# Homework Help: Angle for magnetic flux

1. May 19, 2017

### watthappening

1. The problem statement, all variables and given/known data
I had know that for finding magnetic flux is NBA cos theta. The answer is C though, so why is it sin theta instead of cos theta? I wonder

2. Relevant equations
BA cos theta
NBA cos theta

3. The attempt at a solution

2. May 19, 2017

### Staff: Mentor

This is the problem with memorizing formulas without making sure you know what the variables represent. Try to find in your notes or textbook where that formula was defined and see how the angle θ is defined there.

3. May 19, 2017

### watthappening

I did know what the variables represent, the angle is between the normal of the area of the surface and magnetic field lines. But i still didnt get the reason.

4. May 19, 2017

### Staff: Mentor

Okay, the angle in the given diagram does not subtend the area normal and the magnetic field direction. How does this given angle relate to the angle used in the definition? (It may help to use a little triangle geometry)

5. May 19, 2017

### watthappening

It's doesnt? oh well, I didnt know that

6. May 19, 2017

### Staff: Mentor

The required area normal is perpendicular to the area of the coil "face", that is, it's perpendicular to the plane of the coil. The angle shown in the diagram subtends the magnetic field and the plane of the coil itself.

7. May 19, 2017

### watthappening

is this correct?

8. May 19, 2017

### Staff: Mentor

Sure. That'll work. You should be able to see the relationship between the θ given in the problem figure to the one you've just drawn in.

9. May 19, 2017

### watthappening

alright thanks for the help, mate