# Average emf

1. Apr 3, 2009

### synergix

1. The problem statement, all variables and given/known data
1. When a 100 loop coil of wire of radius 2.00 cm is rotated in a uniform magnetic field,B=0.500T , so that it goes from maximum flux to zero flux in 0.0320 s, find the average emf induced between the terminals of the coil.

2. Relevant equations

Max Flux=BAcos(0)=Change in flux

E=-N change in flux/change in time

3. The attempt at a solution

E=[-N(BAcos0)]/t

E=[-100(0.500T pi(2.00cm2) cos 0o)]/0.0320s

How do I calculate average emf? should I just divide by 0.0320s again?

2. Apr 3, 2009

### Redbelly98

Staff Emeritus
Dividing by 0.0320 s just one time, as you have already done, will give average EMF.

Avg EMF = N Δ(Flux) / Δt​

3. Apr 4, 2009

### synergix

Thank you.
Is it possible to calculate instantaneous emf? and would there ever be a use for such a calculation?

Last edited: Apr 4, 2009
4. Apr 4, 2009

### Redbelly98

Staff Emeritus
Yes and yes.

Have you had calculus? Instantaneous emf is the derivitive (w.r.t. time) of N x flux, where
flux = B A cos(ωt)​

It's useful for power generators, which operate on the principle of induced emf.

5. Apr 6, 2009

### synergix

No I have not had any calculus. I am taking physics 12 right now. I am going to university next year to start an engineering degree so I will be into it soon enough.

6. Apr 6, 2009

### Redbelly98

Staff Emeritus
Okay.

Instantaneous EMF can be approximated by
EMF = N Δ(Flux) / Δt
as long as Δt is a small amount of time, a lot smaller than one oscillation period.

The smaller Δt is, the better the approximation.

In the above homework, Δt corresponded to 1/4 of a period (the time to go from maximum to zero). It would have to be a lot smaller than that in order to get a decent approximation.