Which of the following is not permitted, based on the second law of thermodynamics?
A. Heat flowing from cold to hot
B. A machine that is 80% efficient
C. Heat being completely converted into mechanical energy
D. Heat flowing from hot to cold

A. Heat flowing from cold to hot

Explanation:

The second law of thermodynamics speaks about entropy and directions of processes. These directions go only in one direction, just as time moves only in one direction, as we know it. It is impossible to see time moving forward in a different sense. In the same way, the processes associated with heat transfer, go in a single direction this direction is associated with bodies at different temperatures. Where heat is transferred from a higher temperature body to a lower temperature body.

Heat transfer processes from a cold body to a hot body, do not exist and can not be achieved in a natural way.

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