# If the sun’s surface radiates heat at $$6.3\times 10^{7}~\text{Wm}^{-2}$$ ${}^{}$then the temperature of the sun, assuming it to be a black body, will be: $$\left(\sigma = 5.7\times 10^{-8}~\text{Wm}^{-2}\text{K}^{-4}\right)$$ 1. $$5.8\times 10^{3}~\text{K}$$ 2. $$8.5\times 10^{3}~\text{K}$$ 3. $$3.5\times 10^{8}~\text{K}$$ 4. $$5.3\times 10^{8}~\text{K}$$

Subtopic:  Stefan-Boltzmann Law |
59%
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A block of metal is heated to a temperature much higher than the room temperature and allowed to cool in a room free from air currents. Which of the following curves correctly represents the rate of cooling?

 1 2 3 4
Subtopic:  Newton's Law of Cooling |
78%
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A black body has a maximum wavelength at a temperature of 2000 K. Its corresponding wavelength at temperatures of 3000 K will be:

 1 $${3 \over 2} \lambda_m$$ 2 $${2 \over 3} \lambda_m$$ 3 $${4 \over 9} \lambda_m$$ 4 $${9 \over 4} \lambda_m$$
Subtopic:  Wien's Displacement Law |
86%
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Taking into account the radiation that a human body emits which of the following statements is true?

 1 The radiation is emitted only during the day. 2 The radiation is emitted during the summers and absorbed during the winters. 3 The radiation emitted lies in the ultraviolet region and hence is not visible. 4 The radiation emitted is in the infra-red region.
Subtopic:  Radiation |
59%
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On a clear sunny day, an object at temperature T is placed on the top of a high mountain. An identical object at the same temperature is placed at the foot of the mountain. If both the objects are exposed to sun-rays for two hours in an identical manner, the object placed on the top of a mountain will register a temperature:

 1 higher than the object at the foot. 2 lower than the object at the foot. 3 equal to the object at the foot. 4 none of the above.

Subtopic:  Radiation |
52%
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While measuring the thermal conductivity of a liquid, we keep the upper part hot and the lower part cool, so that:

 1 convection may be stopped. 2 radiation may be stopped. 3 heat conduction is easier downwards. 4 it is easier and more convenient to do so.

Subtopic:  Convection |
62%
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Air is a bad conductor of heat or partly conducts heat. Still, a vacuum is to be placed between the walls of the thermos flask because:

 1 it is difficult to fill the air between the walls of the thermos flask. 2 due to more pressure of air, the thermos can get cracks. 3 by convection, heat can flow through the air. 4 on filling the air, there is no advantage.
Subtopic:  Convection |
79%
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One likes to sit in the sunshine in the winter season, because:

 1 The air around the body is hot, and the body absorbs heat from it. 2 we get energy from the sun. 3 we get heat by conduction from the sun. 4 None of the above

Subtopic:  Conduction |
64%
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Mud houses are cooler in the summer and warmer in the winter because:

 1 the mud is a superconductor of heat. 2 the mud is a good conductor of heat. 3 the mud is a bad conductor of heat. 4 None of the above

Subtopic:  Conduction |
81%
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When two ends of a rod wrapped with cotton are maintained at different temperatures and, after some time, every point of the rod attains a constant temperature, then:

 1 Conduction of heat at different points of the rod stops because the temperature is not increasing 2 The rod is a bad conductor of heat 3 Heat is being radiated from each point of the rod 4 Each point of the rod is giving heat to its neighbour at the same rate at which it is receiving heat
Subtopic:  Conduction |
75%
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