Question

The efficiency of a heat engine will be always less than 100% this is a conseqence of

Answer 1

the efficiency of a heat engine will be always less than 100% this is a consequence of loss of energy and force of friction occuring between them
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Answer 2

Solids conduct heat better than liquids, which are in turn better conductors than gases. The particles in a solid are most tightly bond and their positions are more or less fixed relative to each other (Fig. 5a). The force between adjacent particles is strong, making heat transfer by collision the most efficient.