Spontaneous processes in nature occur only in one direction. Heat always flows from a body at a higher temperature to a body at a lower temperature. If we want to transfer equal heat from lower temperature body to higher temperature body as a spontaneous process, we cannot do it because some work is to be done. First law does not restrict on direction of conversion of heat to work. Heat cannot be converted to equal amount of work. i.e., Q > W. Some heat is to be rejected. The amount of heat rejected is Q – W.
Fig. 6.2. shows that process 1 to 2 is carried out by injecting heat Q1 – 2. If the process 2 to 1 is obtained by doing some work on it i.e., W2-1, it is found that
Q1-2 > W2-1·