Consider a wire through which an AC current is passing. Because of the AC current there is an alternating magnetic field present around the wire and because of the alternating magnetic field there exists an electric field perpendicular to the magnetic field, and this produces a propagating EM wave.

If them magnetic field is creating electric field and they combine to form EM waves, why does a compass show a magnetic field around the wire?

My reason for asking this is:

If the electric and magnetic fields combine into a EM wave and we detect the presence of the magnetic field, then why can't we detect the presence of the electric field perpendicular to it?

  • $\begingroup$ Well, I think we should we able to. Most of the times we put the needle perpendicular to the direction of the E field you talk about, but I can see no reason why it wouldn't affect the needle, as it would induce an electric dipole-like charge distribution on the needle. $\endgroup$
    – guillefix
    Oct 9, 2014 at 18:45
  • $\begingroup$ You can buy very cheap electric field meters or build your own from components for $10-20 yourself. Most tap switches are based on detecting the change in electric field of a hand touching them. Your tablet/cell phone touchscreen works using the change of an electric field. So the answer is "Yes, we can detect the electric field.". $\endgroup$
    – CuriousOne
    Oct 10, 2014 at 5:37
  • $\begingroup$ @CuriousOne really interesting answer, i will check it. if electric field is present(created by magnetic field) then my question had answer.. if possible explain EM wave origin from wire in your words plz... $\endgroup$ Oct 10, 2014 at 19:23
  • $\begingroup$ @lingarajaKN: It's simply the potential difference to ground. If you have an AC wire with 120V AC on it, you can easily produce electric field gradients of 100V/mm across the insulating material. That's a rather strong field. The audio folks call it hum and it's really not easy to deal with in high end audio equipment. So rather than not being able to detect it, that electric field is mostly in our way. $\endgroup$
    – CuriousOne
    Oct 10, 2014 at 21:19
  • $\begingroup$ @CuriousOne, I might be wrong but, as I read the question, the OP is asking how to measure the induced electric field (due to the alternating magnetic field). But the electric field you're referring to is due to the potential difference between the wire and ground, i.e., it is due to charge on the wire and is thus conservative. $\endgroup$ Oct 10, 2014 at 22:49

1 Answer 1


If them magnetic field is creating electric field and they combine to form EM waves, why does a compass show a magnetic field around the wire?

To produce EM waves, we do need the current to be time varying such that the magnetic field is time varying which induces a time varying electric field etc.

However, it isn't that case that the entire magnetic field produced by the time varying current is associated with EM radiation.

In the reactive near field close to the wire, there are time varying electric and magnetic fields that are not associated with EM waves (which transport energy away) but are, rather, associated with energy storage. From the linked article:

For example, current flowing in the antenna creates a purely magnetic component in the near-field, which then collapses as the antenna current begins to reverse, causing transfer of the field's magnetic energy back to electrons in the antenna as the changing magnetic field causes a self-inductive effect on the antenna that generated it. This returns energy to the antenna in a regenerative way, so that it is not lost.

This is all quite complicated in general but, for low frequency, e.g., 60Hz AC current along a wire in a circuit, the sinusoidally time varying magnetic field dominates.

Energy is alternately stored in this field (as the current magnitude increases) and returned (as the current magnitude decreases).

  • $\begingroup$ @ Alfred Centauri your answer is acceptable but antenna is used to increase power of EM waves,you are saying that it produce purely magnetic field at antenna(which is regenerative). Still expecting full answer for the question asked..... $\endgroup$ Oct 10, 2014 at 5:20
  • $\begingroup$ @lingarajaKN, a wire with a time varying current is, in effect, an antenna even if not an intentional one. Moreover, I specifically and properly addressed the question I quoted at the top of my answer. $\endgroup$ Oct 10, 2014 at 10:54
  • $\begingroup$ @ Alfred Centauri yes you specifically assigned your answer to part of my question. I will try to capture your answer. $\endgroup$ Oct 10, 2014 at 15:53

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