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



What about in a dc motor? In that case, the induced emf is a "back emf", causing a drop in the existing current rather than a current of its own. So even if agree on what we mean by the direction of the induced emf and the direction of the current, would the DC motor be a counterexample?
________________________________________
From: phys-l-bounces@carnot.physics.buffalo.edu [phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of LaMontagne, Bob [RLAMONT@providence.edu]
Sent: Friday, October 09, 2009 5:01 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] induced emf and induced current

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.

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).

Notice that the AP objectives consider a loop. Even with a scalar current, can it not be considered clockwise or counterclockwise around the loop just as the emf is?

Bob at PC


-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of carmelo@pacific.net.sg
Sent: Friday, October 09, 2009 6:12 AM
To: phys-l@carnot.physics.buffalo.edu
Subject: Re: [Phys-l] induced emf and induced current

Hi Bob,

In AP Physics's objectives, for example, the students need to
calculate the magnitude and direction of the induced emf and current
in a loop of wire or a conducting bar. Although the current is usually
considered a scalar, eddy current (or Foucault current) can be
clockwise or anticlockwise which is circular in nature. There could be
alternative conception such as induced emf is in the same direction as
current for eddy current?

On a side note, for Faraday's disc or homopolar generator, the induced
emf could be considered as towards the centre, and similarly for the
induced current? Or some may think that the induced current spiral
into the centre? Some may also teach that induced emf may not
necessarily have induced current depending on the resistance of the
"circuit". There could be many alternative conceptions? Just curious...


Alphonsus

Quoting "LaMontagne, Bob" <RLAMONT@providence.edu>:

Do you mean induced field? EMF is a scalar.

Bob at PC

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu
[mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of
carmelo@pacific.net.sg
Sent: Tuesday, October 06, 2009 11:42 PM
To: phys-l@carnot.physics.buffalo.edu
Subject: [Phys-l] induced emf and induced current

Hi everyone,

Does induced emf *always* have the same direction as the induced current?


Best regards,
Alphonsus

_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l
_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l




_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l
_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l