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 The laws of thermodynamics are about how energy is used and transformed in systems such as machines. It is a complex subject that started with a need to explain how things like steam engines actually worked and particularly how they could be improved. The important laws when thinking about perpetual motion machines are the first and second laws. These laws can help us understand why perpetual motion machines or machines that are more than 100% efficient, are impossible. To read more about energy and power click here.
The first law of thermodynamics says that energy cannot be created or destroyed. It can only be converted from one form, such as heat, to another form, such as the energy of a moving vehicle. This law of conservation of energy is absolutely fundamental to our understanding of science and the way the universe works. No one has ever found an example, anywhere, where this law does not apply.
Doing work such as moving a vehicle, or lifting a weight, requires energy to be converted from one form to another.
The second law of thermodynamics says that it is impossible to build a machine that is 100% efficient. Understanding why this is so is explained below.
Any real machine doing useful work must have a source of energy that can supply the energy used in the work done by the machine plus any energy losses in the machine itself.
At first you may think that making a machine that is 100% efficient is just a matter of making lower friction bearings, or improving the heat insulation, but there is more to it than that.
You may have heard of the concept of entropy. Entropy is a complex subject but basically it has to do with the fact that things that are arranged in a particular way only get that way because work has been done to arrange them. Left to their own devices things tend to become untidy. Things that are highly disorganised are said to have a high entropy level. Things that are carefully arranged have a low entropy level.
Sandcastle EntropyOne simple example is to consider a pile of sand. The grains of sand are arranged randomly and therefore have a high entropy level. To make a simple sandcastle out of the sand we have to do work to make the sand into a more ordered state. We have reduced the state of disorder by doing work on the sand. We have reduced its entropy. Over time the sandcastle will gradually crumble back to a heap of sand and it will be disordered again. The energy we put into the sand castle will be transformed again. It will mostly be converted as tiny amounts of heat as the sand grains roll back down one by one to the beach and rub against each other as they go.

 So it takes more energy (work) to create an ordered system than a disordered one. Put another way round, any system will become more disordered with time unless we put energy into maintaining it.  The probability is that as time passes the pile of sand will become more disordered. We know from experience that it is very unlikely that our random pile of sand will slowly change itself into a sandcastle. But it is not impossible. The shape of the sandcastle is one of the many billions of combinations of the grains of sand that could happen, but on the other hand that also means that the chances of making a sandcastle by accident are billions to one.
So what the second law of thermodynamics says is that there is a very high probability that as time goes by our system will become more disordered, and therefore will have a lower level of energy. Our sand will not turn into a sandcastle.

There may not seem to be much in common between a pile of sand and a car engine but the principle is the same. We start out with an engine and some fuel such as diesel that is in a highly ordered state. The diesel is then burnt in the engine to produce hot gases, water vapour, carbon particles and heat. The gases expand and are expelled from the engine through the exhaust.  It is difficult to imagine all these components and the heat that has passed into the diesel engine’s cooling system reassembling themselves into the diesel fuel that we started with. The total energy is the same but the system is now much more disordered. The expanding gases have done work on the piston. The piston drives a mechanism to increase the kinetic energy of the car by making it go faster. The waste heat has passed via the cooling system into the atmosphere.
The second law of thermodynamics is saying that it is possible for a system (or machine) to return to its original state with no loss of energy, it is just extremely unlikely. ’Extremely’ here means chances so low that it would take lengths of time longer than the age of the universe for us to have a chance of seeing it happen. Think of pouring sand into the shape of a sandcastle.
These laws show the basic reasons why perpetual motion machines cannot be created. If the machine is doing useful work then it is transforming energy from one type of energy to another. So it must have a source of energy. As it works some of that energy will be lost to ‘disorder’, usually in the form of heat caused by friction and therefore it is impossible (by which we really mean very, very unlikely) to build a machine that is 100% efficient.
So in summary any machine that does useful work without losing energy is very, very improbable. We can imagine a machine that runs forever such as a pendulum mounted on frictionless bearings in a vacuum; even if to make one would be difficult. However as soon as we try and get it to do useful work it will start to slow down and eventually stop. This is because we are taking energy out of the system and not putting it back. The energy that we are taking out will be converted into another form of energy such as the energy of motion in whatever we are driving with our pendulum and the losses will most probably end up as heat.
Claimed perpetual motion machines without any source of energy are rare. Although machines that rely on an initial state that has a potential energy, like a falling weight, are common. Much more common are machines that violate the second law and claim to be able to convert energy without losses. See the article on Perpetual Motion Machines for some examples of common examples of these machines and the article on Work and Energy to see how energy is converted.
In summary machines are classified as perpetual motion machines if they do work without a source of energy and/or if they claim to do it without any losses. The first and second laws of thermodynamics say that this is practically impossible, and if it isn’t then we will have to rewrite all the science books.