Understanding Reversible Cycles in Thermodynamics

A reversible cycle is an ideal concept in thermodynamics where both the system and its surroundings can be restored to their initial states without producing any permanent change in the surroundings. For a cycle to be truly reversible, it must consist of only reversible processes. Irreversibilities prevent a process or cycle from being reversible.

Conditions for a Cycle to be Reversible

Let's examine the conditions mentioned in the options and understand why they are necessary for achieving a reversible cycle:

Avoiding Free Expansion and Friction

• Condition: Free expansion or friction resisted expansion/compression processes should not be encountered.

• Explanation: Processes like free expansion (expansion against vacuum) are highly irreversible. No work is done by the system during free expansion, and reversing it would require work input from the surroundings, leaving a change. Similarly, friction during expansion or compression causes energy dissipation (conversion of mechanical energy into heat), making the process irreversible. To be reversible, expansion and compression must occur quasi-statically and without friction.

Temperature Equality During Heat Transfer

• Condition (Heat Absorption): When heat is being absorbed, temperature of hot source and working substance should be same.

• Explanation: Heat transfer occurs due to a temperature difference. However, heat transfer across a finite temperature difference is an irreversible process. To achieve reversible heat transfer, the temperature difference between the heat source and the working substance must be infinitesimal. This means the working substance's temperature should be infinitesimally lower than the hot source temperature when absorbing heat.

• Condition (Heat Rejection): When heat is being rejected, temperature of cold source and working substance should be same.

• Explanation: Similarly, for reversible heat rejection, the temperature of the working substance must be infinitesimally higher than the cold sink temperature when rejecting heat. If there is a finite temperature difference, the heat transfer is irreversible.

Conclusion: All Conditions are Necessary

Based on the analysis of each condition:

• Avoiding irreversibilities like free expansion and friction is essential.
• Heat transfer must occur across an infinitesimal temperature difference, meaning the working substance and source/sink temperatures are practically the same during heat exchange.

Therefore, all the listed conditions are necessary requirements for a thermodynamic cycle to be reversible. If any one of these conditions is not met, the cycle will be irreversible.