 20 19 19 + 20 = 39 2 8 8 1 1 Ca 20 20 20 20 + 20 = 40 2 8 8 2 2 The atomic structure and electronic configuration of the first 20 elements is given below: 1. Hydrogen (H) 1pFig. Atomic number = 1 0n Atomic mass = 1 8.9 Shells KL MN Hydrogen ×× No. of e– 1 × GREEN Science (Chemistry) Book-9 147

2. Helium (He) 2p Fig. Atomic number = 2 2nFig. Fig. Fig. Fig. Fig. Atomic mass = 4 8.10 Helium Shells KL MN ×× 3p No. of e– 2 × 4n 3. Lithium (Li) 8.11 Atomic number = 3 Lithium Atomic mass = 7 4p Shells KL MN 5n ×× No. of e– 2 1 8.12 Beryllium 4. Beryllium (Be) Atomic number = 4 5p Atomic mass = 9 6n Shells KL MN 8.13 ×× Boron No. of e– 2 2 6p 5. Boron (B) 6n Atomic number = 5 Atomic mass = 11 8.14 Carbon Shells KL MN ×× 7p No. of e– 2 3 7n 6. Carbon (C) 8.15 Atomic number = 6 Nitrogen Atomic mass = 12 Shells KL MN ×× No. of e– 2 4 7. Nitrogen (N) Atomic number = 7 Atomic mass = 14 Shells KL MN ×× No. of e– 2 5 148 GREEN Science (Chemistry) Book-9

8. Oxygen (O) Atomic number = 8 Atomic mass = 16 Shells KL MN 8p ×× 8n No. of e– 2 6 8.16 Fig. Fig. Fig. Oxygen 9. Fluorine (F) Atomic number = 9 9p Atomic mass = 19 10n 8.17 Shells KL MN Fluorine ×× No. of e– 2 7 10p 10n 10. Neon (Ne) 8.18 Atomic number = 10 Neon Atomic mass = 20 11p Shells KL MN 12n ×× No. of e– 2 8 8.19 Sodium 11. Sodium (Na) Atomic number = 11 12p Atomic mass = 23 12n Shells KL MN 8.20 1× Magnesium No. of e– 2 8 Fig. Fig. GREEN Science (Chemistry) Book-9 149 12. Magnesium (Mg) Atomic number = 12 Atomic mass = 24 Shells KL MN 2× No. of e– 2 8

13. Aluminium (Al) Atomic number = 13 13p Atomic mass = 27 14n Shells KL MN 8.21 3× Aluminium No. of e– 2 8 Fig. Fig. Fig. Fig. 14p 14. Silicon (Si) 14n Atomic number = 14 8.22 Atomic mass = 28 Silicon Shells KL MN 15p 4× 16n No. of e– 2 8 8.23 15. Phosphorus (P) Phosphorus Atomic number = 15 16p Atomic mass = 31 16n Shells KL MN 8.24 5× Sulphur No. of e– 2 8 17p 16. Sulphur (S) 18n Atomic number = 16 Atomic mass = 32 8.25 Chlorine Shells KL MN 6× No. of e– 2 8 17. Chlorine (Cl) Atomic number = 17 Atomic mass = 35 Shells KL MN Fig. 7× No. of e– 2 8 150 GREEN Science (Chemistry) Book-9