# Magnetic flux on a surface

Along a wire (of radius r) a current i is crossed from one.
Calculate the magnetic flux through the surface S, inner to the wire.

http://img138.imageshack.us/my.php?image=cilindrodb4.jpg

I think

$$\phi_b= \int B dS= BS$$

$$B=\mu_0 i/2\pi r$$

How to calculate S if I have not the length of wire?

The solution of the problem is
$$\phi_b=\mu_0 i /4\pi$$

Last edited:

Related Advanced Physics Homework Help News on Phys.org
Astronuc
Staff Emeritus
If this is a wire 'loop' of radius r, then the length is simply 2$\pi$r, and the area circumscribed by a circle is given by A = $\pi$r2.