Re: [Phys-l] entropy and electric motors

[Brian Whatcott <betwys1@sbcglobal.net>]

At 03:45 PM 5/22/2008,  Kyle, you wrote:

> Does anyone know (or can point me to an explanation) of how the 2nd law
> applies to electric motors and generators?
>
> I think I understand how the 2nd law applies to heat engines and I have
> an elementary understanding of entropy as it relates to chemical
> reactions. These applications explain why heat engines and fuel cell
> efficiencies cannot be 100%. I assume the 2nd law applies somehow to
> motors and generators but how do you find the theoretical limit for
> them? (I know this is quite high since electric motors with 95%
> efficiency are already available.)
>
> kyle


When I stood in the steam engine hall of the Science Museum a week or two ago,
admiring James Watt's efforts, it was salutary that his great improvements
in thermal efficiency resulted in monsters of several tons weight providing
a few HP.
This energy conversion is in stark contrast to the energy transfers involved
 in a tuned circuit which can provide quality factors in the thousands
 (you will also recall that the timing elements of clocks to which 
bc recently alluded
could also reach a Q > 1000 with considerable design effort).

The electric motor is somewhat comparable to the tuned circuit, in 
that the motor too
is in the business of converting electrical to magnetic forces.  Like 
the tuned circuit
and the pendulum, the motor can have very small limiting losses  - in 
this case,
the bearings and the air drag involved.
 The tuned circuit is distinguished by the capability of processing 
exciting signals
 smaller than the amplitude of the ambient thermal electrical noise in fact.


Brian Whatcott    Altus OK    Eureka!