Back to main heat and temperature page Energy.
Energy is the ability to do work.
For instance, if you apply 100 Newtons of force to lift weights 1-meter, you will have done 100 Joules of work. If you apply 10 newtons to lift a pumpkin 1-meter, you will have expended 10 Joules. If you apply a force over a given distance - you have done work. In general, the energy transferred depends on the amount of force and the distance over which that force is exerted. If heat is applied to a piston, the increase in internal energy can cause the piston to move.