# Question #babca

Dec 18, 2014

A reversible process is one that is carried out sufficiently slowly that the entire system is very nearly at equilibrium during the process.

Examples:

Suppose you have two 1 kg blocks of copper (specific heat of 385 J/kg-K). Block 1 is initially at 301 K and Block 2 is at 299 K. Now you bring the two blocks into thermal contact with each other. Heat will flow from the first to the second until both blocks are at 300 K. The temperature difference is so small that for all practical purposes the heat transfer is reversible. This can be demonstrated by the fact that the changes in entropy of the two blocks are (nearly) equal in magnitude and opposite in sign.

Block 1: $\Delta S \cong \frac{- 385 J}{300.5 K} = - 1.281 \frac{J}{K}$
Block 2: $\Delta S \cong \frac{385 J}{299.5 .5 K} = 1.285 \frac{J}{K}$
Block 1: $\Delta S \cong \frac{- 385 J}{349.5 K} = - 1.102 \frac{J}{K}$
Block 2: $\Delta S \cong \frac{385 J}{250.5 K} = 1.537 \frac{J}{K}$