Free Energy

[crgb14]

Some years ago whilst sat in a physics lesson I was told 'you can't make energy from energy'.

My reply was (as I always have an answer for everything), what if you have an electrically powered car and attached 4 dynos to it (one on each wheel) which then recharged the battery and hence....you get energy from energy. Why wouldn't this work?? You'd never have to pay to charge the battery and as long as you use the car it'll just keep on charging.

I'm going to patent this in a second!!

[Loosehead]

What is meant is that no system is 100% efficient so putting in energy E in whatever form will always produce less than E in output in most cases considerably less. That is why purpetual motion is impossible as the initial energy that is input is used up in the workings of the system that uses it. To take your example your electric car presumably starts with charged batteries and the dynamo's on each wheel put some energy back into the batteries viola perpetual motion! Doh! no I'm afraid that some of the initial energy is used in driving the wheels which do indeed put some back but the energy to move is taken out as it goes, result? Smaller amounts at a negatively exponential rate. Your car stops quickly! In fact you'd get a lot further without the dynamos as the batteries would have less weight to push a 4 less things to turn. Back to the drawing board with you!

[crgb14]

And there's me thinking I'd become an instant millionaire.

The world shall just have to wait for my next big breakthrough.

[Peter Pedant]

You basically always lose some as heat. - 2nd Law.

Abers will confirm or deny that this is purely observational - and isnt proved from first principles.

[Loosehead]

Yes Peter, I think you are correct, perpetual motion is not prevented by any equation or theory but all observed or indeed predicted outputs point to it's impossibilty. Essentially if a device could be contstructed that re used all the expelled heat then it might be do-able.

[mibn2cweus]

Theoretically some energy could be returned to this system when the vehicle is going down hill but we need to realize that a generator of electrical power resists turning forces in proportion to the amount of power it generates. Even if you could make a perfectly efficient generator the power output will never exceed the input power.

[OBonio]

We were asked to come up with a perpetual motion machine at school once. I came up with the exact same idea. The teacher said it wouldn't work due to friction. I knew that, but it was the best I could come up with. Why set such a stupid homework assignment?

[MrPahoehoe]

Seriously? Thats brilliant OBonio! What age was this? I can just imagine your report card: "Failed physics as he was unable to circumvent the laws of physics" ;-)