Re: Why has no perpetual motion machine been invented?

From: Richard Bell (rlbell_at_csclub.uwaterloo.ca)
Date: 09/29/04 

Date: Wed, 29 Sep 2004 16:21:11 +0000 (UTC)

In article <2ru39oF1emnr5U1@uni-berlin.de>,
ivan <ivan'H'older@yahoo.co.uk> wrote:
>
>
>I must confess that I find it all very tantalising that modern permanent
>magnets can attract and repel with such enormous force, and yet can't be
>incorporated into a motor that continues to revolve until the magnetism
>wears out!
>
The problem here is that electromagnetism is force that is path invariant and
conservative, it also follows the inverse square law. What this means, among
other things, is that if I set up some unchanging fields (say, a bunch of
permanent magnets) in a device that rotates, the net work done to spin it one
complete revolution is exactly zero (plus friction losses).

Permanent magnets can repel each other with enormous force, but it takes an
equally enormous force to bring them together. Alternatively, the permanent
magnets will attract each other with enormous force, but rotating one of them
so that they will repel each other is a lot of work.

All DC motors use electric current and commutator segments to change the
magnetic field of the armature so that it is always close to 90 degrees off
of the stator's magnetic field. It takes work to force the electric current
to do that, but an amazing portion of that work becomes mechanical output.

The real problem with perpetual motion is that, outside of peculiar quantum
effects near black holes and between parallel flat plates, all of the
measurable forces in nature are either conserve energy, or dissipate it
energy as heat. Even black holes suffer for producing something from nothing,
as they evaporate.