When a heater is immersed in a beaker of water (the system), the
capacity of the system to do work increases because hot water can be used to do more work than the same amount of cold water.
Not all boundaries permit the transfer of
energy even though there is a temperature difference between the system and its surroundings.
Boundaries that do permit the transfer of energy as heat are called diathermic;
those that do not are called adiabatic.