A system executes a power cycle while receiving 2000 Btu by heat transfer at a temperature of 1000 R and discharging energy by heat transfer at a temperature of 500°R. There are no other heat transfers. Applying Eq. 7.2, determine σcycle if the thermal efficiency is
(a) 75%,
(b) 50%,
(c) 25%.
Identify the cases (if any) that are internally reversible or impossible.