The first and second laws of thermodynamics

If a system can return from the final state to the initial state through the same intermediate states, the process is reversible; real processes (friction, heat exchange) are irreversible, and a reversible process is an idealisation. The internal energy U of a thermodynamic system is the sum of the kinetic energies of the random motion of its molecules and their interaction potential energies; it depends only on the state, so what matters in a process is its change ΔU. The first law (conservation of energy): the heat given to a system goes to increasing the internal energy and to the work the system does against external forces: Q = ΔU + A. Heat received Q > 0, heat given away Q < 0; if the system does work A > 0, if work is done on it A < 0. This law shows that a perpetual motion machine of the first kind (one giving work without using energy) is impossible. The second law fixes the direction of processes: Clausius – heat does not pass by itself from a colder to a hotter body; Planck – no process converts heat completely into work. So a perpetual motion machine of the second kind is also impossible.