Question

A perpetuum mobile (Latin for "perpetual motion machine") is an apparatus capable of doing work without supply of energy. It has been the dream of inventors since ancient times and continues to pop up in sketches and drawings of would-be inventors every now and then but in 1775 the Royal Academy of Sciences in Paris issued the statement that the Academy "will no longer accept or deal with proposals concerning perpetual motion". (Why?)

Answer 1

The perpetual motion machines are of course fascinating and gives ideas for doing work infinitely without the supply of energy. Well, you read that right but don't you think there is something wrong with that? Yes, there is!! So, why aren't these machines working in real life??

That is because all these ideas for perpetual motion machines violate one or more fundamental laws of Thermodynamics that relates different forms of energy.

The first law of thermodynamics states that "Energy can't be created or destroyed" . That means you can't get more energy than you put in. That rules out the possibility of perpetual motion machine right away. Because the machine can only produce energy for itself to work. No energy can be taken outside the machine to work on anything else .

Okay, I get the point. What if we want the machines to keep itself working with no energy to be extracted out?

This takes us to the second law of Thermodynamics. The second law says that "The energy tends to spread out through processes like friction".

Any real machine would have moving parts that would have interaction with air or liquid molecules that would generate tiny amounts of friction and heat , even in vacuum. That energy produced through friction is energy escaping from the machine. That energy keeps leaching out until a point where the machine losses the energy itself and inevitably stopped.

So it seems that, for the machine to keep working the input energy is not equal to the output energy due to these tiny losses. The machine has to find more input energy to keep itself working to get a continuous output. This again violates the first law of thermodynamics.

So far, these two laws of thermodynamics have stymied every ideas of perpetual motion and every idea of perfectly efficient perpetual motion machines.