For "closed systems" with no external source or sink of energy, the first law of thermodynamics states that a system's energy is constant unless energy is transferred in or out by mechanical work or <answer>heat</answer>, and that no energy is lost in transfer. This means that it is impossible to create or destroy energy. While <answer>heat</answer> can always be fully converted into work in a reversible isothermal expansion of an ideal gas, for cyclic processes of practical interest in <answer>heat</answer> engines the second law of thermodynamics states that the system doing work always loses some energy as waste <answer>heat</answer>. This creates a limit to the amount of <answer>heat</answer> energy that can do work in a cyclic process, a limit called the available energy. Mechanical and other forms of energy can be transformed in the other direction into thermal energy without such limitations. The total energy of a system can be calculated by adding up all forms of energy in the system.
What can be fully converted into work in a reversible isothermal expansion of an ideal gas?
heat