# Strange Question

Discussion in 'Homework Help' started by Robin Mitchell, May 15, 2011.

1. ### Robin Mitchell Thread Starter Well-Known Member

Oct 25, 2009
738
206
HI everyone. I have tried all ideas to get the answer but with no help. The answer is 34 micro volts but i dont understand where it comes from:

Question:

A metal framed window is 1.3m high and 0.7 m wide. It pivots about a vertical edge and faces due south.

(Horizontal component of earths field is 20 micro tesla, vertical is 50 micro tesla)

Calculate the magnetic flux through the closed window (not a problem, 18Wb)

The window is opened through an angle of 90 degrees in a time of 0.8s

calculate the emf induced

Before you ask this is NOT homework but a question that i am having loads of problems with. I tried thinking of it as a volumetric shape (a quater of a cylinder). I tried thinking about emf's canceling out on the shape etc....

Thanks

2. ### steveb Senior Member

Jul 3, 2008
2,433
469
Simply think about the projection of the area of the window onto a plane that is perpendicular to the horizontal component of the field. At what rate does this area change? Since emf is the (negative) rate of change of flux and the field is constant, the flux only changes because the projected area is changing.

By this way, this isn't such a strange question. The context (with a window turning) is a little strange, but the problem of a spinning loop is the classic motor calculation.

Last edited: May 15, 2011