# Questions on finding average EMF

1. May 13, 2015

### Lilly1

A 34 turn circular coil of wire has diameter 1.07 m. It is placed with its axis along the direction of the Earth's magnetic field of 60.0 µT, and then in 0.202 s it is flipped 180°. An average emf of what magnitude is generated in the coil?

ε = − dΦB dt = −2Nπr 2B/dt = 16.95 when I used 3.14. I changed 3.14 to 3.141 and entered 16.97 I still get the same error message "Your response is within 10% of the correct value. This may be due to roundoff error, or you could have a mistake in your calculation. Carry out all intermediate results to at least four-digit accuracy to minimize roundoff error. mV".
Does anyone what is acceptable answer for the question from webassign?

Thanks!

2. May 13, 2015

### Tom_K

You are making an error in calculation, not in the value of π.

I don’t know where the error is. Are you sure you are dividing by 0.202 seconds and not 0.220?

Last edited: May 13, 2015
3. May 14, 2015

### Lilly1

Yes. I use 0.202. Can you please help on finding my problem?

4. May 14, 2015

### Staff: Mentor

Lilly1, I have moved this thread to the Introductory Physics Homework forum. It would be helpful if you could give more detail on your attempt at solution (i.e., give the numbers you are using for each quantity in the calculation).

5. May 14, 2015

### Staff: Mentor

Two things don't look right in what I've quoted here.

6. May 15, 2015

### rude man

I think you meant to write ε(t) = -N dΦ/dt = -Nπr2 dB/dt which would be right.
However, they're asking for the average emf over the 180 degree rotation, so you write avg ε(t) = -N avg(dφ/dt).
In general, avg f(t) = ∫f(t)dt/∫dt = (1/T)∫f(t)dt. So avg(dφ/dt) = (1/T)∫(dφ/dt)dt = (1/T)∫dφ = Δφ/T
and avg ε = -NΔφ/T. So, what is your Δφ? Then compute avg ε.
Your answer should be a positive number since they're asking for magnitude.
BTW I get nothing like 16.97V. More like 0.1% of that.