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Physics Question - Magnetic fields on a rotating disc

* Copied as the Physics forums seems to be a bit dead, mods please delete the Physics forum version if this gets replies, thankies :smile: *

Hey all

I have a question which reads;
"A plastic disc of radius R has a positive charge Q distributed uniformly over its surface, and rotates an angular frequency omega about an acis through its centre and perpendicular to its plane. Show that the magnetic field at the center of the disc is mu0 omega Q/(2 pi R).

I know that for a loop of current I with radius r,
B = mu0 . I / 2r
but obviously we have a disc. However, a disc can be thought of as the sum of many concentric loops (all rotating at the same frequency but not the same speed so not the same current).

So we need to think of the differential current in one loop and then use the
eqution above to give the differential magnetic field off one loop and
then integrate. However, we are struggling to set up a differential equation based on current. Presumably for the integrating step we need to integrate between 0 and R...

Hope someone can help please!! Charsss...

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