The perpetual motion indicates the idea of a movement (generally periodic), within a system, able to last indefinitely without contribution external of energy or Matière, nor irreversible transformation of the system.

Since the Rebirth, inventors being unaware of of the principles of the Mécanique tried to build mechanical systems ready to perpetuate their movement, thinking that they could constitute a free source of work.

Their mechanisms could not function in accordance with their hopes, because the technical training of the time hardly made it possible to reduce to a significant degree the phenomena of Frottement between the fixed parts and moving parts.

It is known today that if a perpetual motion can exist in theory, it cannot in any event be an energy source, used like engine.

The perpetual motion in nature

In theory a body isolated in uniform rectilinear motion does not release any energy and can thus have a perpetual motion (the forces which is exerted on him cancel and are nondissipative).

Electrons in the atom

A representation is that the electron S turn around the core in the Atome (planetary model); this is impossible because they would then radiate energy with the image of a radio antenna (that is shown starting from the Maxwell's equations); the quantum Mécanique watch makes of it that the electrons form a “electronic cloud” around the core.

See also: Atom

Planets

On a human scale, the trajectory of planets seems to be a perpetual motion (the modifications of periods are detectable, but really very weak). Of course, certain planets will be destroyed when the Sun becomes a Géante red. However, it is exact that the movement of planets around a star is perpetual as long as the system does not undergo modification. This kind of perpetual motion does not contradict the laws of conservation of energy since it does not provide work (the use of the gravitational Assistance for the acceleration of space engines is one of the rare exceptions to the preceding sentence, but taken energy is negligible for such a system).

Attempts at “perpetual” machines with movement, surunity

It is question here of having a machine having a cyclic movement, giving more energy, than it does not receive any, for a use as engine. Knowing that the rise and the cyclic descent of a weight or a float do not give energy, that the compression and the cyclic decompression of a spring either, it is obvious that such machines are impossible. The argument of the Résonance does not hold either, because resonance does not create either energy.

Many scientists are leaning on the perpetual motion, of which Léonard de Vinci.

One of the first perpetual motions suggested is the wheel of Johann Bessler. It is a drum full dug with cavities in which weights rock, by involving the wheel. The shape of the cavities is conceived in such a way that the weights approach the axis of rotation at the time of the rise and move away from there to the descent.

But a rigorous calculation applying the laws of Newton (posterior with the drawing of the wheel) watch which the wheel cannot gain number of revolutions, only to find same speed at the end of a turn, by supposing that there are no frictions.

Use as engine

An old dream to have a free energy is to use a wheel in perpetual rotation (which one would have succeeded in maintaining frictions mechanical on a negligible level) like electric energy source by electromagnetic Induction, i.e. to place a Dynamo on a wheel at perpetual motion. But it is shown that the Force of Laplace behaves then exactly like a force of friction. Electrical energy provided on the whole (once the stopped wheel) corresponds then exactly to the kinetic energy provided by the user at the origin.

Machines with perpetual pseudo-movement

Mechanisms can give the illusion of the perpetual motion. In fact the movement is always maintained by an energy source more or less difficult to detect.

In the Years 1760, the clock and watch maker James Cox invented a clock with beam which does not need to be gone up. Actually its clever mechanism uses a hidden energy source: variations of the atmospheric pressure. It is indeed a mercury column which maintains the mechanism.

Small the driving with water, uses the energy of the evaporation of water to create a movement.

Perpetual machines of second order

The machines described previously are of the machines of first order, violating the First principle of thermodynamics. The machines of second order, would violate the Second principle of thermodynamics. For example, one could imagine a boat being driven by transforming sea water into ice floes.

An example of apparent violation of the second principle is the Démon of Maxwell or the ratchet wheel and pawl of Feynman. Such mechanisms are supposed capacity to transform heat into work during a cycle monotherme. They thus raise the question of the validity of the second principle (i.e. the monotonous assumption of growth of the macroscopic Entropie of the “isolated” systems). In fact, these two mechanisms by no means make it possible to show that the second principle would express only one limitation of our current technological possibilities instead of having to be interpreted like an inviolable physical principle (independently of any technological advancement).

Indeed, the fall of entropy of gas realized by the action of the Démon of Maxwell for example (by opening and closing at the good moment a small door spared in a partition separating a tank from gas in two tight compartments) is possible without violation of the second principle of thermodynamics. Taking into account macroscopic equivalence entropy/lack of information of a macroscopic observer it is enough, to respect the monotonous assumption of growth of the entropy of the “isolated” systems, that this fall of entropy of gas is accompanied by a higher or equal loss of the information held by the demon on the state of gas and its own state.

It is thus known today that the experiment of thought corresponding to the demon of Maxwell does not make it possible to invalidate the assumption according to which the second principle of thermodynamics would present a fundamental character. The physicists today ceased seeking exceptions to the first law of thermodynamics and hardly believe that it is possible, in the future, to violate the second law of thermodynamics thanks to progress in Nanotechnologie S.

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