EMF in loop, given magnetic field

[RKOwens4]

Homework Statement

In the figure below, a circular loop of wire 12 cm in diameter (seen edge on) is placed with its normal at an angle θ = 30° with the direction of a uniform magnetic field B of magnitude 0.60 T. The loop is then rotated so that N rotates in a cone about the field direction at the rate 160 rev/min; angle θ remains unchanged during the process. What is the emf induced in the loop?

Figure: http://imageshack.us/photo/my-images/854/webassign.jpg/

Homework Equations

EMF = BAcos(theta)

or

EMF = d(BAcos(theta))/dt

The Attempt at a Solution

I converted 160 rev/min to rev/sec and then to rad/sec, getting 16.755 rad/sec. I then plugged the given data into the 2nd equation above (trying both positive and negative signs) and it's still incorrect. Any help would be appreciated.

 

[Delphi51]

Interesting. The EMF is proportional to the change in flux through the loop. Is there a change? You have the flux as BAcos(theta), which looks correct to me. The B is constant. The A is constant. So is theta. Looks like the flux is constant, so its derivative will be zero.

 

[RKOwens4]

I'm confused. Are you saying the EMF is zero? I only have one guess left before the problem is marked wrong.

 

[RKOwens4]

Anyone have any ideas?

 

[Delphi51]

Yes, it looks like EMF = 0 to me. There are some B field lines cutting through the wire, but it looks like an equal number cutting in and out, so that approach also yields zero for the EMF.

 

[RKOwens4]

That was the right answer, but could you please explain the reasoning again? Is it that magnetic flux has to be increasing/decreasing to create an EMF, and if the flux is constant then EMF is zero? Because I had another homework problem which I already solved that said, "An electric generator contains a coil of 110 turns of wire formed into a rectangular loop 60.0 cm by 30.0 cm, placed entirely in a uniform magnetic field with magnitude B = 3.50 T and with B initially perpendicular to the coil's plane. What is the maximum value of the emf produced when the coil is spun at 1000 rev/min about an axis perpendicular to B?" For this one I just used the formula EMF=NABomega (where omega = 104.7) and got the correct answer of 7,257 V. Can you explain why the two problems are different?

 

[Antiphon]

In one problem the net flux linking the loop changes with time. In the other it doesn't.