electric motort

[csking]

why can't you use an electric motor to power a generator that in turn creates electricity to run the electric motor?

[factor-fiction]

How ill you start the motor?

[New Judge]

You can start it by rotating the motor or the generator by hand.

However, it will not work because the power produced by the generator will be insufficient to power the motor. This is because there is a loss due to friction of the working parts in the generator.

[ChuckFickens]

http://en.wikipedia.org/wiki/Perpetual_motion

Explains why things such as you have suggested are impossible

[rov1200]

Perpetual motion talks about closed systems as your question suggests. If you could use an external initial force then there is no reason why it should not happen. ie take wind, wave or nuclear power.

[Peter Pedant]

Because there is energy loss in every process - the lost energy is found as heat.

The people answering above seem all to be English graduates or economic historians who cant add.

The motor will require energy E and this will come out of the rotor as torque or turning energy T.
It doesnt matter wh units (always use MKS)
T is less than E. always. T could be equal to E but this only occurs in a reversible process and this isnt one.

T is used to turn a generator and will generate electrical energy Z
This generator is not a reversible process, and so z must be less than T, and the difference will heat the generator up.

and that's why you cant do it -because you always lose a bit of energy as heat. In fact in this it is quite a lot.
unless you add in energy (wind more electric or muclear etc see above)
and the amount of energy you have to add in would be the energy represented by the heating effect in the motor and generator. actually it is just a little bit more, as the addition in this way is not a reversible process.

This question is meant to introduce you to the principles of heat and its conversion to energy and is called thermodynamics. This discussion by the way leads almost directly to the second law.

[jake-the-peg]

Thats simply not true rov

In any closed or open system there is friction - it may be large or it may be small.

That friction causes energy to be lost and that has to be replaced to keep something moving.

In wave power that energy is replaced by the sun.

In nuclear power matter is "burnt" to provide the energy

[chakka35]

There were two basic snags in the classical 'perpetual motion machines'.
One was that they could not turn, self-powered, for ever, for the reasons already given.
The other is that they were meant to power other machines to do work! Where that energy came from allowing the pm machine to give it up while continuing to rotate was never explained.

[R1Geezer]

aah bless, the old purpetual motion question! go with he first law of thermo dynamics mate!