# Magnetic flux formula confusion

1. Jul 16, 2008

### sameeralord

Hello everyone,

In the magnetic flux formula in our textbook it says Ø = BAcosø and somewhere else I saw it as Ø=BAsinø. I don't know which one to use. If F=BIL sinø
why does magnetic flux have cos in it formula. If something is perpendicular don't we have to use sine. Any help would be apppreciated. Thanks

Last edited: Jul 16, 2008
2. Jul 16, 2008

### tuananh3ap

formula $$$$\phi$$ = BAcos$$\alpha$$$$ is magnetic flux
it like $$$$\phi$$=$$\oint$$EdA$$
formula $$F=ILBsin\alpha$$ was deduced from F=qvB or F=qvbsin$$\alpha$$

Last edited: Jul 16, 2008
3. Jul 16, 2008

### Domenicaccio

Those 2 formulas might both be fine, it just depends on how is the angle ø defined.

They are however just special cases, when the area considered is flat and B is constant.

The real definition of magnetic flux, which is valid for every surface and in every case, is:

$$\Phi = \int_A B_n dA$$

which means that you have to sum (integral) the contributes of the perpendicular (normal) component of B in every point of the surface A.

When B is constant everywhere (does not vary in different points of the surface) and A is flat, the integral goes away and the normal component of B is Bcosø if ø is defined as the angle between B and an axis perpendicular to A (or Bsinø if ø if the angle between B and the surface A).

4. Jul 17, 2008

### sameeralord

I got it. You are right it depend on the angle they give. Thanks a lot .