Question

Choose the correct statements from the following :

• A. Good reflectors are good emitters of thermal radiation
• B. Burns caused by water at 100${}^o$ C are more severe than those caused by steam at 100${}^o$ C
• C. If the earth did not have atmosphere , it would become intolerably cold
• D. It is impossible to construct a heat engine of 100 % efficiency

Answer 1

Answer: (C), (D)

The correct options are
C If the earth did not have atmosphere , it would become intolerably cold
D It is impossible to construct a heat engine of 100 % efficiency

Good reflectors are bad absorbers of radation, and the ratio of emissive and absorptive is a constant. Hence a good reflector would be a bad emitter of radiation. Hence option A is wrong.
The burns caused by steam at ${100}^{\circ }C$ would cause a larger burns than that of water at ${100}^{\circ }C$ because of higher kinetic energy of the particles. Hence option B is wrong.
If the earth did not have an atmosphere, the heat absorbed by the suns radiation would be a lot lesser and hence it would become intolerably cold. Hence option C is true.
From the second law of thermodynamics, we know that it is impossible to make the perpetual motion machine of the first kind. In other words, a machine with 100% efficiency. Hence option D is correct.