Energy Transfer Across the System Boundary (Heat and Work)

Energy transfer across the boundary of a closed system may occur in the form of heat and work. When a closed system is left in a medium of different temperature, energy transfer takes place between the system and the surrounding until thermal equilibrium is reached. The direction of energy transfer is always from the higher temperature side to the lower temperature side. Once the temperature equilibrium is established, energy transfer stops. In the processes described earlier, energy is said to be transferred in the form of heat. Heat is defined as the form of energy that is transferred between two systems or between a system and its surroundings by virtue of a temperature difference.

During adiabatic process heat transfer is negligible. A process can be adiabatic when either the system is well insulated so that only a negligible amount of heat can pass through the boundary or both the system and the surroundings are at the same temperature. Even though there is no heat transfer during an adiabatic process, the energy content, and thus the temperature of a system can still be changed by other means such as work, i.e., the heat can be transformed into work. If the energy crossing the boundary of a closed system is not heat, it must be work. Heat is easy to recognize as its driving force is a temperature difference between the system and its surroundings. Then we can simply say that an energy interaction that is not caused by a temperature difference between a system and its surroundings is work.