Answer: When heat is supplied to a gas, the energy goes into the random motion of its molecules, so their kinetic energy and hence the internal energy of the gas increases (and its temperature rises).
Consider a gas enclosed in a cylinder with a fixed piston, placed in contact with a hotter body (a flame or a hot reservoir). Because of the temperature difference, energy flows as heat from the hotter body into the gas through the walls of the cylinder.
- The molecules of the hot walls vibrate vigorously; gas molecules colliding with the walls bounce off with greater speeds.
- Thus the average kinetic energy of the gas molecules increases, i.e. the internal energy \(U\) of the gas increases and its temperature rises.
- If the piston is fixed (no work done), all the heat supplied goes into internal energy: \(\Delta U = Q\). If the gas is allowed to expand, part of the heat is used to do work, and \(\Delta U = Q - W\).
Conversely, if the gas is placed in contact with a colder body, heat flows out of it and its internal energy decreases.
