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Old May 2nd 2009, 03:21 PM   #1
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Question the effect of change in area of a wire loop...?

A circular loop in the plane of the paper lies in a .75 T magnetic field pointing into the paper. If the loops diameter changes from 20.0 cm to 6.0 cm in .50 s, (a) what is the direction of the induced current? (b) what is the magnitude of the average induced emf? (c) if the coil resistence is 2.5 ohms, what is the average induced current?


I though that a changing magnetic field, B, induced current...how does a changing area affect anything except flux..?
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Old May 2nd 2009, 07:52 PM   #2
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Faraday's Law of electromagnetic induction says that change in magnetic flux causes induced emf and the magnitude of emf is given by change in flux/time.
This flux f is defined as-

f = B.A Where B is the magnetic field intensity and A is the area. If the angle between B and A be @ then

f = BA cos@

So if any one out of B, A and @ changes, emf will be induced. Note, if A changes we call it motional emf. Change of @ is used in generators where a coil is rotated in the magnetic field.

Let diameter changes from d to d' in time t then magnitude of

induced emf = change in flux divided by time
= {II(d/2)^2*B - II(d'/2)^2*B}/t
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