Re: [Phys-l] Question about Electromagnetic Induction

[Brian Whatcott <betwys1@sbcglobal.net>]

Fakhruddin, Hasan wrote:
> Hey folks,
>
> The problem I posed looks innocent but is not a physical possibility.  Magnetic field lines form closed loops.  Hence, the space between the two circles cannot be devoid of B field. 

The fact that stray field exists is not on account of the closed 
circuit nature of magnetic flux, so much as the limited confinement 
which the magnetic material usually used in magnetic circuits offers.
The comparable electric circuit  usually exhibits very low stray 
electric current on account of the
high resistance of a typical insulator such as air.
> In fact, the field through this space should be directed opposite to the one within the inner circle. Thus, larger the radius 'a' LESSER is the NET flux through it and correspondingly lesser the dB/dt. Thus induced emf would decrease as 'a' increases.  
>   
I don't follow this  sequence - depending on the arrangement of the two 
circuits I conceive the magnetic
flux may increase or decrease.
> It is inconceivable to think that emf would stay constant even if 'a' increases indefinitely!
>
> Yesterday, I used a length of wire. Voltmeter, and the 'jumping ring' apparatus and verified that the induced emf indeed decreased as 'a' increased.  Thanks,
>
> ~Hasan Fakhruddin
In a flux though free air, I can easily see that the flux coupled into a 
conductive ring would decrease.
If one chose instead a complete high permeability circuit, I believe I 
could demonstrate a contrasting result.

Brian W