Question

A thermodynamic system undergoes a cyclic process as shown in figure. Over one complete cycle, determine the direction of heat flow.

• A. 0
• B. >0
• C. <0
• D. Nothing can be said

Answer 1

The correct option is B >0

The first law of thermodynamics gives us the relation:

$\delta q=\Delta U+w$

For a cyclic process the change in internal energy is zero, because internal energy is a state function and doesn't depend on path.

i.e. $\delta q=w$
For a cyclic loop as shown in the diagrams. The work done is area under a P-V graph.

With an application of calculus we understand that for a clockwise loop, work done is positive and equal to the area enclosed by the loop. For an anticlockwise loop, work is negative but the magnitude is equal to that of the area enclosed.

The given diagram consists of 2 loops, one clockwise and one anticlockwise. The area enclosed by the clockwise loop is larger. Hence the work done is positive.

The heat supplied to the system must also be positive. Hence heat flows into the system.