# How do I compute magnetic flux?

1. Aug 23, 2009

### austin007

A circular loop (with a radius of 10 cm) rotattes in a constant magnetic fiels, which has a magnitude of 1 T. At an instant when the angle between the magnetic field and the normal axis (to the plane of the loop) is equal to 10 degrees and is increasing at a rate of 20 degrees/s, what is the magnitude of the induced emf in the loop?

How do I compute magnetic flux? ( I know that farady's law can be used to find emf)

2. Aug 23, 2009

### Fightfish

Re: Emf

Firstly, what do you understand by "magnetic flux"?

3. Aug 23, 2009

### austin007

Re: Emf

It is the product of magnetic field, B and perpendicular surface area to B, A.

I know B which is 1 T, how do I get A especially the angle.

4. Aug 23, 2009

### Fightfish

Re: Emf

Consider the projection of the surface area of the loop onto the plane perpendicular to the magnetic field (sort of like the 'shadow'). (You can consider the loop in 2D - a straight line to simplify the analysis)

5. Aug 23, 2009

### austin007

Re: Emf

So is A = pi * r * r cos(10 deg + 20 deg * t) correct?

6. Aug 23, 2009

### Fightfish

Re: Emf

That would be the correct A (perpendicular) for any time t if the rate of increase remains constant over time. However, it would not help you solve the problem. You should formulate it in a generic fashion: A = pi * r * r cos (theta), solve for dA/dt, before substituting the relevant values in to obtain the solution at that point.

7. Aug 24, 2009

### rl.bhat

Re: Emf

At any instant the magnetic flux φ = B*A*cosθ.
So induced emf e = - dφ/dt = -(-B*A*sinθ*dθ/dt )