# Magnitude and Direction of a Magnetic Field at the Circumference of a Disk

1. May 2, 2012

### GermanMC

1. The problem statement, all variables and given/known data

A very thin disk of non-conducting material initially holds a charge QO = +5 μC that decreases with time t as

Q(t) = QO e-t/τ
where τ = 10 seconds.
If the disk has a radius of 0.10 m, what is the magnitude and direction of the magnetic field at the circumference of the disk as a function of time?

2. Relevant equations
Assume at any time t that the displacement vector D is uniform across the disk.
There is no free current.

∇$\bullet$E = (1/εO

∇×B = μOJ + μOεO (∂E/∂t)
3. The attempt at a solution

I tried to just work out the curl of B and I immediately ran into problems and it's not correlating to the problem.

Last edited: May 2, 2012
2. May 2, 2012

### vela

Staff Emeritus
Show your work. Don't just describe what you did generally.