It has been suggested that a perpetual motion machine could be constructed in th

Question

It has been suggested that a perpetual motion machine could be constructed in the following way. A long vertical tube contains water. At the bottom, the water escapes through a turbine which drives an electrical generator, and the electrical power is used to electrolyse the water to gaseous hydrogen and oxygen. By increasing the height of the column of water, the energy delivered to the turbine by the water may be increased indefinitely so that, by making the column sufficiently tall, more than enough power may be obtained to electrolyse all the water passing through. Excess power is used to obtain useful work in the surroundings. The whole system is enclosed in another vertical tube, closed at the top, and the gases diffuse up this to the top of the water column where they are recombined by a catalyst and replenish the supply of water as quickly as it is used. The device therefore runs indefinitely and provides a perpetual source of power. Where is the fallacy?

Explanation / Answer

There is a scientific consensus that perpetual motion in an isolated system violates either the first law of thermodynamics, the second law of thermodynamics, or both. The first law of thermodynamics is a version of the law of conservation of energy. The second law can be phrased in several different ways, the most intuitive of which is that heat flows spontaneously from hotter to colder places; relevant here is that the law observes that in every macroscopic process, there is friction or something close to it;

In other words: In any isolated system, one cannot create new energy (law of conservation of energy)

The above suggested system is isolated, so it will nor create new energy.

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