# Homework Help: Average Back EMF Induced in Coil in Loudspeaker

1. Mar 31, 2012

### equinox2012

Hi,
I would appreciate any help with the following:

The coil in a loudspeaker has an inductance of L = 112 microH. To produce a sound of frequency 40kHz, the current must oscillate between peak values of + and - 4.4 A in half a period. What average back emf is induced in the coil during this variation?

My main confusion is, why do we need to know the half period thing...I have no idea how to incorporate it into the solution.

Here's what I have now:

Xl = inductive reactance = 2 pi f L = 2 pi 40 x 10^3 x 112 x 10^-6 = 28.15 Ohm.

Then I rms = I/sqrt 2 = 4.4/sqrt 2 = 3.11 A. Am I correct in looking at rms values here?

Then V rms = I rms x Xl = 3.11 x 28.15 = 87.5 V.

Is that all? Is this V rms the average induced back emf they are asking for? Or am I wrong? I'm really confused about this one.

Any help would be much appreciate! Thanks!

2. Mar 31, 2012

### Staff: Mentor

Hi equinox2012, Welcome to Physics Forums.

The back-emf created by an inductor depends upon the rate of change of the current flowing through it. Thus for an inductance L,
$$V = L \frac{dI}{dt}$$
That's why the information about the change in current over a particular period of time was given.

3. Mar 31, 2012

### equinox2012

Thank you for the help! ;)