15 - THERMAL AND CHEMICAL EFFECTS OF CURRENTS Page 1 ( Answers at the end of all questions )

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1 5 - THERMAL AND CHEMICAL EFFECTS OF CURRENTS Page A heater coil is cut into two equal parts and only one part is now used in the heater. The heat generated will now be four times doubled halved ( d one-fourth [ AIEEE 005 ] Two voltameters, one of copper and another of silver, are joined in parallel. When a total charge q flows through the voltameters, equal amount of metals are deposited. If the electrochemical equivalents of copper and silver are and respectively, the charge which flows through the silver voltameter is q q q ( d q [ AIEEE 005 ] Time taken by a 836 W heater to heat one litre of water from 0 C to 40 C is 50 s 00 s 50 s ( d 00 s [ AIEEE 004 ] 4 The thermo-emf of a thermocouple varies with the temperature θ of the hot junction as E = a θ + b θ in volts where the ratio a / b is 700 C. If the cold junction is kept at 0 C, then the neutral temperature is 700 C 350 C 400 C ( d no neutral temperature is possible for this thermocouple [ AIEEE 004 ] 5 The electrochemical equivalent of metal is kg per coulomb. The mass of the metal liberated when a three ampere current is passed for seconds will be kg kg kg ( d. 0-7 kg [ AIEEE 004 ] 6 A 0 volt, 000 watt bulb is connected across 0 volt mains supply. The power consumed will be 000 watt 750 watt 500 watt ( d 50 watt [ AIEEE 003 ] 7 An electric motor operates at 0 rev / sec. What will be the approximate power delivered by the motor, if it supplies a torque of 75 N-m? 550 W 4040 W 7730 W ( d 940 W [ AIEEE 003 ] 8 The negative Zn pole of a Daniell cell, sending a constant current through a circuit, decreases in mass by 0.3 g in 30 minutes. If the electrochemical equivalents of Zn and Cu are 3.5 and 3.5 respectively, then increase in the mass of positive Cu pole in this time is 0.4 g 0.80 g 0.4 g ( d 0.6 g [ AIEEE 003 ] 9 litre water kept in a kettle is heated by KW power source. Kettle is open and it loses heat at the rate of 60 J/s. The time taken for the temperature of kettle to change from 7 C to 77 C is 8 min 0 sec 6 min 0 sec 5 min ( d 7 min [ IIT 005 ] 0 An ideal gas enclosed in an insulated vessel is heated through a coil of resistance 00 Ω carrying current of A for 5 minutes. Then change in its internal energy will be 30 KJ 0 KJ KJ ( d 3 KJ [ IIT 005 ]

2 5 - THERMAL AND CHEMICAL EFFECTS OF CURRENTS Page The three resistors of equal value are arranged in the different combinations shown below. Arrange them in increasing order of power dissipation. III < II < I < I II < III < I < I I < I < III < II ( d I < III < II < I [ IIT 003 ] A 00 W bulb B and two 60 W bulbs B and B 3 are connected to a 50 source as shown in the figure. Now, W, W, and W 3 are the output powers of the bulbs B, B and B 3 respectively. Then W > W = W 3 W > W > W 3 W < W = W 3 ( d W < W < W 3 [ IIT 00 ] 3 A wire of length L and 3 identical cells of negligible internal resistances are connected in series. Due to the current, the temperature of the wire is raised by T in time t. A number N of similar cells is now connected in series with a wire of the same material and cross-section but of length L. The temperature of the wire is raised by the same amount T in the same time. The value of N is ( d 9 [ IIT 00 ] 4 A constant voltage is applied between the two ends of a uniform metallic wire. Some heat is developed in it. The heat developed is doubled if both the length and the radius of the wire are halved both the length and the radius of the wire are doubled the radius of the wire is doubled ( d the length of the wire is doubled [ IIT 980 ] 5 In the circuit shown in the figure, the heat produced in the 5 Ω resistor due to the current flowing through it is 0 cal / s. The heat generated in the 4 Ω resistor is cal / s cal / s 3 cal / s ( d 4 cal / s [ IIT 98 ]

3 5 - THERMAL AND CHEMICAL EFFECTS OF CURRENTS Page 3 6 Two resistors R and R when connected across 0 consume power at the rate of 5 W and 00 W respectively when connected in series and parallel across the same 0 line. Then the ratio of power consumed by R to that by R is : : : ( d : 4 7 Two electric bulbs, one rated P watt at volt and the other rated P watt at volt are connected in parallel across volt mains, being less than and, then the total power consumed by them is P + ( P P + P + P + ( d ( P 8 Find the power wasted in the transmission cables of resistance 0.05 Ω when 0 kw is transmitted at kw 0.5 kw 5 kw ( d 37.5 kw 9 Two electric bulbs, one rated P watt at volt and the other rated P watt at volt are connected in series across volt mains, being such that no bulb fuses, then the total power consumed by them is P P + P ( + P P ( + P ( d P ( + P 0 A house is served by a 0 supply line. In a circuit protected by a fuse marked 9 A, the maximum number of 60 W lamps in parallel that can be turned on is ( d 33 A tap supplies water at C. A man takes litre of water per minute at 37 C from the geyser. The power of geyser is 00 W 75 W 050 W ( d 575 W A wire of uniform cross-section when connected directly across a 00 volt mains produces x cal / s of heat. If the wire is cut into 0 parts of equal length which are then connected in parallel to 00 volt mains, the heat produced in cal / s will be 5 x 50 x 75 x ( d 00 x

4 5 - THERMAL AND CHEMICAL EFFECTS OF CURRENTS Page 4 3 A cell sends a current through a resistance R for time t. Then the same cell sends current through another resistance R for the same time t. If the same amount of heat is developed in both the resistances, then the internal resistance of the cell is R + R R - R R R ( d R R 4 The power of a heater is 500 W at 800 C. What will be its power at 00 C if α = per C? 484 W 67 W 56 W ( d 6 W 5 Three resistors are connected as shown in the adjoining figure. The maximum power consumed by each resistor is 8 W. Then the maximum power consumed by the combination is 54 W 7 W 36 W ( d 8 W 6 The E. C. E. of silver is kg / C. Its atomic weight is 08. The Avogadro number is per gm. mole. The charge on one Ag ion is C C C ( d C 7 Material of heating element of electric heater should have high resistivity and high melting point high resistivity and low melting point low resistivity and low melting point ( d low resistivity and high melting point 8 An electric kettle has two coils. When one is switched on, water in the kettle boils in 6 min. With the other it takes 3 min. When they are connected in series and used together, time taken to reach the boiling point is x min. When both are used together in parallel connection, time taken is y min. Then x / y = 0.5 ( d Two heating wires have lengths in the ratio :. They are used by first connecting in series and then in parallel. The ratio of heat produced in the two cases will be : 5 5 : : 9 ( d 9 : 30 Resistivity of iron is 0-7 Ω-m. The resistance of an iron wire is Ω. If its diameter and length are both doubled, the resistivity in Ω-m will be equal to ( d 4 0-7

5 5 - THERMAL AND CHEMICAL EFFECTS OF CURRENTS Page 5 3 How much current should pass through acidulated water for 00 s to liberate 0.4 litre of hydrogen?.4 A 9.3 A 9.65 A ( d A 3 If 00 kwh of energy is consumed at 33 in a copper voltameter, what is the mass of copper liberated? Take E. C. E. of copper as kg / C. 3.6 kg 3.3 kg kg ( d mg 33 Which of the following is not reversible? Seebeck effect Peltier effect Joule s effect ( d Thomson effect 34 A copper-iron thermocouple is connected to an external battery of emf E. On doubling the emf of the battery, the heat produced at a junction remains same becomes half doubles up ( d becomes four times 35 What is the nature of graph between temperature and thermo-emf? hyperbola straight line parabola ( d some other curve 36 The unit of Peltier coefficient is J / C J / A J / ( d None of these 37 The unit of Thomson s coefficient is J / C J / A J / ( d None of these 38 Which is a characteristic temperature of thermocouple? cold junction temperature hot junction temperature inverse temperature ( d neutral temperature 39 During electroplating,.5 g of silver is deposited in 5 min. The ammeter connected in series reads A. Approximately by what percentage is the ammeter reading incorrect? 0 % less than correct value % more than correct value 3 % less than correct value ( d % more than correct value Answers b a c d a d d d a a a d b b b a c b a d c a c d b a a d c a b a c c c a d d d

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Downloaded from CHAPTER 12 ELECTRICITY Electricity is a general term that encompasses a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena such