# Am I missing something obvious here? - EM induction

## Homework Statement

A square coil of wire has sides of length 10mm, consists of 2000 turns and has a resistance of 250$$\omega$$. The coil is placed in a magnetic field such that the normal to the plane of the coil is parallel to the direction of the field. When the coil is rotated through 180 degrees and total charge of 120[tex]\mu[/texC flows through the coil. What is the magnitude of the magnetic field? If the rotation of the coil takes 0.2s what is the maximum voltage induced? (assume the coil is rotated at a constant rate)?

## Homework Equations

Not sure - if I knew them I could probably do it!

## The Attempt at a Solution

I'm not sure - most of the equations for this kind of situation don't seem to make sense with the variables given. For the first part I think I could probably get an answer if I had the current flowing, but I don't. I know I=Q/t, but again in the first part I don't have the time, so that approach is out.
I've just had another look, and I still can't see the answer, or how to approach it.Any hints?

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andrevdh
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http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html" [Broken]

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I'm sound on the second part of the question, the voltage produced, its the first part that I'm most stuck on and that doesn't seem to help me. Or not that I can see at all.

andrevdh
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http://www.saburchill.com/physics/chapters/0056.html" [Broken]

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andrevdh
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http://www.saburchill.com/physics/chap02.html" [Broken]

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I get how to calculate the flux, but I'm not given that in the question (or at least I don't think I can calculate it with what I'm given). I understand that I could calculate the B field with that, but as I said, I'm not given it am I?

Ok I think its just come to me:

emf = -NBA/t = IR = QR/t

I have all those variables, so that should be able to be solved to find B, yes?

andrevdh
Homework Helper
It is the rate of change of the magnetic flux that induces an emf in the coils. In this particular case the change is produced by changing the exposed area to the constant magnetic field.

I would think that these concepts would take quite some time to convey to students. So you must have missed quite a few lectures?

I did miss a couple, but I was in the lectures for Magnetic Inductance. I understand what you're getting at there, I just didn't think that I had the variables given to me to calcuate the flux, and hence the magnetic field.

andrevdh
Homework Helper
Since we agree that the magnetic field remains constant what then causes the induced emf in the coils?

Yes, you can calculate the induced emf (which causes the current to flow since the coils now acts as an electric power supply) with the current and resistance. The problem suggests that the end of the coils are tied together so the only resistance in the circuit is that of the coils themselves.