You can transfer energy into a system by the mechanical process of doing work on the system. But that can’t be all there is to energy transfer. What happens when you place a pan of water on the stove and light the burner? The water temperature increases, so . But no work is done and there is no change in the water’s mechanical energy . This process clearly violates the energy equation . What’s wrong?
Nothing is wrong. The energy equation is correct as far as it goes, but it is incomplete. Work is energy transferred in a mechanical interaction, but that is not the only way a system can interact with its environment. Energy can also be transferred between the system and the environment if they have a thermal interaction. The energy transferred in a thermal interaction is called heat.
The symbol for heat is Q. When heat is included, the energy equation becomes
Heat and work are both energy transferred between the system and the environment.
NOTE We’ve dropped the subscript “ext” from W. The work that we consider in thermodynamics is always the work done by the environment on the system.
We’ll return to Equation 19.3 in Section 19.4 after we look at how work is calculated for ideal-gas processes and at what heat is.