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CHAPTER – 12, , ELECTRICITY, Think life without “electricity” in this modern society. Is it possible to survive, without electrical energy in world of technology. Since we are science student, so it, is necessary to understand the basic concept behind the word “electricity”, Charge®, (q), It is a very small particles present in an atom it can be either negative (electron) or, positive (proton), “Coulomb” is the SI unit of charge, represented by C., Net charge (Q)– Total charge, 1018, IC Net charge is equivalent to the charge contained in nearly electrons 6´, Q = ne (n is no: of electron), If Q = 1C, e = 1.6 ´, 10–19C (negative charge on electron), n=, , Q, 1, =, e, 1.6 ´, 10–19, 100, =, ´, 1018 = 6.2 ´, 1018, 16, n=6´, 1018 electron, , Current (I), Rate of flow of net charge is called current. Denoted by (I), I =, , Q, t, , t is time, , SI unit of current is “Ampere” rep. by A., Ampere ®, Defined as one coulomb of charge following per second., 1A =, , 1C, 1s, , 124, , X-Science

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In an electric circuit the electric current flow in the opposite direction of the flow of, electron (–ve charge) conventionally. It flows from the +ve terminal of battery or, cell to –ve terminal., Small quantity of current are expressed in, –3, , mA (milli Ampere) = 10 A, uA (micro Ampere) = 10–6 A, Ammeter– It is an instrument used to measure the electric current in a circuit., It is always connected in series m a circuit, It is represented by the symbol ––––––––+ A – ––––––– in an electric circuit. It has, low resistance., Electric Circuit– It is a closed path along which an electric current flow., , Bulb, The arrow shows the direction, of electric current in circuit., , –, Ammeter A, , +, +, , –, , Key, , In this circuit ammeter is, connected in series., , Battery, (Having 3 cells), , The electron can only flow when there is difference of electric pressure. For, example “water flowing through a tube” It is only possible when there high, pressure at one side and low at another side, then it will move from high pressure to, low pressure., In case of electric current, the flow of charge is made possible due to chemical, action with in a cell that generates the potential difference across the terminals of, the cell., 8. Electric potential Difference– It is defined as the work done in carrying a unit, charge from one point to another between the two points of an electric circuits., V – Potential Difference, W – Work, Q – Net Charge, SI unit of potential difference – Volts rep. by “V”, V=, , W, Q, , 125, , X-Science

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One Volt ®, When 1 Joule of work is done to carry one coulomb (1C) of charge, from one point to another of a current carrying conductor then the potential, difference is send to be IV., IV =, , 1J, 1C, , Voltmeter ®, It is an instrument, used to measure the potential difference and, +V –, represented by the symbol, in an electric circuit. It is always, connected in parallel across the points between which the potential difference is to, be measured. It has high resistance., Symbols for some commonly used instrument in circuit diagrams, +, , (1) Cell, , (2) Battery, , –, , +, , –, , (3) Key (switch) open, , OR, , (4) Key (Close), , OR, , (5) Joint wire, , (6) Wires with no join, , (7) Bulb, +, , (8) Ammeter, , +, , (9) Voltmeter, , 126, , A, , V, , –, , –, , X-Science

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Georg Simon Ohm (physicist) 1787 – 1854, Found the relationship between the current (I) flowing through a conductor and, potential difference (V) across the terminals of a conductor using the circuit, diagram., + V–, , GRAPH, , –, A, +, , V, (V), +, , O, , –, , Va, I, I (A), , In this circuit diagram we come across two new symbols, RESISTANCE (R), OR, , RHEOSTAT (Variable Resistance), , Ohm’s Law ®, He stated that the electric current flowing through a conductor is, directly proportion at to the potential difference across its ends, provided the, temperature remain constant, Va, I, V = IR, Where “R” is the proportionality constant for the given metal at given temperature, and is said to be resistance, the graph between V and I is always straight line., Resistance– It is the property of a conductor that opposes the flow of current. It is, represented by ‘R’ and symbol is, SI unit of resistance “Ohm” OR W, 1 Ohm – The resistance of a conductor is said to be one Ohm, when the potential, difference across the conductor is 1V and the current flowing through it is 1A., V = IR, \, R= V, I, 1 Ohm or 1 W, = 1V, 1A, 127, , X-Science

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Rheostate–, As we know that, V = IR, , {, , Shows that current through, \, 1= V, conductor resistor is inversely, R, proportional is its resistance, , }, , So to increase or decrease the current accordingly in the circuit a component is, used is called “Rheostat”, that regulates the current without changing potential, difference. Represented by “Rh”, Its symbol is, , OR, , it a conductor has less Resistance, then more current will flow through it., FACTORS ON WHICH RESISTANCE OF A CONDUCTOR DEPENDS–, (1) On its length (l), (2) On its cross sectional area (A), (3) On the nature of material, (Resistance), , Ra, l, 1, Ra, A, l, Ra, A, R= l, A, , (Directly prop. to length), (inversely prop to cross-sectional area), , Where “ ” (rho) is a proportionality constant known as resistivity of the material, of conductor., 11. Resistivity ( ) – the resistance offered by a wire of unit length and unit crosssectional area is called resistivity., l, Since R =, Its SI unit is W, m, A, R.A, W, ., m2, \, ==, l, m, \, SI unit of =, W, m., For a material irrespective of length and area, the resistivity is a constant., 128, , X-Science

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Resistantly of a material vary with temperature, Resistivity of an alloy (homogeneous mixture of metals) is generally higher than, of its constituent metals. Example Constantan (alloy of Cu & Ni), Alloys have high resistivity and do not oxidise (burn) readly at high temperature,, for this reason they are commonly used in electrical heating devices, like electric, iron, heater, toasters etc. For example “Tungsten” as filament of electric bulb., Resistance in Series– (Maximum Effective Resistance), Let us take three resistance R1, R2 and R3 that are connected in series in a circuit., R1, , R2, , R3, , V1, , V2, , V3, , I, +, , –, V, , Ohm’s low stated, V = IR, , The current (I) flowing through the resistance in series will remain same, where as, the potential difference (V) across each resistor will be different., V = IR, V1 = IR1 , V2 = IR2 , V3 = IR3, Total potential difference (V) = V1 + V2 + V3, V = IR1 + IR2 + IR3, , the value of, } Putting, V, V , V & V, 1, , 2, , 3, , I R = I (R1 + R2 + R3), Reff = R1 + R2 + R3, Thus, we conclude that effective Resistance of the several resistors joined in series, is equal to the sum of their individual resistance., Resistance in Parallel (Minimum Effective Resistance), Let us take three R1, R2 and R3, that are connected in parallel in the electric circuit., R1, , I1, I2, I1, , R2, , I3, , R3, , +, , –, V, , 129, , X-Science

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Now,, , I1 = V , I2 = V , I3 = V, R1, R2, R3, , Total current (I) = I1 + I2 + I3, substitute the value of I1 , I2 , I3 and I, V = V + V + V, R, R1, R2, R3, V = V, R, , 1 + 1 + 1, R1, R2, R3, , 1 = 1 + 1 + 1, Reff, R1, R2, R3, Thus, we conclude that the reciprocal of total effective resistance of the several, resistors connected in parallel is equal to the sum of the reciprocals of the, individual resistance., Disadvantage of series connection in on electric circuit :–, 1., , In series connection if any of the component fail to work, the circuit will break, and then none of the component (ex. TV, bulb, fan..) will work., , 2., , It is not possible to connect a bulb and a heater in series, because they need, different value of current to operate properly., , Hence, to overcome this problem we generally use parallel circuit., Heating effect of Electric Current :, Explanation® Battery or a cell is a source of electrical energy., Battery or Cell, (Chemical reaction in, it will produce potential, difference at its, two terminals), Rest of energy of source is, converted into heat, that, raises the temperature, of gadget., This is known as heating, effect of electric current, , Electron, will come in motion, to flow current, through resistor, Part of this energy is, consumed in useful, work, (like rotating of fan), , To maintain this current,, the source has to keep, expending its energy, , This effect is utilized in, devices such as electric, heater, iron etc., , 130, , X-Science

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Mathematical Expression :–, Let us suppose that current (I) is flowing through a resistor of resistance (R) for the, time (t). The potential difference across the resistance is (V)., W, V= Q, \, Work done in morning the charge Q will be, W = VQ, W, Then power, P = t [Rate of change of work done], VQ, = t, Q, P = VI, I= t, – (1), Heat energy supplied by the source for time t will be, Energy or Work, H=P´, t, P=, – (2), time, Put equation (i) in equation (2), H = VIt, = (IR) It, , \, V = IR Ohm’s Law], , H = I2Rt, This is known as Joule’s Law, The law stated that the heat produced in a resistor is, (i) directly proportional to square of the current(I), (ii) directly proportional to resistance (R) for given current, (iii) directly proportional to time (t) for which current flow through resistor., Application of Heating Effect of Electric Current :–, (1) Used in electric iron, toaster, oven, heater etc., (2) It is also used in bulb to produce light., (Filament of bulb is made of strong metal with high melting point such as, tungsten (m.pt = 3380°C). This filament can retain as much of the heat, generated as possible, to become very hot and emit light), (3) It is also used in the “fuse connected in an electric circuit {Fuse a safety, device, protect the circuits and appliance by stopping the flow of high current., The wire of fuse is made of an alloy of metals for ex Aluminium Copper, Iron, 131, , X-Science

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lead etc. The alloy should be of low m.pt and high resistivity, fuse is always, connected in series circuit. When large current flow through the circuit, the, temperature of fuse wire will increase. This melts the fuse wire and break the, circuit., “ Fuses” used for domestic purposes are rated as 1A, 2A, 3A, 5A, 10A etc. for, various operation depending upon the power of appliance using., Example- let us consider an appliance “electric Iron” which consume 1KW, electric power, at 220V, P = VI, I = P = 1KW = 1000W, V, 220V, 220V, I = 4.54A, In this case a 5A fuse is required., Electric Power :– In case of electricity, it is defined as the rate of change electrical, energy dissipated or consumed in an electric electrical energy dissipated or, consumed in an electric circuit., P = VI, or P = I2R, V2, or P =, R, , ( V = IR Ohm’s Law), ( I= V ), R, , or P = Electrical Energy (E), time (t), SI unit of electric power is “Watt” (W)., 1 Watt®, Defined as the power consumed by a device, when 1A of current passes, through it at the potential difference of 1V., P = VI, 1 Watt = 1 Volt ´, 1 Ampere, 29 Electrical Energy–, P= E, t, , E – Electrical Energy, t – time, , \, E=P´, t, , 132, , X-Science

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EXERCISE, (Question Bank), Very Short Answers (1 Mark), 1., , Define the SI unit of (one mark each), (a), , Current, , (b), , Potential Difference, , (c), , Resistance, , (d), , Electric Power, , (e), , Electric Energy (Commercial), , 2., , What is the conventional direction of flow of current?, , 3., , Define the term resistivity?, , 4., , On what factors does the resistance of a conductor depend?, , 5., , How is the voltmeter and ammeter connected in the electric circuit., , 6., , Heating effect of current carrying conductor is due to –, (Ans : loss of kmectic energy of electron), , 7., , Why the filament of bulb has high melting point?, , 8., , How does use of a fuse wire protect electrical appliance?, , 9., , What is the relationship between power, current and potential difference, (Ans : P = VI), , 10., , How many joules are there in 1KWh?, , Short Answer (2-3 marks) type Questions, 1., , Draw a schematic diagram of a circuit consisting of a battery of six cell of, 1.5V each, three resistor each of 3W, in series and a plug key., , 2., , State Ohm’s law. Draw the graph between V&I?, , 3., , What is joule’s Heating effect of current, derive its expression?, , 4., , A wire of length L and R is stretched so that its length’s doubled and the area, of cross section is halved. How will its, (i), , Resistance change, , (ii), , Resistivity change., , 134, , X-Science

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3., , (1), , Calculate total effective resistance, , (2), , The total current through the circuit., , (3), , Potential difference across 4W, and 2W, ., , Three resistance of 2W, , 3W, and 5W, are connected in the electric circuit., Calculate the, (1), , Maximum effective resistance, , (2), , Minimum effective resistance, , 136, , X-Science