# EMF, curled or straight antenna

1. Jun 2, 2007

### vs5813

hmm just a topic for discussion..im revising for an exam and i found an old paper asking to find rms values of electric field and magnetic field from power...and that's easy, using the poynting vector. Then it asks, given a piece of wire of length l in these fields, you can either use it as a straight antenna or as a curled antenna: what is EMF in the wire in each case?

Ok, so i was thinking about how id answer, i know that if i were to use it as a straight antenna, i'd be causing it to respond to the E, while if i used it curled it would be responding to B. But how could i go about calculating it?

An equation i could use is EMF = - B (dA/dt)..but how do i relate it to the electric field in the straight antenna case?

Any thoughts greatly appreciated..!

2. Jun 2, 2007

### ice109

im pretty sure that if you were going to use it as a straight wire you will just have an e field * the length of the wire, gives the voltage drop across the wire

3. Jun 2, 2007

### vs5813

oh! i just realized...the straight antenna case is actually easier to think about:

EMF = integral_from_0_to_l[E*dl]

which in this case just means: EMF = El if the two of them are parallel.

but the curled antenna...wouldn't the emf just be zero? The B field is not changing, and the area can't be changing either if the loop is of constant radius....

4. Jun 2, 2007

### Meir Achuz

"The B field is not changing,"
Why do you say that?

5. Jun 2, 2007

### vs5813

hmm...should i not assume that? because in the first part of the problem they gave us the emitted power and area so we could find the poynting vector, and from that we can find the numerical value of the rms E and B fields..so i assumed they would stay constant...

6. Jun 2, 2007

### ice109

lol you figured it out or i told you

what does RMS mean and why do you take the RMS of a function over a period

Last edited: Jun 2, 2007