EMF, curled or straight antenna

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    Antenna Emf
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Discussion Overview

The discussion revolves around the calculation of electromotive force (EMF) in a wire used as either a straight or curled antenna in the context of electromagnetic fields. Participants explore the theoretical implications of using the Poynting vector to derive the relationship between electric and magnetic fields and their effects on the wire's EMF.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification, Debate/contested

Main Points Raised

  • One participant suggests that for a straight antenna, the EMF can be calculated using the equation EMF = integral_from_0_to_l[E*dl], leading to EMF = El if the electric field and wire are parallel.
  • Another participant questions the assumption that the magnetic field (B) is not changing in the curled antenna case, prompting a discussion about the implications of this assumption.
  • There is a consideration that the EMF in the curled antenna might be zero due to the constant radius of the loop and the assumption of a constant B field.
  • Participants discuss the relationship between the Poynting vector, emitted power, and the calculation of root mean square (RMS) values for electric and magnetic fields.
  • One participant expresses confusion about the meaning of RMS and its relevance to the discussion.

Areas of Agreement / Disagreement

Participants express differing views on the behavior of the magnetic field in the curled antenna case, with some assuming it remains constant while others challenge this assumption. The discussion remains unresolved regarding the implications of these assumptions on the EMF calculations.

Contextual Notes

Participants rely on the definitions of electric and magnetic fields derived from the Poynting vector and the context of the problem, which includes emitted power and area. There are unresolved questions about the changing nature of the magnetic field and its impact on the EMF in the curled antenna scenario.

vs5813
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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..! :redface:
 
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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
 
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...
 
"The B field is not changing,"
Why do you say that?
 
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...
 
vs5813 said:
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...

lol you figured it out or i told you
vs5813 said:
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...

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

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