A) we cannot prevent radiation
B) we cannot find ideal sources
C) we cannot reach absolute zero
D) we cannot eliminate friction
Correct Answer: C
Solution :
Efficiency of ideal heat engine \[\eta =1-\frac{{{T}_{2}}}{{{T}_{1}}}\] where, \[{{T}_{2}}=\]temperature of sink \[{{T}_{1}}=\]temperature of source For 100% efficiency\[\eta =1\] \[\therefore \] \[1=1-\frac{{{T}_{2}}}{{{T}_{1}}}\] \[\Rightarrow \] \[\frac{{{T}_{2}}}{{{T}_{1}}}=0\] \[\Rightarrow \] \[{{T}_{2}}=0\,K\] This implies that temperature of sink is absolute zero which practically cannot be obtained. So, ideal heat engine cannot give 100% efficiency.
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