buku physics11

When enough electrons build up in the sphere, which represents theupper atmosphere in a lightning storm, the resulting repulsion of like nega-tive charge causes electrons to be repelled from the sphere in a fashion sim- Fig.15.13 Telephone, coaxial,ilar to a lightning bolt. and fibre optic cablesMany safety systems use a grounding mechanismthat provides a path for excess electrons to flow tothe ground, or that allows a deficit of electrons to beneutralized by transferring electrons from theground. Materials made up of atoms with looselybound electrons that can be easily transferred toneighbouring atoms are called conductors. Othermaterials have atoms with tightly held electrons thatare not transferred anywhere. These materials are For imagecalled insulators and are often used to protect peo- see studentple or property from electric shock. Figure 15.13 text.illustrates how both insulators and conductors areused in modern wiring. The copper in the telephonecable conducts electrons, and the outer plastic coatinginsulates the surroundings from electron transfer.These wires make perfect “electron tunnels” thatallow transfer of charge from one place to another,but keep the effects of the environment out.1. State the rules of reaction between positives and negatives. Is there gpplyin another force that behaves similarly? Co a the2. How do neutral objects interact with charged objects? Explain the tsncep processes.Neutral ObjectsA neutral object (its total charge is zero) doesn’t mean that the objecthas no charge. In fact, a neutral object is a dynamic system of balance.It has the same number of positive charges as negative charges.Because they are in balance, we cannot detect them. Similarly withwhite light. White light is composed of many colours, each having adifferent wavelength. As long as all the colours are present, we seewhite light and not its component colours. chapter 15: Electrostatics 529

15.4 Measurement of Charge Early studies of electricity required scientists to quantitatively measure amounts of electric charge. Because it’s impossible to count individual elec- trons, scientists grouped electrons into “bundles” containing more or less equal amounts of electrons. One bundle of electrons or charge, symbolized by Q, was given the unit name of coulomb (C), after Charles de Coulomb, a French scientist and inventor (1736–1806). This approach is similar to consistently filling an egg carton with a dozen eggs, but not knowing that there are 12 eggs in a dozen. In 1911, Robert Millikan performed his famous oil drop experiment. He estimated that the number of electrons in one coulomb of charge is 6.25 ϫ 1018.Fig.15.14 In 1911, American scientist R.A. Millikan succeeded Atomizer 800 V DCin measuring the charge on a single oil droplet. A droplet of Oilradius 10Ϫ6 m falls through the air at a constant speed and Brass platespicks up a charge by contact with an ion. The droplet then fallsbetween two brass plates. The upper plate is connected to abattery that gives the plate a charge opposite to that on thedroplet. Because the droplet is charged, it can be acted on byan electric force. The electric force causes the droplet to moveupwards. When the battery is switched off, the droplet falls. Ifthe distance and time of the droplet’s motion are known, itsspeed can be calculated. If we know the calculated speeds, thevoltage of the battery, and the properties of oil and air, we candetermine the charge on the oil droplet.To convert between charge and The overall charge on an object may be determined by the equationnumber of electrons, Q ϭ Ne1 C ϭ 6.25 ϫ 1018 e or1 e ϭ 1.602 ϫ 10Ϫ19 C where Q is the amount of charge in coulombs, N is the total number of electrons in either deficit or excess, and e is the charge on an electron, 1.602 ϫ 10Ϫ19 C. e x a m p l e 2 Static cling Jordan put her wool socks and a silk shirt together in a tumbling clothes dryer after washing them. When she took them out, she noticed that the wool sock and whatever was left of the silk shirt were stuck together because of an electrostatic force of attraction. If 3.7 ϫ 1024 electrons were transferred between the wool and the silk, what is the amount of charge on the silk shirt?530 u n i t e : E l e c t r i c i ty a n d M a g n et i s m

Solution and Connection to TheoryGivenN ϭ 3.7 ϫ 1024 electrons Q ϭ ?The charge on the silk is a result of the transfer of electrons. Whateverthe amount of charge is on the silk, the wool has the same amount, onlyof opposite charge. One would be due to an excess of 3.7 ϫ 1024 electronsand the other would be due to a deficit.Q ϭ Ne ᎏ(1.602 ϫᎏ10Ϫ19 C) ϭ 3.7 ϫ 1024 e e ϭ 5.9 ϫ 105 CTherefore, the charge on either the wool or the silk is 5.9 ϫ 105 C. In Table 15.1, we see that wool is higher on the list than silk. We know gpplyinthat the wool would lose electrons and silk would gain them. Therefore, the Cowool has a charge of ϩ5.9 ϫ 105 C and the silk has a charge of Ϫ5.9 ϫ 105 C. a the tsncep If any two of these materials come into contact, electrons are passedfrom the item with a weak attraction for electrons to the one with thehigher attraction for electrons. The electron receiver becomes negative andthe electron donor becomes positive. 1. What is the fundamental charge carrier? Which charge is the mobile one? How do you produce a net positive and a net negative charge? 2. a) How many coulombs are there in one fundamental charge? b) How many charges are there in one coulomb? 3. State the law of conservation of electric charge. What other conser- vation laws do you know? 4. How is charge transferred from one object to another? What dic- tates which way the charge moves? 15.5 Force at a Distance A field is a volume of three- dimensional space in which aIn Chapter 4, we learned that a force is defined as a push or pull on an object. certain property or quantityOne difference between the forces we have studied and electrostatic forces is is distributed.that electrostatic forces can occur between charged objects that are some dis-tance from one another. These forces are said to work “at a distance.” In gen-eral, the farther apart the charges, the weaker the forces between them. When we wish to describe our position in relation to certain geographicalareas, we use a map. To describe the forces that exist in areas around electro-static charges, we use a field map. A field map describes an electric field, thespace around a single charge or an array of charge(s) in which electric forces act. chapter 15: Electrostatics 531

Fig.15.15 A field map analogy The quantity that is distributed in an electric field is force. If a positive test charge is placed inside the electricTest charge Charge creating field field of another charge, the two charges will experience a force of attraction or repulsion. Just as your road map helps you choose the direction in which you should travel, a field map tells you not only the strength of a force, but also the direction in which it will act. ϩϩ The “hot/cold” game is a good analogy for mapping out a field structure. One person is blindfolded and sets out to find an object. He or she walks around the room, and his or her dis- tance from the object is mapped by others shouting “warmer” or “colder,” depending on whether the person is getting closer or farther away from the object. Figure 15.15 illustrates how an Field map electric field relays its information to us in a similar way. The closer the positive test charge (analogous to the blindfolded partici- pant) comes to the positive object, the stronger the repulsive force becomes. The field map shows the relative value of force strength based on how close the field lines in the map are to each other. The direction of the field lines (as shown by the arrows) shows the direction of the force. The “warmer” theFig.15.16 Field maps arounddifferent charge configurations For image ϩ see student text.(a) Single positive charge For image ϩ see student ϩ text.(b) Equal like charges(c) Equal and opposite charges For image ϩ532 see student Ϫ text. unit e: Electricity and Magnetism

test charge is to the charged object, the closer together the field lines. By con-vention, the arrows on the lines of force indicate the direction a positive testcharge would take if placed at that spot in the electric field. Figure 15.16shows examples of field maps around charges. Each line represents the pathof a positive test charge if it were placed in the field and released. Other forces that act at a distance include magnetic forces (see Chapter17) and the force of gravity (see Chapter 5). These two force fields are illus-trated in Fig. 15.17.Fig.15.17 Magnetic and gravitational fields. The arrowsindicate the direction(s) of motion of objects in that field.N S a ts Test g Test mass magnet Co pplyin(a) (b) the ncep 1. What other forces act without contact? 2. How does the magnitude of the electrostatic force vary with dis- tance? Does any other force act the same way? 3. Compare the electrostatic force to the gravitational force. 15.6 Applications of Electrostatics and Charge TransferThe concepts of force at a distance and static electric charge have manypractical applications. Table 15.2 describes many examples of how electricforce at a distance and electrostatics may be applied.chapter 15: Electrostatics 533

Table 15.2 Applications of Electrostatic ForcesApplication and description Schematic diagram Photo(a) Electrostatic Precipitator Exhaust gases enter Insulator For imagethe precipitator cylinder that is made of a grounded see studentmetal conductor. A centrally placed, highly charged Cleansecondary conductor ionizes the gases (splits them gas out text.into charged particles) and electrical “lightning-like”discharges occur. The solid and liquid ions are Negative For imagerepelled to the outer wall, where they are neutralized electrode see studentand collected. The resulting cleaner gases arereleased through the top. Weight text. Dirty gas in Collected gas(b) Electrostatic Air Cleaner The particles in air are PsrcerfeiletenrChargin؊g ؉ Charcoalionized (given an electrostatic charge) by corona section Collecting filterwires as they enter the air cleaner at one end. The col-lecting plates at the other end have an opposite sectioncharge to the particles, which are attracted and gath-ered there. Cleaner, particle-free air is then forcedback out of the unit.(c) Electrostatic Painting The item to be painted is Ϫ Positivelygiven a positive charge and the paint is given a nega- chargedtive charge as it leaves the spray nozzle. The paint is Nozzle of targetelectrostatically attracted to the area being painted. spray gun ϩ Negatively charged paint droplet Electrified metal screen(d) Bug Zapper A black light attracts flying insects Insect tries (high voltage AC)through a highly charged mesh screen. A groundedcentral core means that the bugs are hit by tiny light- to fly towards For imagening bolts when they enter the zapper. see student UV light Ultraviolet text. light to attract insects Collecting pan(e) Photocopier The drum, which is coated with sele- 4. Transfer 6. Clean Light Originalnium, is positively charged. An image of an original copydocument is shone on the drum, leaving the areas that Cleaningreceived bright light (the non-print areas) neutral. The Light imagingremaining areas, which are still positively charged, pick Paper 5. Fuse Cleaning lamp the P/Rup negatively charged toner (made of carbon and car- path coronaried by plastic beads) in the shape of the print. Fuse CleaningPositively charged paper rolls over the drum, attracting brushthe negatively charged toner. The sheet is then heatedto “set” the image on the paper. (ϩ) Paper Transfer 1. Charge For image coronϩaϩϩϩϩϩϩPdhϩrouptmoocsboitenivcdeoulmycteivseϩpcCohdosarirturoigmvneeasly see student picks up (Ϫ) toner to text. make image Magnetic Ϫ Ϫ ϩϩϩ cϩhaϩrgeϩd ϩϩ Mirror brush Ϫ Ϫ White light Ϫ Black print neutralizes charge development ϪϪ ϪϪ leaves ϩ charge system 3. Develop 2. Expose534 u n i t e : E l e c t r i c i ty a n d M a g n et i s m

The force of repulsion between electrons causes electrons to flow throughconductors, creating current electricity. Current electricity is what runs allour modern conveniences on which we’re so dependent. In the next chapter,we will study the concept of current electricity so that we can understandhow it has become such an important commodity for modern living.1. Describe the differences between charging by induction and charg- gpplyin ing by contact. Co a the2. Describe the hydrogen atom in terms of the electrostatic force and tsncep the force of gravity. Which force dominates? Why? (You may wish to look up the mass of an electron and proton.)Ink Jet PrintersThe inkjet printer uses charged plates and ink droplets, which getcharged by the plates as they leave the print head. The drops areextremely small (1 ϫ 10Ϫ4 m in diameter) and travel relatively quicklytowards the paper (65 km/h). A computer connected to a sensor in theprint head detects areas with print on the original document. As thedrops pass by the charging plates, the plates are turned OFF, therebyallowing the spray of ink to fly directly onto the paper. When the sen-sor detects areas without print, the computer turns the plates ON,thereby charging the drops of ink. The charged deflection plates attractthe unwanted drops, which go into a reservoir (Fig. 15.18A). Fig.15.18A Stream of Fig.15.18B ink dropletsPrinthead Deflection plates Uncharged drops F yzks Nozzle Paper Charging electrode Charged (controlled by computer) drops Inksupply These machines need constant upkeep and maintenance. A college 535course trains students to become technicians in this area.3. Compare the resolution and speed of the ink jet printer, the laser printer, and the dot-matrix printer.4. The photocopy machine also uses electrostatics through a process called xerography. Use Table 15.2 to explain how a photocopier works. Draw a block diagram illustrating the process. chapter 15: Electrostatics

S T S c i e n c e — Te c h n o l o g y — S o c i ety —S E Environmental InterrelationshipsFig.STSE.15.1 The Sparktec Pulse Power Technologies (Sparktec)SPK-8000 Can physics triumph over chemistry to protect property and the environment? Pulse Power Technologies (Sparktec Environmental Inc.) of Stoney Creek, For image Ontario, thinks that it can. This Canadian company has developed a device see student that generates a pressure wave in fluids using a high-voltage spark. The device, shown in Fig. STSE. 15.1, is the Sparktec plasma sparker (SPK-8000). Like text. a car spark plug, it produces a spark with a direct current voltage of about 8000 V for short time periods. The spark produces the usual ultraviolet lightFig.STSE.15.2 Zebra mussels and ozone gas (O3). When submerged in water, it also produces a high-energy pressure wave, which can be used in water management and treatment systems. For image see student The infestation of zebra mussels in the Great Lakes region has created serious problems for industries that maintain water intake systems. Water text. intake systems can become blocked by zebra mussels as well as other natu- ral organic growth, as shown in Fig. STSE. 15.2 and 15.3. As Sparktec’sFig.STSE.15.3 A zebra-mussel- pressure wave travels down the pipe system (Fig. STSE.15.4), it dislodges the zebra mussels and other debris. Field tests (Fig. STSE.15.5) have shownencrusted intake pipe excellent results. For image The current practice is to clean the pipes using chlorine, which poses a see student health hazard. At an estimated maximum of $200 per year, the cost of run- ning a Spartec device seems very reasonable, as it requires only a standard text. 120/240 V electrical service.Fig.STSE.15.4 A pressure wave Sparktec is currently testing the effectiveness of using the system to kill fecal streptococci and more resilient bacteria, such as cryptosporidium, intravels down the pipe municipal water treatment applications. Although this system shows much promise, there may be resistance to its widespread acceptance as a physics alternative to a traditional chemical application. Power Consumption of the SPARKTEC Pulsed Power Sparker Voltage (V) Energy Average power Maximum power Estimated yearly (kWh/year) consumption consumption cost of energy during discharge during discharge cycle (W) cycle (W) 6000 1224 140 280 $61.20 7000 1666 190 380 $83.30 8000 2177 314 629 $108.85 9000 3401 388 776 $170.05536 u n i t e : E l e c t r i c i ty a n d M a g n et i s m

Design a Study of Societal Impact Fig.STSE.15.5 The difference The most feared spark is, of course, lightning. What devices are cur- Sparktec can make rently on the market that could either prevent lighting from striking or at least manage/control the strike? What would be the environ- For image mental or social impacts if lightning was not controlled? see student Research the physics behind voltage surge suppressors. Contact text. insurance companies to find out how much money is spent annually to replace electronic equipment that has been damaged by electrical surges. Fig.STSE.15.6 The spark unit of Taking preventative measures would have been much cheaper an SPK-8000 than fixing the zebra mussel problem. What other environmental dis- asters could we minimize or prevent by acting sooner?Design an Activity to Evaluate Sparks are used as ignition devices in many natural gas and propane burning appliances such as barbecues. Obtain an old hand-held piezo- electric barbecue lighter and take it apart. Mount each piece in an appropriate spot on a display board with appropriate labels to explain to others how this simple device works. Use the piezo-electric cell from the dismantled barbecue lighter or a compression piezo-electric cell from a scientific supply company. Set up the cell (from whatever source) by attaching either end of the two electric leads to a voltmeter (analog, dig- ital, or the voltmeter setting on a computer data interface. Be sure to check that the device may register or not be damaged by high voltages.). Conduct a correlation study of the electric potential (voltage) that can be created across the spark gap versus the force that is applied to compress the cell. Conduct a similar study of potentials created across the spark gap of a standard two-cycle engine spark plug. The engine can be cranked by hand after the voltmeter leads are attached, or the spark plug could be set off in the physics lab using a standard tesla coil. Variables that could be adjusted could be spark gap, relative humidity, etc.Build a Structure Build a Sparktec-like prototype device from the basic materials of a dismantled barbecue lighter. The basic components could also be fab- ricated after researching the design of electronic camera flashes. If stu- dents are successful at creating a reliable underwater spark, a competition could be started to see which design team could create the best/strongest pressure wave.chapter 15: Electrostatics 537

S U M M A RY S P E C I F I C E X P E C TAT I O N S You should be able to Understand Basic Concepts: Describe the current model of the atom and outline the role of each sub- atomic particle in the net electric charge on an object. Compare and contrast the function of conductors and insulators and out- line their role in the transmission of electrical energy. Outline the steps to successfully charging an object by either friction, contact, or induction. Relate how the repulsion of electrons is the basis of current electricity. Define concepts and units relating to electrostatic charges and simple field theory such as electric charge, coulomb, and electric field. Describe and illustrate the three-dimensional properties of an electric field, including its shape, direction, and strength. Model the concept of force at a distance, with examples. Develop Skills of Inquiry and Communication: Design and conduct an experiment to verify the law of electric charges. Verify physical data that have been accepted by the scientific community, such as the electrostatic series, through experimentation. Analyze experimental observations to verify personal predictions about experimental outcomes. Relate Science to Technology, Society, and the Environment: Analyze the operation of various technological devices, such as an elec- troscope, an electrostatic air cleaner, a bug zapper, a photocopier, and an ink jet printer, and identify the principles of electrostatics in each. Relate how the acceptance of certain conventions, such as positive and negative electric charge (the coulomb), are important in the scientific community. Recognize how electrostatic discharges are being used to provide environ- mental and financial benefits to traditional chemical water treatment. Equations Q ϭ Ne538 u n i t e : E l e c t r i c i ty a n d M a g n et i s m