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A friend has been talking to me about using various electric generators in order to create a perpetuum mobile that can be used for various especially in cars. Of course I instantly said that it doesn't work and mentioned the following reasons that I think contradict his claim that it must work:

  • Assuming the generator is based on induction to generate electricity I said that most likely the conductors will inherently heat up which is a loss of power.
  • Furthermore the generated current will adhere to Lenz's law and I assume the magnetic field induced by the generated current might cause loss of power as well.
  • Ultimately the rotor in such a system will not be frictionless and cause heat as well.

Unfortunately though, my friend still insists on his idea and boldly mentioned the following things:

  • He claims that he has seen an electric generator that is being sold that an be plugged into the wall socket and that consumes $x$ kW and generates $y$ kW where $x<y$.
  • He thinks it must be possible somehow to use a fuel based generator in order to power an electric car since it takes $x$ hours to power the battery while the fuel based generator can power the battery in less time thus generating more current/power than inserted into it.

For the first bullet I am actually struggling to believe him since I don't see any reason why I would like to plug something into the socket in order to get current that I would have gotten without the generator in the first place.

Nonetheless, I am looking for futher proof why it must be impossible to have such a generator.

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    $\begingroup$ Your friend is crazy. $\endgroup$
    – Hot Licks
    Commented Feb 10, 2018 at 20:12
  • $\begingroup$ Think you mean perpetual motion rather than perpetuum mobile. $\endgroup$
    – MaxW
    Commented Feb 10, 2018 at 20:18
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    $\begingroup$ Ask him for a link to that generator, and post it here. Many of us will want to buy one. $\endgroup$
    – garyp
    Commented Feb 10, 2018 at 21:45
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    $\begingroup$ @Countto10. No of course I did not buy it!!!! Better I delete my comment or the little reputation I have here will be lost :))) Just saw it somewhere and I was surprised that people argue is real or not. $\endgroup$
    – Alchimista
    Commented Feb 10, 2018 at 22:26
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    $\begingroup$ @Countto10 Noooo! Then I will start to think on expansion etc etc. :))) $\endgroup$
    – Alchimista
    Commented Feb 10, 2018 at 22:40

2 Answers 2

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We can't prove that perpetual motion devices are impossible; physics doesn't even really do "impossible".

Rather, what physics says is that our current models don't predict a way to do this. Practically, the distinction here is that we can't prove that perpetual motion machines are impossible to someone who's skeptical of our current models.

He claims that he has seen an electric generator that is being sold that an be plugged into the wall socket and that consumes $x$ kW and generates $y$ kW where $x<y$.

If this were true, then your friend have the generator loop back to feed itself, then take the difference in energy, $\left(y-x\right)\mathrm{kW}$, as net production.

So, it seems like either:

  1. your friend doesn't actually believe what they're saying; or

  2. your friend should corner the market on electricity now!

You might press your friend to either follow through or else explain to you why they don't want infinite wealth.

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    $\begingroup$ Fully agree: physics is not scolastic, so experiment trumps theory. If somebody would demonstrate a perpetuum mobile then the theories would hv to be adjusted. Currently its just that in evert corner we are looking laws of thermo serm to hold. $\endgroup$
    – lalala
    Commented Feb 10, 2018 at 20:55
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Any perpetual motion machine necessarily requires the nonconservation of energy- that is, to keep it running, energy has to come out of nowhere at no cost. Despite vigorous searches, this is simply not observed to ever happen. So the fundamental issue becomes this: why is energy conserved in the first place?

By Noether's Theorem, energy is conserved because the laws of physics do not exhibit time dependence. If they did, it would be easy to build perpetual motion machines, which grabbed energy for free on a Wednesday afternoon when we broke the laws, and used it to propel the machine on Thursday morning when we turned the laws back on.

I do not know if your friend would find this line of reasoning to be dispositive and convincing, but this is the way the math works, and if he ever wants to correctly understand the way things work, he'll have to understand the math.

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