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21.

The temperature of two outer surfaces of a composite slab, consisting of two materials having coefficients of thermal conductivity K and 2K and thickness x and 4x, respectively are T2 and T1 (T2 > T1). The rate of heat transfer through the slab, in a steady state isopen parentheses fraction numerator straight A left parenthesis straight T subscript 2 minus straight T subscript 1 right parenthesis straight K over denominator straight x end fraction close parentheses straight f with f, equal to

  • 1

  • 1/2

  • 2/3

  • 2/3

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22.

A light ray is incident perpendicular to one face of a 90° prism and is totally internally reflected at the glass-air interface. If the angle of reflection is 45°, we conclude that the refractive index n

  • n<2

  • straight n greater than square root of 2
  • straight n space greater than space fraction numerator 1 over denominator square root of 2 end fraction
  • straight n space greater than space fraction numerator 1 over denominator square root of 2 end fraction
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23.

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

  • 50 s

  • 100 s

  • 150 s

  • 150 s

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24.

The thermo emf of a thermocouple varies with the temperature θ of the hot junction as E = a θ + bθ2 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

  • 1400°C

  • 1400°C

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25.

Initially a beaker has 100 g of water at temperature 90°C. Later another 600 g of water at temperature 20°C was poured into the beaker. The temperature, T of the water after mixing is

  • 20°C

  • 30°C

  • 45°C

  • 55°C


26.

A metallic bar of coefficient of linear expansion 10-5 K-1 is heated from 0°C to 100°C. The percentage increase in its length is

  • 0.1 % 

  • 1 % 

  • 10 %

  • 0.01 %


27.

Two perfectly black spheres A and B having radii 8 cm and 2 cm are maintained at temperatures 127°C and 527°C, respectively. The ratio of the energy radiated by A to that by B is

  • 1 : 2

  • 1 : 1

  • 2 : 1

  • 1 : 4


28.

Hot water in a vessel kept in a room cools from 70°C to 65°C in t1 minutes, from 65°C to 60°C in t2 minutes and from 60°C to 55°C in t3 minutes then,

  • t1 < t2 > t3

  • t1 = t2 = t3

  • t1 < t2 < t3

  • t1 < t2 = t3


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29.

If the time taken by a hot body to cool from 50°C to 40°C is 10 min when the surrounding temperature is 25°C, then the time taken for it to cool from 40°C to 30°C when the surrounding temperature is 15°C, is

  • 40 min

  • 10 min

  • 5 min

  • 15 min


30.

1 cc of water becomes 1681 cc of steam when boiled at a pressure of105 Nm-2. The increasing internal energy of the system is (Latent heat of steam is 540 cal g-1 , 1 cal = 4.2 J)

  • 300 cal

  • 500 cal

  • 225 cal

  • 600 cal


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