Perpetual motion machine is a hypothetical device that can continuously produce work with no energy input, continuously convert energy completely into work, or continuously produce more energy than it consumes. No one has ever succeeded in building a perpetual motion machine, and almost all scientists and engineers believe no one ever will.
For a machine to achieve perpetual motion, it would have to violate one or both of two laws of thermodynamics. These laws summarize how machines work. The first law states that energy cannot be created or destroyed. Energy may change form–for example, from internal energy to mechanical motion–but the total energy of any system remains the same. A system may be anything from a simple object to a complex machine. The second law says that heat, by itself, can flow only from a hot object to a colder object. See Thermodynamics .
Inventors have proposed at least two kinds of perpetual motion machines. The first kind would run forever with no energy input. The second kind would continuously convert energy completely into work. A machine that would continuously yield more energy than it consumes is considered by some experts to be a third type of perpetual motion machine. Other experts class this device with the first kind of perpetual motion machine.
The first kind of perpetual motion machine violates the first law of thermodynamics. This machine will not work, because resistance opposes the moving parts of all machines. To keep running, the machine must use energy to overcome this resistance. Without energy input, therefore, the machine soon stops.
The second type of perpetual motion machine violates the second law. This type of machine traditionally does work by exploiting the natural flow of heat from a hot region to a cooler region. But once the flow of heat has warmed the cool region to the same temperature as the hot region, the machine stops. Some inventors have proposed machines that would continuously produce work by converting all the energy of the randomly moving molecules in the sea or atmosphere. However, no machine has been able to accomplish this, either.
A machine that would continuously produce more energy than it consumed also violates the first law of thermodynamics. This machine would have to create energy.
Some people have considered artificial satellites as possible sources of perpetual motion. But atmospheric friction limits the lifetimes of artificial satellites that orbit relatively close to the earth. Scientists expect that even satellites that orbit the sun may eventually be pulled into it. Nuclear energy has also been considered a possible source of perpetual motion. Uranium and other nuclear fuels do contain tremendous amounts of energy for their size. But after this energy is used up, the remaining matter must be replaced with fresh fuel. See Nuclear energy .
