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Re: [femm] [Faraday's law ?]

Can I add some comments?

At 21:57 21/07/01 +0000, you wrote:

To Dear Everybody:

How are you? it seems like that I always ask something not contributing
anying. Feeling guilty, I would like to ask
onemore and promise as time goes, I will contribute my knowledge
to this very useful group.

While I try to model crosstalk thing (and waiting Cornelius tutorial,I
requested tutorial to him in person thinking to send e-mail to group is not
good), I would like to understand more basics.

Let's say there is one thin conductor whose current is time-varying and
there is conductor loop whose conductivity is high (i.e good conductor),
then Faraday's law says there is induced emf and current in the passive
loop. Current is time-varying (let's say as always sinusoid), to get the emf
voltage, we need to calculate d(flux)/dt. Therefore in the expression there
is always frequency term. HERE IS MY QUESTION. Does it mean induced
(magnitude of) current is linear function of source frequency ? I think
answer is NO because if it is true, the induced current can be higher than
source current. If somebody knows anything, I would like to here any
explanation. Books, papers Ok, too.

You have to be careful when talking about "induced current". Faraday's law says nothing about current it only says that any conductor located in a changing magnetic field will have induced in it an emf which is equal to the rate-of-change of flux-linkage (flux-linkage note not flux - it's effectively the total flux linking the conductor). The induced emf can cause current to flow but the current is not induced it is as a result of induced emf. Pedantic I know but an important distinction.

If a current is allowed to flow in the second conductor then Lenz's law implies that the effects of the current will tend to oppose the original cause of it. In effect this means that the mmf of the second conductor opposes the first. The effect of the current flow in the second conductor depends on how the first conductor is supplied. At constant voltage (like in a transformer) the current in the first conductor will increase to compensate and try to keep the flux constant. At constant current the flux and the induced emf will fall (I think).

Anyway, to the point, the induced emf is a linear function of frequency but the current depends on other things and is affected by frequency in a more complicated manner.

Keith.


Keith Gregory
Department of Electronic & Electrical Engineering
Loughborough University
Phone: 01509 227025 Fax: 01509 227014
Department web: http://www.lboro.ac.uk/departments/el/