Page 126 - Physics - XI

Q4. Hot water cools from 60°C to 50°C in the fi rst 10 minutes and to 42°C in the next 10 minutes.
The temperature of the surroundings is

a. 25°C b. 20°C c. 15°C d. 10°C

Q5. Certain quantity of water cools from 70°C to 60°C in the fi rst 5 minutes and to 54°C in the
next 5 minutes. The temperature of the surroundings is

a. 42°C b. 20°C c. 45°C d. 10°C
Q6. The two ends of a metal rod are maintained at temperatures 100°C and 110°C. The rate of
heat fl ow in the rod is found to be 4.0 J/s. If the ends are maintained at temperatures 200°C

and 210°C, the rate of heat fl ow will be
a. 44.0 J/s b. 16.8 J/s c. 4.0 J/s d. 8.0 J/s

Q7. The power radiated by a black body is P and it radiates maximum energy at wavelength λ .
0
If the temperature of the black body is now changed so that it radiates maximum energy at
3
wavelength λ , the power radiated by it become nP. The value of n is
4 0

a. 256 b. 4 c. 3 d. 81
81 3 4 256
Q8. When 1 kg of ice at 0°C melts to water at 0°C, the resulting change in its entropy (taking latent
heat of ice to be 80 cal/°C) is

a. 273 cal/K b. 8 × 10 cal/K c. 80 cal/K d. 293 cal/K
4
Q9. A slab of stone of area 0.36 m and thickness 0.1 m is exposed on the lower surface to steam at
2
100°C. A block of ice at 0°C rests on the upper surfaces of the slab. In 1 hour, 4.8 kg of ice is
melted. The thermal conductivity of slab is (Given latent heat of fusion of ice = 3.36 × 10 J kg )
–1
5
a. 1.02 J/s/°C b. 1.29 J/s/°C c. 1.24 J/m/s/°C d. 2.05 J/m/s/°C
Q10. Two rods A and B of diff erent materials are welded together as shown in the fi gure. Their
thermal conductivities are K and K . The thermal conductivity of the composite rod will be
1 2

A K 1
T T
1 2
B K
2
d

K + K 3(K + K )
a. 1 2 b. 1 2 c. K + K d. 2(K + K )
2 2 1 2 1 2
Answer key

1. (d), 2. (d), 3. (b), 4. (d), 5. (c), 6. (c), 7. (a), 8. (d), 9. (c), 10. (a)

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