Question

Total energy change for a reversible isothermal cycle is:

• A. always positive
• B. zero
• C. always negative
• D. always $100kJ$ per degree

Answer 1

The correct option is B zero

Since reversible isothermal cycle means there is no change in temperature.
Hence, $\Delta U=nR\Delta T=0$
Also, it will follow the same path for process 2-3 in reverse direction as it has followed in process 1-2 to complete the cycle in isothermal step.
Hence, the value of work in 1-2 is negative of work in 2-3 process.
So, net work done in the cycle is also zero.
Since, $q=W+\Delta U=0+0=0$
Hence, energy change for a reversible isothermal cycle is also 0.
Hence, the answer is $B$.