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CURRENT ELECTRICITY, , , , , , , , Syllabus :, , (i) Simple electric circuit using an electric cell and a bulb to introduce the idea of current (including its, relationship to charge); potential difference; insulators and conductors; closed and open circuits; direction, of current (electron flow and conventional)., , Scope — Current electricity : brief introduction of sources of direct current — cells, accumulators (construction,, working and equations excluded); electric current as the rate of flow of electric charge (direction of current —, conventional and electronic), symbols used in circuit diagrams. Detection of current by galvanometer or ammeter, (functioning of meters not to be introduced). Idea of electric circuit by using cell, key, resistance wire/resistance, box/rheostat qualitatively; elementary idea about work done in transferring charge through a conductor wire;, potential difference V = W/q; resistance R from Ohm’s law V/I = R; insulators and conductors. (No derivation, of formula) simple numerical problems., , (ii) Efficient use of energy., , Scope — Social initiatives : improving efficiency of existing technologies and introducing new eco-friendly, technologies, creating awareness and building trends of sensitive use of resources and products e.g. reduced, use of electricity., , , , , , , , , , (A) ELECTRIC CURRENT,, , , , 9.1 SOURCES OF DIRECT CURRENT Ina cell, chemical energy changes into the, , We are familiar with cells used in a torch to, light up its bulb. The cells are the source of direct, current. They provide direct current (abbreviated, as d.c.) to the bulb. The current flows from the, positive terminal of cell to its negative terminal*, through the bulb of the torch (Fig. 9.1). The, magnitude of current given by a cell remains, constant for a sufficiently long time. When the, cell gets discharged, it stops giving current and, becomes useless. Thus we can define direct current, (d.c.) as follows :, , , , Direct current (d.c.) is a current of constant, magnitude flowing in one direction., , , , , , , , Brass cap +, Dry cell Bulb, Switch, Zinc case =, , Fig 9.1 A dry cell used to light up a bulb, , * In a dry cell, brass cap projecting out at the centre, acts as the positive terminal of cell and base of zinc, case acts as the negative terminal., , electrical energy when it sends current in a circuit., A cell basically consists of two conducting, rods, called the electrodes, at some separation, placed immersed in a solution (or jelly), called, the electrolyte, kept in a vessel., , Kinds of cells : The cells are of two kinds :, (1) The primary cells, and, (2) The secondary cells or accumulators., , (1) Primary cells : These cells provide, current as a result of irreversible chemical, reaction. The cells are discarded after use when, entire chemical energy in them has converted, into the electrical energy. Thus these are ‘use, and throw' type of cells and cannot be, recharged. Depending upon the material of, electrodes and electrolyte, we have different, types of primary cells e.g. simple voltaic cell,, Leclanche cell, Daniel cell, dry cell, etc., , (2) Secondary cells or accumulators :, Secondary cells also provide current as a result, of chemical reaction. In these cells, chemical, reaction is reversible and so they can be recharged, after use. After taking the electrolyte and, electrodes in a vessel, the cell is first charged by, connecting it to a direct current source. During, , 179

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charging, the electrical energy changes into the, chemical energy and gets stored in the cell. When, cell is used to send current in a circuit, the, chemical energy changes into the electrical, energy. These cells can thus be used for a very, long period because they can be recharged again, and again. Lead (or acid) accumulator and Ni-Fe, (or alkali) accumulator are the two types of, secondary cells commonly used to provide low as, well as high current. We use lead accumulator as, battery in a car as it is capable of giving high, current. Low current rechargeable cells are widely, used in toys and mobile phones. These cells are, also called the storage cells., , Difference between the primary cell and secondary cell, , , , , , Primary cell Secondary cell, , 1. Chemical reaction is|1. Chemical reaction is, irreversible. reversible., , 2. Chemical energy is|2. Electrical energy, converted into} converts into chemical, electrical energy when energy when current is, current is drawn from| passed in it (i.e., during, it charging), while, , chemical energy, , converts into electrical, energy when current is, drawn from it (ie.,, It can be recharged., Its internal resistance is, low., , It can provide low as, , 3. It can not be recharged. | 3., 4. Its internal resistance | 4., , is high., 5. It is capable of giving | 5., , weak current only. well as high current., 6. It is light and cheap. | 6. It is heavy and costly., Examples : Simple voltaic | Examples : Lead (or acid), cell, Leclanche cell, accumulator, Ni-Fe (or, Daniel cell, dry cell. alkali) accumulator, Li-H, battery., , 9.2 ELECTRIC CURRENT (AS RATE OF, FLOW OF ELECTRIC CHARGE), , We have read that when two bodies are, rubbed together, they get charged due to transfer, of electrons from one body to the other. The, body gaining electrons becomes negatively, charged while the body loosing electrons gets, positively charged. A positively charged body has, a deficit of electrons, while a negatively charged, body has an excess of electrons. The charges can, , , , , , , , , , be made to flow between them by suitable, arrangements. For example, if two charged bodies, are joined by a metallic wire, the electrons flow, from the body having more electrons, to the body, having less electrons. The flow of electrons, constitutes an electric current. Thus current flows, due to motion of charges. The rate of flow of, charge gives the magnitude of current. Thus, , Current is the rate of flow of charge across a, cross-section normal to the direction of flow of, current., , , , , , , , , , If charge Q flows through the cross-section, of a conductor in time f, then, , Current oa, , The current is a scalar quantity., , , , 9.1), , , , , , , , In metals (or conductors), current flows due to, the movement of electrons, while in electrolytes,, current flows due to the movement of both, the, positive and negative ions. An electron carries a, negative charge equal to — e, where e = 1-6 x 10", coulomb is the electronic charge., , If n electrons pass through the cross section, of a conductor in time f, then, , Total charge passed Q=n x e, and current in conductor J = 2. me, , , , (9.2), , , , , , , , Note : In an electrolyte, the current flows due to the, movement of both the positive ions (i.e., cations) and, negative ions (i.e., anions). In one second, if n, positive, ions each carrying a charge g, move in one direction and, Nn, negative ions each carrying a charge q, move in opposite, direction, the total current will be J = n,(+ 4) — ny(— 92), = (mq, + Mg), because direction of current is taken, positive in the direction of flow of positive ions, and, negative in the direction of flow of negative ions., , Direction of current (conventional and, electronic), , Conventionally, the direction of current is, taken positive in the direction of flow of positive, charge. Therefore, conventionally, the current will, be negative in the direction of flow of electrons., The rate of flow of electrons in a direction is, called the electronic current in that direction. The, conventional current (or simply current) is in a, direction opposite to the direction of motion of, , 180

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electrons. The magnitude of conventional current, is the rate of flow of total charge across the given, cross-section., , Unit of current : From relation J = 2, , Unit of charge, , Unit of time *, The S.I. unit of charge is coulomb and S.I. unit, of time is second, so the S.I. unit of current is, coulomb per second which is called ampere., It is denoted by the symbol A. Thus., , Unit of current =, , , , 1 ampere (A) = 9.3), Thus current is 1 A, if the rate of flow of, charge is 1 coulomb per second. Since 1 coulomb, , 1, of charge is carried by Texio® = 625% 10'8, electrons, therefore if 1 ampere current flows, through a conductor, it means that 6:25 x 10!8, , electrons pass in 1 second across that cross section, of conductor., , Note : Weak current is measured in units, milli-ampere (mA) and micro-ampere (1A), where, , , , , , , , , , , , , , , , , , , , 9.3 SYMBOLS IN CIRCUIT, , DIAGRAMS, , An electric circuit has different electric, components connected to a current source., Fig. 9.2 shows some of the electric components, which are used in an electric circuit in the, laboratory., , The symbols and functions of various, electric components are given below., , (1) Source of current : There are, two types of current sources : (i) alternating, current* (a.c.) source such as mains in our house, and a.c. generator, and (ii) direct current (d.c.), source such as a cell or a battery. The purpose, of source of current is to supply electric current, in a circuit. In laboratory, Leclanche cell, , USED, , * An alternating current (abbreviated as a.c.) is the, current in an element (say, bulb) for which both the, magnitude and direction change with time. The current, repeats its value after a fixed time. The number of, times current repeats its value in one second, is called, the frequency of alternating current. The frequency, of a.c. obtained from our mains is 50 Hz., , i +, ¥, , ‘= Leclanche cell, , , , @ as ey: (e) Rheostat, (f) Resistance box (g) Ammeter, (h) Voltmeter (i) Galvanometer, , , , Fig. 9.2 Different electric components, , [Fig. 9.2(a)], Daniel cell [Fig. 9.2(b)] and lead, accumulator (storage cell) [Fig. 9.2(c)] are used., , A dc. sources (cell) is represented by two, vertical lines of unequal lengths. The longer one is, marked ‘+’ to represent anode, while the shorter is, marked ‘~’ to represent cathode [Fig. 9.3 (a)]. When, current is drawn from a cell, it flows from the, positive terminal to the negative terminal through, the external circuit and from the negative terminal, to the positive terminal inside the cell through its, electrolyte so as to maintain a continuous flow., , —oO, (c), BATTERY A.C.SOURCE, , Fig. 9.3 Symbolic representation of source of current, , When a strong current is needed, either we, join a number of cells together in series (cathode, of one cell connected to the anode of another, cell), to form a battery or we use a storage cell., Fig. 9.3(b) represents the symbol for a battery, (using three cells)., , An alternating current (a.c.) source (mains), is represented by a sine curve (~) within a circle, as shown in Fig. 9.3 (c)., , (2) Key : It is used to put the current on, and off in a circuit. It may be either a plug key,, , 181

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~ a switch or a tapping key. Fig. 9.4 shows the, symbolic representation of these keys when they, are open and when they are closed. A key is, usually represented by the symbol K., , In laboratory, generally we use a plug key, shown in Fig. 9.2(d). When key is open (i.e.,, the plug is taken out), the circuit is incomplete, and is called an open circuit. When key is closed, (i.e., the plug is put in), current flows through, the circuit and is said to be closed. A tapping, key is used when current is momentarily needed, in the circuit., , When key is open, =f Ere, (a), PLUS KEY, When key is closed, , (a) (b), PLUG KEY SWITCH, , (b) (c), SWITCH TAPPING KEY, , (c), , TAPPING KEY, Fig. 9.4 Symbolic representation of different keys, , (3) Resistance wire, resistance box,, rheostat or variable resistance : A resistance, wire is generally made from an alloy, called, manganin* and it has a fixed resistance, depending upon its length and thickness. It is, used as a standard resistance., , A rheostat [Fig. 9.2(e)] is a device by which, resistance in a circuit can be varied continuously. It, is used to adjust the magnitude of current in a, circuit by changing the length of resistance wire, included in the circuit. As shown in Fig. 9.5, it, consists of an enamelled constantan wire wound in, form of a coil (single layer) over an asbestos pipe, fitted in an iron frame and provided with a sliding, , Jockey, , Brass Rod, —, , , , Fig 9.5 Rheostat, , * Manganin is an alloy which is used for preparing, wires in a resistance box and rheostat. It is composed, of 84% copper, 12% fmmnganere and 4% nickel., , =a, , , , contact (jockey) on a brass rod. It has three, terminals A, B and C, out of which two terminals, A and B are at the ends of resistance wire and the, third terminal C is at the end of brass rod, connecting the jockey. The enamel of the coiled, wire is removed at the places where jockey, touches the wire., , The wire between the two terminals A and, B provides a fixed resistance equal to the, resistance of entire length of wire. When rheostat, is used as a fixed resistance R, connections are, made with the terminals A and B as shown in, Fig. 9.6 (a). Symbol R is used for a fixed resistance., , The main use of a rheostat is as a smoothly, varying resistance. For this, connections are made, between the terminals C and either of the, fixed terminal A or B at the end, as shown in, Fig. 9.6 (b). The jockey connected to the middle, terminal C can be slided and by making its contact, at different points, a variable resistance can be, obtained. The current then enters at the positive, terminal A (or B), it flows in the resistance wire of, length up to the jockey and then leaves at the, terminal C. Thus rheostat provides a continuously, varying resistance to adjust current of a desired, value in the circuit. Symbol Rh is used for a, variable resistance., , JOCKEY Cc JOCKEY, BA B e, Rh Rh, , (b), VARIABLE RESISTANCE, , , , (a), FIXED RESISTANCE, , Fig 9.6 Symbolic representation of a rheostat, , A resistance box [Fig. 9.2(f)] is provided, with several standard resistances (say 1, 2, 2, 5,, 10, 20, 20, 50, .... ohm) connected in series, between the two terminals. A resistance of any, desired value between 1 ohm to its maximum, value (i.e., sum of all resistances) can be used in, step of one ohm by taking out the plug of that, particular resistance from the resistance box., , (4) Ammeter : An ammeter [Fig. 9.2(g)] is, an instrument used to measure the magnitude of, current flowing in a circuit. It is joined in series, in a circuit so that the entire current of the circuit, passes through it. It is symbolically represented, by the letter A enclosed in a circle as shown in, Fig. 9.7. The + sign marked at one terminal of, , 182

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it indicates that current must enter it through this, terminal. Therefore this terminal is connected to, the higher potential terminal of the current, source. An ammeter must have a very low, resistance so that the resistance of the circuit (or, the current to be measured) may not alter when, it is joined in the circuit. It has a dial on which, deflection of needle shows the magnitude of, current flowing in that circuit., , >—@)——2, , Fig. 9.7 Symbolic representation of an ammeter, , Different kinds of ammeter are required to, measure the alternating current and the direct, current*., , (5) Voltmeter : A voltmeter shown in, Fig. 9.2(h) is used to measure the potential, difference between two points of a circuit. It is, connected across the two points between which, potential is to be measured (i.e., in parallel to the, flow of current). It is symbolically represented, by the letter V enclosed in a circle as shown in, Fig. 9.8. The + sign marked at one terminal of, it indicates that current must enter it through, this terminal. Therefore this terminal is, connected to the point at higher potential. A, voltmeter usually has a very high resistance so, that it does not draw an appreciable current from, the circuit. It has a dial on which deflection of, needle shows the potential difference across the, points of circuit in between which voltmeter is, , connected., o—_ v }——2, , Fig. 9.8 Symbolic representation of a voltmeter, Different kinds of voltmeter are required to, , measure the alternating and the direct potential, difference*., , (6) Galvanometer : A galvanometer, [Fig. 9.2(i)] is used when we want to detect the, presence of very weak current in an electric circuit, or to know only the direction of flow of current in, a circuit. It does not measure the magnitude of, current in a circuit. It is joined in a circuit in series, just like an ammeter. It is symbolically represented, , * An ammeter or voltmeter used to measure alternating, current is based on heating effect of current so it has, graduations of unequal spacing while to measure direct, current is based on magnetic effect of current so it, has graduations of equal spacing., , SS ae 183, , by the letter G enclosed in a circle as shown in, Fig. 9.9. Current can enter or leave through any of, its terminal, therefore no +/— signs are marked at, its terminals. It has a dial with zero mark at its, centre. The deflection of needle on it shows the, presence of current and the direction of deflection, indicates the direction of flow of current., , o——{(@)——o, , Fig. 9.9 Symbolic representation of a galvanometer, (7) Load : An appliance which is connected, in a circuit is called a load. It may be just a, resistance (e.g., bulb, heater etc.) or a combination, of different electrical components. It can be, represented by the symbol L as shown in Fig. 9.10., , L L, (a) (b), , BULB HEATER, Fig. 9.10 Symbolic representation of a load, , (8) Connecting wires : These are the wires, used to connect various electrical components., They are made of highly conducting metal such, as copper (or aluminium because of its low cost), so that they are of negligible resistance and do not, change the resistance of the circuit. A connecting, wire may be a thick wire or several fine wires, twisted together. Generally, we use thick insulated, copper wire as a connecting wire which is, represented in a circuit diagram simply by a line., , 9.4 SIMPLE ELECTRIC CIRCUIT, , In our daily life, we use a torch in which a, cell (or a combination of two or more cells) is, connected to a bulb. This is the simplest electric, circuit which uses only three components viz cell,, bulb and switch. Fig. 9.11 shows a line diagram, representing the simple electric circuit containing, , ® BULB, , 2 (=:, +, CELL, , Fig. 9.11 An electric circuit of a cell, bulb and switch, , , , , , , , , , SWITCH, , a