Perpetual Motion Machines
Perpetual motion is the motion of bodies that continues forever. A perpetual motion machine is a hypothetical machine that can do work infinitely without an energy source. This kind of machine is impossible, as it would violate the first or second law of thermodynamics.
These laws of thermodynamics apply regardless of the size of the system. For example, the motions and rotations of celestial bodies such as planets may appear perpetual, but are actually subject to many processes that slowly dissipate their kinetic energy, such as solar wind, interstellar medium resistance, gravitational radiation and thermal radiation, so they will not keep moving forever.
Thus, machines that extract energy from finite sources will not operate indefinitely, because they are driven by the energy stored in the source, which will eventually be exhausted (e.g., ocean currents, which derive energy from the sun).
These perpetual motion machines come in many forms.
Perpetual motion machines of the first kind violate the First Law of Thermodynamics. They produce mechanical work (i.e. movement) without any energy being input; this violates the principle of the conservation of energy.
Perpetual motion machines of the second kind violate the Second Law of Thermodynamics. They convert thermal energy directly into mechanical work, with no exhaust heat being emitted; this violates the rule of the production of entropy, that entropy in a system must always increase.
Perpetual motion machines of the third kind do not, as their name suggests, violate the Third Law of Thermodynamics. Rather they claim to maintain motion (once started) forever, and do so by assuming that frictional forces can be eliminated completely (often through operating in a perfect vacuum, which is also not possible).
In 2017, new states of matter, time crystals, were discovered in which the component atoms are (microscopically) in continual repetitive motion, thus satisfying the literal definition of “perpetual motion”. However, these do not constitute perpetual motion machines in the traditional sense or violate thermodynamic laws because they are in their quantum ground state, so no energy can be extracted from them; they exhibit motion without energy.
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