Perpetual Motion Machine
Imagine a device that uses an electric motor to turn a wheel. This wheel in turn runs an electric generator to generate the electricity. The electricity can be used to power a house as well as the electric motor turning the wheel. Once this device is set in motion, it will continue to run forever because it supplies its own electricity as well as creates enough electricity to run a house. No more electric bills!
Because this device, once set in motion, would never stop it is called a perpetual motion machine. Many would be inventors have dreamed of building a perpetual motion machine, but none have succeeded.
Enough of these devices could run a city or the entire world. The inventor of such a device could solve all the world's energy problems. Selling this device would make its inventor rich enough to laugh at Bill Gates' poverty.
Sound to good to be true? It is!
First Law of Thermodynamics
The first law of thermodynamics is the law of conservation of energy applied to heat engines. It states that the work output from an engine cannot exceed the energy input.
The perpetual motion machine described above violates the first law of thermodynamics. The generator portion generates enough electricity to run other devices as well as power the generator. Hence once this perpetual motion machine is set in motion, it produces useful work without any energy input. Free work out with no energy in violates the first law of thermodynamics. Energy is being created from nothing.
Perpetual motion machines like the one described above are perpetual motion machines of the first kind. They violate the first law of thermodynamics.
Perpetual Motion Machine of the Second Kind
The first law forbids perpetual motion machines that create extra energy, but imagine disconnecting the portion of the machine that powers the house. The motor powers a generator which supplies the electricity needed to run the motor. Such a machine would not supply free energy, but once set in motion it would still continue to run forever.
The inventor of such a machine would not make untold riches by solving the world's energy problems. It might however be possible to make a living selling them as novelty devices.
This second kind of perpetual motion machine does not violate the first law of thermodynamics. Is it possible?
Second Law of Thermodynamics
The second law of thermodynamics says that an engine or process of any type must always have an efficiency of less than 100%. A perpetual motion machine that uses a generator to power the motor that runs the generator requires both the generator and motor to operate with 100% efficiency. This type of perpetual motion machine does not violate the first law of thermodynamics, but violates the second law of thermodynamics. It is a perpetual motion machine of the second kind because it violates the second law of thermodynamics.
Not even the cleverest engineer or inventor can build a perpetual motion machine because it would violate either the first or second law of thermodynamics, which are fundamental laws of physics.
Further Reading
Hecht, E., Physics: Algebra/Trig, Brooks/Cole, 1997.
Paul A. Heckert - I have a Ph.D. in astrophysics, over 30 years experience teaching physics and astronomy, and over 60 published research articles.
