# Homework Help: Rotating coil

1. May 9, 2010

### trix312

1. The problem statement, all variables and given/known data
A flat square coil of N turrns and area A is rotated at an angular velocity w in a uniform magnetic field of strength B. The rotation axis is perpendicular to the magnetic field direction. The coil is connected to a Galvonometer using suitable slip ring connectors and the total resistance in the coil and meter is R. Show that the detected current is I = -NBAcos(wt) / R

2. Relevant equations

Fardays 's law of induction *N
I = V/R

3. The attempt at a solution
I get I = NBAwsin(wt) / R
I dont see why i am wrong. The B field and area is uniform. So I only differentiate cos(wt) and divide the emf by R to obtain the current.

Any help or suggestions would be very welcome.

2. May 9, 2010

### phyzguy

Both answers can be right, depending on the orientation of the coil at time t=0. Since this isn't given in the problem, all you can say is that I ~ Cos[w(t+t0)]. If t0 = 0, you get a Cos solution, if t0=-pi/(2w), you get a Sin solution, and if t0 is something else, you get something in between.

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