# Magnetic dipole moment help

1. Dec 25, 2006

### stunner5000pt

1. The problem statement, all variables and given/known data
A phonograph record of radius R carrying a unifrom surface charge sigma is rotating at a cosntant angular velocity omega. Find its magnetic dipole moment

2. Relevant equations
$$m = int I \bullet da$$

3. The attempt at a solution
Need to find the current first

well the sruface charge density is
$$K = \sigma \omega R$$

$$K = \frac{dI}{dl_{perp}}$$

where dl perp is the infinitesimal widthruning parallel to the flow of current
so here dl perp refers to circumference
$$I = \int K \bullet 2 \pi r dr$$??

is this even close to being right??
I fear i may have made some geomterical error..

Last edited: Dec 25, 2006
2. Dec 25, 2006

### stunner5000pt

ahh i got the answer using the equation

$$\vec{m} = \frac{1}{2} \int_{S} \vec{r}' \times \vec{K}(\vec{r'}) da'$$

$$da = r dr d\theta$$

thanks anyway!