# Magnetic Flux through a solenoid

1. Apr 1, 2009

### Interesting

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

The magnetic field inside a cylindrical solenoid of area 4 cm^2 is 0.15 T along the axis of the solenoid. What is the magnetic flux through a disk of radius 3 cm placed perpendicular to the solenoid axis?

2. Relevant equations
$$\phi = \zeta B * dA$$

3. The attempt at a solution

Well the area is 4 cm^2 and B is 0.15, so 6E-5 Wb is the answer, but I'm a little confused here. I thought flux for a closed surface was always 0 (gauss' law for magnetism)? What's going on here?

2. Apr 1, 2009

### alphysicist

A closed surface means that it encloses a volume. A sphere, cube, etc. would be a closed surface, but a disk would not. If you constructed a cylinder with the disk in the problem at one end, and calculated the flux through the cylinder then the flux would be zero.