Electricity X

Electricity ________________________________________________________________________________________________________________________________________________________________ Electric Current and Circuit Switch - It makes a conducting link between the cell and the bulb. Electric Circuit - A continuous and closed path of an electric current. Electric Charge - Intrinsic property of atoms. Represented by Q. Charge of Electrons = 1.6 x 10-19 C SI unit = 1 Coulomb (C) = 6 x 1018 electrons Electric Current - It is the rate of ow of electric charges. It is expressed by amount of charge owing through a particular area in a unit time. Represented by I. Current I = QT TCihmarege Direction of Current = opposite of ow of electrons (negative charge). SI unit = Ampere (A) named after French scientist, Andre-Marie Ampere (1775-1836) Device to measure current = Ammeter is always connected in series 1A=1C 1S 1 Milliampere (mA) = 10-3 A 1 Microampere (uA) = 10-6 A Electric Potential and Potential Difference Electrons only move when there is a difference of electric pressure along the conductor. Potential Difference The difference can be produced by batteries, consisting one or more electrical cells. Working of a Battery in Creating Potential Difference 1 The chemical action within a cell generates the potential difference across the terminals of the cell (even without current being drawn from it). 2 When the cell is connected to a conducting circuit element, the potential difference sets the charges in motion in the conductor and produces an electric current. 3 In order to maintain the current in the circuit, the cell has to expend its chemical energy stored in it. Potential difference V = W Work Done Q Charge SI unit = Volt (V) named after Italian physicist, Alessandro Volta (1745-1827) V = 1 Joule 1C Work done/ Energy SI unit = Joules (J) 1 lf lflf

Ohm’s Law The potential difference (V) across the ends of a given metallic wire in an electric circuit is directly proportional to the current owing through it, provided its temperature remains the same. - Georg Simon Ohm, German physicist, 1827 Derivation of Ohm’s Law V∝I or, V/I = constant or, V/I = R Hence, V=IR Resistance It is the property of a conductor to resist the ow of charges through it. SI unit = Ohm R is constant R∝ 1 I So, 2R = 1/2 x I 1Ω=1V/1A Variable Resistance - A component used to change regulate current without changing the voltage source. Eg Rheostat - Used to change the resistance in the circuit. Electrons are not completely free to move within a conductor. They are restrained by the attraction of atoms among which they move. Thus, motion of electrons is retarded by its resistance. Good conductor - A component of a given size that offers a low resistance. Resistor - A conductor having some appreciable resistance. Insulator - A component of identical size that offers a higher resistance. Factors on which Resistance Depends Derivation of Resistance ∝R length R∝1 A (= cross section area) ∝So, R length A Thus, R = ρ /A (p is the electrical resistivity. It is constant.) SI unit of resistivity = Ω Resistivity of - (Vary on temperature) • metals and alloys = 10-8 - 10-6 Ωm (Low resistivity) • insulators = 1012 - 1017 Ωm (High resistivity) 2 lf lf

Alloys They are used in electrical heating devices like iron, toaster. because 1 Resistivity is higher than its metals. 2 Don’t oxidise or burn readily at high temperatures. Tungsten (Metal) is used almost exclusively for electric bulb laments because it has high melting point. Copper and Aluminium are used for electrical transmission lines. Resistance depends on length, cross section area and resistivity. Resistivity depends on temperature. Resistance of a System of Resistors Resistors in Series I is constant. V = V1 + V2 + V3 So, IR = IR1 + IR2 + IR3 Thus, R = R1 + R2 + R3 Resistors in Parallel I = I1 + I2 + I3 I1 = V/R1 I2 = V/R2 I3 = V/R3 So, V/R = V/R1 + V/R2 + V/R3 Thus, 1= 1 + 1 + 1 R R1 R2 R3 Series Circuits Disadvantages 1 The current is constant, thus, it is impracticable to connect an electric bulb and heater in series because they need current of widely different values to operate properly. 2 When one component fails the circuit is broken and all the components stop working. Eg- In fairy lights, to detect the dead bulb as each bulb has to be tested. Parallel Circuits Advantage 1 The total resistance decreases which helps when each gadget has different resistance and requires different current to operate properly. 3 if

Heating Effect of Electric Current The chemical reaction within the cell generates the potential difference between its two terminals that sets the electrons in motion to ow the current through a resistor or a system of resistors connected to the battery. In order to maintain the current in the circuit, the cell has to expend its chemical energy stored in it. In order to maintain the current in the circuit, the cell has to expend its chemical energy stored in it. A part of the source energy in maintaining the current may be consumed into useful work. Rest of the source energy may be expended in heat to raise the temperature of gadget. For example, an electric fan becomes warm if used continuously for longer time etc. Heating Effect of Electric Current - If the electric circuit is purely resistive, that is, a con guration of resistors only connected to a battery; the source energy continually gets dissipated entirely in the form of heat. This effect is utilised in devices such as electric heater, electric iron etc. Joules Law of Heating The law implies that heat produced in a resistor is ∝1 H I2 2 H∝R 3 H∝t Hence, H = I2Rt SI unit of Heat = Joules (J) Applications 1 Electric Applications The electric laundry iron, electric toaster, electric oven, electric kettle and electric heater are some of the familiar devices based on Joule’s heating. 2 Electric Bulb • The electric heating is also used to produce light, as in an electric bulb. Here, the lament must retain as much of the heat generated as is possible, so that it gets very hot and emits light. It must not melt at such high temperature. • A strong metal with high melting point such as tungsten (melting point 3380°C) is used for making bulb laments. The lament should be thermally isolated as much as possible, using insulating support, etc. • The bulbs are usually lled with chemically inactive nitrogen and argon gases to prolong the life of lament. Most of the power consumed by the lament appears as heat, but a small part of it is in the form of light radiated. 3 Fuse • Fuse protects circuits and appliances by stopping the ow of any unduly high electric current. The fuse is placed in series with the device. • It consists of a piece of wire made of a metal or an alloy of appropriate melting point like aluminium, copper, iron and lead. • If a current larger than the speci ed value ows through the circuit, the temperature of the fuse wire increases. This melts the fuse wire and breaks the circuit. The fuse wire is usually encased in a cartridge of porcelain or similar material with metal ends. The fuses used for domestic purposes are rated as 1 A, 2 A, 3 A, 5 A, 10 A. 4 lfif lf ififif ifif if ifl f

• Eg- For an electric iron which consumes 1 kW electric power when operated at 220 V, a current of (1000/220) A, that is, 4.54 A will ow in the circuit. In this case, a 5 A fuse must be used. Electric Power Power - Rate of doing work and rate of consumption of energy. P = VI or, P= I2R or, P = V2 / R SI unit = Watt (W) IW=1VA 1 Kilowatt = 1000 Watts (unit of Electrical Energy) 1 kWh = 1000 watt x 3600 seconds = 3.6 x 106 J Name Representation SI unit Resistance R Ohm (Ω) Current I Ampere (A) Potential Difference V Volt (V) Electric Charge Q Coulomb (C) Resistivity ρ Ohm (Ω) Heat H Joules (J) Power P Watt (W) 5 lf