The idea of perpetual motion sounds straightforward. An electric car powered this way could recharge its own batteries forever, and a clock could automatically rewind itself for years. But there is a reason why perpetual motion machines remain in the realm of fantasy; it's the Laws of Thermodynamics. Some inventions may appear to run by perpetual motion, but they usually rely on a hidden source of external energy.
Both the First and Second Laws of Thermodynamics keep the perpetual motion car in the garage. According to one portion of the First Law, energy cannot be created or destroyed, only changed to different forms. The batteries that power an electric car only contain a fixed amount of energy. Most of this energy goes into propelling the electric motor, but some is inevitably lost through friction and the recreation of momentum after a stop. The car's recharger would have to create more energy in order to keep the batteries at full capacity. No such power generator exists, nor can one be built if the Laws of Thermodynamics are true.
The Second Law of Thermodynamics also prevents perpetual motion machines from becoming reality. Part of the Second Law states that heat energy inevitably seeks out cold areas, eventually creating a neutral temperature state called entropy. This means the car will eventually conk out from a lack of usable heat energy. The motor casing gets hot during work, and some of that heat energy would be dissipated into the air, not back into the battery system. Since external factors such as gravity and friction would be constantly pulling on the machine, eventually all of the usable energy would be lost.
Perpetual motion machines would only be possible if a substance could be found that generated more energy than it used. Some inventors hoped that radioactive materials would prove to be useful in this way, but their energy is still considered finite. Magnets have also been used to power would-be perpetual motion machines, but their continued operation often requires some external energy source. Gravity is usually considered a force hostile to perpetual motion, but some inventors use gravity to their advantage when creating theoretical machines.
Because scientific laws and theories generally deem perpetual motion impossible, patent offices are extremely reluctant to grant patents for such machines. Proposed machines are the only devices that require a working model at the time of patent application. To date, no inventor has successfully submitted a working model of such a machine.