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Suppose I have a electronic car. It runs by electrical energy. So when wheels rotate, mechanical energy is produced. Then why can't we convert it back to electric energy and produce somewhat unlimited energy?

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Your question is similar to the common question of putting a wind turbine on an electric car to generate electricity while driving and provide power, and is based on the same conceptual misunderstanding. To understand why it is not possible you should read about why perpetual motion machines are not practically possible.

Specifically relating to your question, the short answer is "because then there would be no power left to push the car". The principle of conservation of energy implies that you can only take out what you put in, or you'll be draining the car of energy. That's the first law of thermodynamics.

An ideal generator could, at best, produce electrical energy at the rate it takes kinetic energy from the car. In reality it would lose some energy in the process.

But in order to keep the car moving, the motor would have to push the car harder to counteract this loss. Even if the generator was 'ideal' there would still be friction losses, mechanical losses, aerodynamic drag, and a number of other losses that would have to be counteracted.

Regenerative brakes do exist, and are designed to convert some of the car's kinetic energy to electrical energy during braking, but are limited by efficiency, and by the batteries' maximum rate of charge.

It might also be worth reading about the second law of thermodynamics, which states that entropy always increases in a closed system, so you can't exchange energy indefinitely in a closed system without eventually needing to dissipate some, to keep the energy 'useful'.

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In an electronic car, as you say, or any other device that uses electrical energy to produce some mechanical work, [the term "uses" means transforms one kind of energy-one kind of microscopical interactions(here electromagnetic)- to another], what happens? You have an electrical energy source that, through a mechanism, gives you mechanical work, so the car starts moving. This work is the mechanical energy the car has for moving, it is expressed as a kinetic energy. If you wanted that energy to become electrical again, assuming you have a mechanism that does that, then the car would lose kinetic energy via a transformation to electrical. So the car in the end wouldn't move.

Also, any transformation of energy has a waste heat, a part of energy emitted to the environment(as radiation ). Add also the waste energy from friction(a part of the mechanical energy that moves the car is lost doe to friction of the car with the road- friction is an electrostatic interaction ). This heat has not sufficient energy to produce new work, so it's, up to this date, lost to the environment.

I hope this answer helps you with your question.

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