Re: [Phys-l] induced emf and induced current

["LaMontagne, Bob" <RLAMONT@providence.edu>]

Sorry, my last email must have been sent when I lost my wireless connection. Only the first sentence survived.

The voltage measured by the volmeter for a spinning metal disc in a magnetic field is the result of a real electric field set up by induced static charges. The electrons in the spinning disk will migrate to the center or the edges of the disk depending on the set up. This is a result of simple v cross B forces. As the resulting positive and negative charges build up at the center and edges an electrostatic field will build up. Eventually, this field becomes so large it overcomes the v cross B force and there is no further flow of charge. The static electric field from the separated charges produces a voltage (not an emf in this case) that can be measured by a meter.
I think this is a classic case of talking past each other - and I apologize if I am doing that. We probably all learned different (or no) definitions of emf so we interpret it differently. Voltage needs static fields to have meaning, emf is not so rigorous.
Bob at PC

________________________________________
From: phys-l-bounces@carnot.physics.buffalo.edu [phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of carmelo@pacific.net.sg [carmelo@pacific.net.sg]
Sent: Tuesday, October 13, 2009 1:51 AM
To: phys-l@carnot.physics.buffalo.edu
Subject: Re: [Phys-l] induced emf and induced current

Quoting "LaMontagne, Bob" <RLAMONT@providence.edu>:
> I'm not sure what is meant by "the induced emf could be considered
> as towards the centre". The induced emf, as I understand it, is the
> result of integrating the induced electric field around a closed
> path encompassing a changing magnetic field. It doesn't really exist
>  from point to point because magnetic flux cannot be defined
> relative  to an open path.

The induced emf for this spinning disc is B(pi)r^2 (frequency). There
is a voltage difference between the rim and the centre of the disc.

> I'm not sure the differential approach helps either, i.e., curl E =
> -dB/dt, because the integral of the curl from point to point along
> an open path doesn't have a physical meaning (that I am aware of).

For open path, there can be induced emf or potential difference. The
physical meaning could be "Hall voltage", and the measured voltage may
depend on the orientation of your voltmeter.


Best regards,
Alphonsus

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