Perpetual Motion

A general definiton of perpetual motion machines is that these machines produce more energy or work than the amount of energy they consume. A perpetual motion machine could extract the needed energy for its motion from the energy it produced, this explains why it's called perpetual motion. Such a machine would mean the end of all energy problems. However, it's physically impossible to create a machine like that. There are two fundamental laws that counter the existence of these kind of machines: the first and second law of thermodynamics. The first law of thermodynamics expresses the conservation of energy. Energy can transform from one form into another, but it can't be destroyed or created. The second law of thermodynamics states that no machine can be more efficient than a Carnot heat engine. A heating engine extracts heat from a hot reservoir at temperature Th and gives this heat to a cold reservoir at temperature Tc, with the purpose of heating the cold reservoir. A cooling engine works in the same way, but with the purpose of cooling the hot reservoir. The efficiency of a Carnot heat engine is then given by:

e = 1-Tc/Th

The temperatures Tc and Th are expressed in Kelvin. This means that Tc/Th is always positive, and e always less than or equal to 1. The efficiency e can be equal to 1 if Tc is equal to zero. There are however theoretical reasons for believing that an absolute temperature of 0 Kelvin can't be attained experimentally (e.g. the third law of thermodynamics). We conclude that e is always less than 1, and a machine can never produce as much energy as it consumes and perpetual motion machines are not very likely to exist.