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Ok, so, I was in my physics lesson yesterday and we're doing about magnetic flux. I know that you get a p.d. whenever you move a conductor in a magnetic field.

What I'm wondering about is - if you have a perfect band/circle and you spin it around its own axis (so that whenever you look at it, it is exactly the same in all directions), would that create a p.d.?

I'm confused because on the one hand you are moving it, which would suggest yes, but conversely it's exactly the same whenever it's turned round, so you aren't really changing anything.

Re: Change in magnetic flux -
May 27th 2009, 12:54 PM

I learned magnetic flux as the number of magnetic field lines per a certain area. To have a change in magnetic flux, you have to have a change in the number of magnetic field lines in the area. The equation is magnetic flux = B*A*cos(theta), where B is the magnetic field strength, A is the area of the loop, and theta is the angle of the loop to the magnetic field lines. So if you have a circle that's rotating about it's own axis (like how a hula-hoop is- imagine a "perfect" hula hoop going at a constant pace around your hips without moving up or down), I think there would be no change in magnetic flux.

(P.S.- if this is wrong, I'm so sorry- I just took the AP Physics B exam, but electricity and magnetism was the hardest part for me).

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Re: Change in magnetic flux -
May 30th 2009, 08:56 AM

Are you using conceptual- or calculus-based physics?

Since your disc is not a closed surface, a change in magnetic flux can cause a force, which by Faraday, can be reduced to a simple derivative of the flux through the surface. If the flux through the surface is zero or constant, the derivative is zero and therefore a force will not result. I assume you did not learn it this way since looking at it from a calculus standpoint, the magnetic flux is a surface integral of the magnetic field itself, and the resulting force is a path integral which reduces to the derivative of the surface integral.

One neat formula is by Gauss: If the surface integral is around a closed surface then it is equal to zero and there is no magnetic flux.