Magnetic Permeability ... I can't find anyone talking about this !!!!!

[The Observer]

Hello,

Been Busy + my transmission just went out.

Anyways,  back EMF is darn interesting.

I am with Charlie that I would like to know exactly how this works and what is going on.
I need to know what the waves are doing.

I did find an experiment that could shed some light on this.

       http://www.physics.brown.edu/physics/demopages/demo/em/demo/5j1023.htm

The peak of the back emf is always greater than the applied voltage.
By the way, the very nature of this guy's experiment shows back emf can be harnessed.

Be Well,
             The Observer

[fritz]

OK.

You have an ideal current source with 1mA. (at this point you should
now what an ideal current source is).
Then you connect a capacitor with capacity C in Farad.
What happens ?
The voltage rises
Well, if you reached 100Volts you disconnect.
Now you have W= 1/2 C*U^2 energy in the cap.
Fine, the voltage on the cap is still 100V.
What happens if you short-circuit the cap ?
You got a pretty(relatively) infinite current pulse in the opposite
direction. In practic you got propably 20 Amps current pulse in the
opposite direction.

OK.
Now we take a coil with inductivity L.
We connect the coil to an ideal voltage source with 10 Volts.
What happens ?
The current rises.
Well, now we short circuit the coil if it reached 1 Amps.
What happens ? - the current goes down slowly.
(on a superconducting coil - the current would stay at 1 Amp)
The energy in the coil at this point in time is W = 1/2 *L*I^2.
If you open now the current loop - you got a very high and
negative voltage - lets say 200V, depending on material.

What does this all mean ?

Capacitors and inductors can store energy.
The cap stores the energy if left open, the coil stores
the energy if shortened.
The capacitor can release this energy in a negative current pulse
with current multiple of the charging current. (theoretical infinite)
The inductor can release this energy in a negative voltage pulse
with voltage multiple of the charging voltage (theoretical infinite).

If it comes to DC and you have no superconducting coil - the coil
is a lousy energy storage (because of wire resistance) - but there
are MEMS - superconducting storage with powerplants on coil
basis - so its no theory.

Thats ___________very_______________ basic.

Better we avoid "Back EMF" because everbody means something
different.
In the posts before I meant the inner voltage of a dc motor caused
by the inerta of its rotating mass - which can act as a generator -
the stuff mentioned here is something completly different - but also
effects the dc motor on commutation - you turn off the current in the
coils - so you have some energy to vanish ... (....)

[Charlie_V]

I don't think you need super conductors to make coils store more energy, you just need bigger coils with thicker gauge wires - of course you won't ever be able to match the field strength of a super conductor.  Besides, the energy storage in a coil is around 80-99 percent, not too shabby if you ask me.  Plus, if you start factoring in all the support equipment required to maintain 30 degrees Kelvin, the superconducting coil is probably worse than a regular coil. 

I'm not sure how else back-EMF is defined other than the way I defined it earlier.  Its the coil producing a magnetic field that opposes the applied field.  In a motor it causes the current in the coil to go to zero, by inducing a current in the coil that is in the opposite direction from the applied source current.  When the motor is loaded, the rotation slows down (known as slip).  This causes the back-emf to no longer cancel as much of the source current as it did at no load, and the source current is allowed to flow through the coil (consuming power from the source).  In generators it tries to stop the shaft rotation (which is almost always some sort of rotor with either magnets or electromagnets). 

What are some other ways of describing back-emf? 

[Charlie_V]

Besides, the energy storage in a coil is around 80-99 percent, not too shabby if you ask me.

By this I mean the coil's efficiency at storing energy.  It can be really good for properly designed coils.  Of course this value can be a lot lower too, depending on how the coil is made.

[fritz]

A capacitor can maintain a charge for hours,
a "loaded", then shortened coil for seconds.
Because of the resistance - the current will
always decay (if not zero).

so what ?

If you measure after an hour - the capacitor has 99% charge,
the inductor 0% charge - efficency -infinite.

If I measure after 1ms, I got 100%:99.9%; depends on.

I