For a thermodynamics process to be reversible, the temperature difference between hot body and the working substance should be

In thermodynamics, a reversible process

is a process

whose direction can be "reversed" by inducing infinitesimal changes to some property of the system via its surroundings, while not increasing entropy. Throughout the entire reversible process

, the system is in thermodynamic

equilibrium with its surroundings, which means that the hot body and surroundings are at same temperature.