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Saturday, 9 January 2016

THERMAL AND CHEMICAL EFFECTS OF CURRENTS mcq

THERMAL AND CHEMICAL EFFECTS OF CURRENTS
1 ) 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

( a ) four times 
( b ) doubled 
( c ) halved 
( d ) one-fourth                                                       [ AIEEE 2005 ]

2 ) Time taken by a 836 W heater to heat one litre of water from 10° C to 40° C is

( a ) 50 s ( b ) 100 s ( c ) 150 s ( d ) 200 s                [ AIEEE 2004 ]

3) The thermo-emf of a thermocouple varies with the temperature θ of the hot junction as E = a θ + b θ2 in volts where the ratio a / b is 700° C. If the cold junction is kept at 0° C, then the neutral temperature is
( a ) 700° C 
( b ) 350° C 
( c ) 1400° C 
( d ) no neutral temperature is possible for this thermocouple
 [ AIEEE 2004 ]

4 ) The electrochemical equivalent of metal is 3.3 × 10 - 7 kg per coulomb. The mass of the metal liberated when a three ampere current is passed for 2 seconds will be

( a ) 19.8 × 10-7 kg 
( b ) 9.9 × 10-7 kg 
( c ) 6.6 × 10-7 kg 
( d ) 1.1 × 10-7 kg [ AIEEE 2004 ]

5) A 220 volt, 1000 watt bulb is connected across 110 volt mains supply. The power consumed will be

( a ) 1000 watt 
( b ) 750 watt 
( c ) 500 watt 
( d ) 250 watt  [ AIEEE 2003 ]

6) An electric motor operates at 20 rev / sec. What will be the approximate power delivered by the motor, if it supplies a torque of 75 N-m ?

( a ) 1550 W 
( b ) 4040 W 
( c ) 7730 W 
( d ) 9420 W [ AIEEE 2003 ]

7 ) The negative Zn pole of a Daniell cell, sending a constant current through a circuit, decreases in mass by 0.13 g in 30 minutes. If the electrochemical equivalents of Zn and Cu are 32.5 and 31.5 respectively, then increase in the mass of positive Cu pole in this time is

( a ) 0.242 g 
( b ) 0.180 g 
( c ) 0.141 g 
( d ) 0.126 g [ AIEEE 2003 ]

8) 2 litre water kept in a kettle is heated by 1 KW power source. Kettle is open and it loses heat at the rate of 160 J/s. The time taken for the temperature of kettle to change from 27° C to 77° C is 
( a ) 8 min 20 sec 
( b ) 6 min 20 sec 
( c ) 5 min 
( d ) 7 min [ IIT 2005 ]

9 ) An ideal gas enclosed in an insulated vessel is heated through a coil of resistance 100 Ω carrying current of 1 A for 5 minutes. Then change in its internal energy will be

( a ) 30 KJ 
( b ) 20 KJ 
( c ) 2 KJ 
( d ) 3 KJ [ IIT 2005]

10 ) 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 ΔT 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 2L. The emperature of the wire is raised by the same amount ΔT in the same time. The value of N is

( a ) 4 
( b ) 6 
( c ) 8 
( d ) 9 [ IIT 2001 ]

11) 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
( a ) both the length and the radius of the wire are halved
( b ) both the length and the radius of the wire are doubled
( c ) the radius of the wire is doubled

( d ) the length of the wire is doubled 

12) Two resistors R1 and R2 when connected across 120 V consume power at the rate of 25 W and 100 W respectively when connected in series and parallel across the same 120 V line. Then the ratio of power consumed by R1 to that by R2 is

( a ) 1 : 1 
( b ) 1 : 2 
( c ) 2 : 1 
( d ) 1 : 4 

13) Find the power wasted in the transmission cables of resistance 0.05 Ω when 10 kW is transmitted at 200 V.

( a ) 0.0125 kW 
( b ) 0.125 kW 
( c ) 25 kW 
( d ) 37.5 kW

14 ) A house is served by a 220 V 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

( a ) 44 
( b ) 22 
( c ) 55 
( d ) 33

15 ) A tap supplies water at 22° C. A man takes 1 litre of water per minute at 37° C from the geyser. The power of geyser is
( a ) 2100 W 
( b ) 725 W 
( c ) 1050 W 
( d ) 1575 W


16 ) A wire of uniform cross-section when connected directly across a 200 volt mains produces x cal / s of heat. If the wire is cut into 10 parts of equal length which are then connected in parallel to 100 volt mains, the heat produced in cal / s will be

( a ) 25 x 
( b ) 50 x 
( c ) 75 x 
( d ) 100 x

17 ) The power of a heater is 500 W at 800° C. What will be its power at 200° C if  α = 4 × 10 - 4 per ° C ?

( a ) 484 W 
( b ) 672 W 
( c ) 526 W 
( d ) 611 W

18 ) Material of heating element of electric heater should have
( a ) high resistivity and high melting point 
( b ) high resistivity and low melting point

( c ) low resistivity and low melting point 
( d ) low resistivity and high melting point

19 ) 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 =

( a ) 1 
( b ) 2 
( c ) 0.5 
( d ) 4.5


20 ) Two heating wires have lengths in the ratio 1 : 2. They are used by first connecting in series and then in parallel. The ratio of heat produced in the two cases will be

( a ) 2 : 5 
( b ) 5 : 2 
( c ) 2 : 9 
( d ) 9 : 2

21 ) How much current should pass through acidulated water for 100 s to liberate 0.224 litre of hydrogen ?

( a ) 22.4 A 
( b ) 19.3 A 
( c ) 9.65 A 
( d ) 1 A

22) Which of the following is not reversible ?

( a ) Seebeck effect 
( b ) Peltier effect 
( c ) Joule’s effect 
( d ) Thomson effect 

23 ) 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
( a ) remains same 
( b ) becomes half 
( c ) doubles up 
( d ) becomes four times


24 ) What is the nature of graph between temperature and thermo-emf ?

( a ) hyperbola 
( b ) straight line 
( c ) parabola 
( d ) some other curve

25) The unit of Peltier coefficient is
( a ) J / C 
( b ) J / A 
( c ) J / V 
( d ) None of these

26 ) The unit of Thomson’s coefficient is
( a ) J / C 
( b ) J / A 
( c ) J / V 
( d ) None of these

27) Which is a characteristic temperature of thermocouple ?
( a ) cold junction temperature 
( b ) hot junction temperature
( c ) inverse temperature 
( d ) neutral temperature

28 ) During electroplating, 1.5 g of silver is deposited in 25 min. The ammeter connected in series reads 1 A. Approximately by what percentage is the ammeter reading incorrect?
( a ) 10 % less than correct value 
( b ) 11 % more than correct value

( c ) 13 % less than correct value 
( d ) 12 % more than correct value



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