# Heat transfer from, or to, a large heat reservoir is always reversible?

A heat reservoir (Figure above) is a constant temperature heat source or sink. Because the temperature is uniform, there is no heat transfer across a finite temperature difference and the heat exchange is reversible. From the definition of entropy ( $$dS = dQ_\textrm{rev}/T$$ ), $$\displaystyle \Delta S = \frac{Q}{T},$$

How is the heat exchange reversible if a reservoir is at constant temperature? Can anyone please help me. I'm getting confused. Thank you.

Similarly done by "Blundell and Blundell" pg 142 ,2 ed,They take the heat flow between the system and large reservoir to be reversible.

• What is it that you don’t understand? The answer is basically written above. Commented Jan 30, 2021 at 8:30
• If a large reservoir has constant temperature how does that mean that the process is reversible? Commented Jan 30, 2021 at 8:31
• Heat transfer is irreversible when it occurs over a temperature gradient. There are no temperature gradients in an ideal thermal reservoir. Commented Jan 30, 2021 at 11:41
• Yes but there does exist a gradient between reservoir and the system. Won't that finite difference cause the process to be irreversible? Commented Jan 30, 2021 at 17:28