Choose the correct statements from the following:

Options

A. Good reflectors are good emitters of thermal radiation
B. Burns caused by water at `100^@C` are more severe than those caused by steam at `100^@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

C, D

Explanation

Statement (a) is incorrect. According to Kirchhoff's law, the ratio of the emissive power e and absorptive power a is constant for all substances at any given temperature and for radiation of the same wavelength,
i.e., `(e)/(a)=`constant
Thus, if e is large, a must also be large i.e, if a body is a good emitter of a radiation of a particular wavelength it is also a good absorber of that radiation. Conversely, if a body is a poor emitter of radiation it is also a poor absorber (and hence a good reflector) of the radiation. Statement (b) is also incorrect The latent heat of steam at `100^@C` gives out `2.25xx10^6J` of heat energy to convert into 1 kg of boiling water at `100^@C`. Hence, the burns caused by steam are more severe than those caused by boiling water. For the same reason, heating systems based on circulation of steam are more efficient in warming a house than those besed on circulation of hot water.
Statement (c ) is correct. The thermal radiation from the sun warms the earth during the day, since, air a poor conduction of heat, the atmosphere acts as a blanket for the earth and keeps the earth warm during the night. Moon is very cold because it has no atmosphere.
Statement (d) is also correct. The efficiency of an ideal heat engine is given by `eta=1(T_2)/(T_1)`
where `T_2` is the temperature of the sink. To have an efficiency `eta=1(or100%)`,`T_2=0K`. Since absolute zero cannot be achieved even an ideal heat engine cannot have a `100%` efficienty.