Ways to change internal energy
(1) Doing work can change the internal energy of an object.
When an external force does positive work on an object, the internal energy of the object increases, and vice versa.
(2) Heat transfer can change the internal energy of an object.
Three forms of heat transfer: heat conduction, heat convection (generally found in gases and liquids) and heat radiation. The condition of heat transfer is that there must be a temperature difference between objects.