The dream of perpetual motion


Perpetual motion machines are, alas, no more than a beguiling fantasy, writes Alan Finkel.


Everlasting energy? Not quite. – Atmos

I have an Atmos clock in my office, built by the Swiss company Jaeger-LeCoultre. The impish side of me points out to visitors that I have not wound it for nearly 10 years, that I never will and that it has no hidden batteries. Yet my clock will run forever. Some of my perplexed visitors think they are looking at a perpetual motion machine. They are not, of course; the clue to how the clock operates lies in its name – Atmos.

I make my perpetual motion claims for entertainment value, but others who make preposterous claims have less innocent intent.

I recall many years ago the premier of Queensland singing the praises of a local inventor whose car ran on water. In a staged presentation for the television cameras the inventor poured a glass of water into the fuel tank but refused to allow the vehicle to be properly examined. This was a case of deliberate fraud, with the premier being the dupe.

In the case of perpetual motion, the machines are more often earnestly presented by someone who doesn’t understand the laws of thermodynamics.

When the industrial revolution in England was in its infancy, engineers and scientists hoped to create ideal machines that could convert the chemical energy of coal into mechanical output without any losses. Somewhat sadly, they came to realise that whenever heat was used to make steam to run a machine, some of the heat was permanently lost to the environment.

Eventually, in 1824, French scientist Sadi Carnot concluded that no device could ever convert 100% of the input energy into mechanical energy.

Many scientists extended Carnot’s work and generalised it to the second law of thermodynamics, which applies to all systems including the Universe as a whole.

In particular, the second law of thermodynamics says that energy lost as heat cannot be completely recovered in a useful form. Thus perpetual motion is impossible. The only thing perpetual about perpetual motion machines are the claims that they exist.

When I was working for an electric car company I received an email from a naïve inventor suggesting that we install a wind turbine on every car. He imagined that as the car drove along the highway the wind turbine would generate enough electricity to keep the battery perpetually charged.

Sounds great, but the car would have to provide extra power to push the wind turbine through the air. If the car motor ordinarily used 10 kilowatts to maintain a constant speed, it might use 18 kilowatts to push the wind turbine through the air at the same speed.

Of the extra eight kilowatts, around five would be irrecoverably lost as heat due to increased air friction leaving three kilowatts to be returned as electricity to the battery. The net result is it would cost an extra five kilowatts to attach your wind turbine – the exact opposite of what the inventor imagined.

A less innocent example was announced in a recent press release from a start-up company making a nano-flow battery. It stated that its batteries “are able to be recharged from the recovered energy of the moving car, meaning that future cars will not need to stop to be externally recharged, giving them the potential to reach unlimited mileage”.

Hang on. This statement claims that the battery will only need to be charged once, after which the car will run forever. That’s perpetual motion.

Perpetual motion is physically impossible because most of the energy used by the car is irreversibly lost as heat from air friction, friction in the moving parts, friction in the brakes and friction in the tyres. The company’s claims are pure fraud. The naïve and fradulent ideas described above should not be confused with regenerative braking, which can be used to recover kinetic energy, increasing the driving range by a few per cent.

In short, there is no such thing as a free ride.

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Alan Finkel is an electrical engineer, neuroscientist and Chief Scientist of Australia.