# Magnetic and Electric flux

1. Oct 4, 2009

### epilot

Hello guys,

just have a question.
are these definitions true for the magnetic flux and electric flux??:
Magnetic flux: The number of magnetic filed lines which are passing through a CLOSED surface.

Electric flux: The number of electric field lines which are passing through a CLOSED surface.

Thanks.

2. Oct 4, 2009

### Staff: Mentor

In general, when calculating the flux of a field, the surface can be either open or closed.

However, in some applications, most notably Gauss's Law, you specifically need to use a closed surface.

3. Oct 8, 2009

### epilot

Thanks,

So when we want to define those ones then what do we tell??

4. Oct 9, 2009

### Born2bwire

A closed surface is a surface that fully encloses a volume. More specifically, of the two normals that the surface has, one will always be inside a fully enclosed volume while the other will always point into the open space.

5. Oct 9, 2009

### pseudophonist

Flux can be calculated for either. You can calculate the electric flux through either an open or closed surface, but the closed surface is usually more useful since it is related to the charge enclosed by Gauss's Law.

Conversely, magnetic flux is more often calculated for open surfaces since (ignoring the possibility of magnetic monopoles) the magnetic flux through a closed surface is zero. The rate of change of flux through an open surface however gives you the induced potential.