Magnitude of Magnetic Field

1. Jan 26, 2005

wisper

I understand that it needs to relate to the following equation but that is it. $$B=\frac{\mu_{0} I}{2\Pi r}$$

I am unsure how to derive an answer for this, I am drawing a blank on solving this. Can anyone give me any helpful hints?

Last edited: Jan 26, 2005
2. Jan 26, 2005

vincentchan

what is the direction of the current???? post the whole question pls....

3. Jan 26, 2005

wisper

That is the full question, the current is running through a hollow cylinder. From how I understand the current is flowing through direction of the hieght of the cylinder.

4. Jan 26, 2005

vincentchan

find the $\ver{A}$ first, then B

5. Jan 26, 2005

clive

$$\oint{\vec{B}\vec{dl}}=\mu_0\int{\int_S{j}}ds$$
and from this you can simply derive

$$B(r)2\pi r=\mu_0 j \cdot \pi(r^2-a^2)$$

where
$$j=\frac{I}{\pi (b^2-a^2)}$$.

6. Jan 26, 2005

wisper

Thanks to both of you. Clive you pointed out a part in which I made an error in my thinking of my previous answer. Appreciate the help.