# EM field and EM wave

• I
• crocodile_
crocodile_
Hello, I'm new to this forum. I have a short question that I can't solve on my own, I've consulted many books but I can't find solutions, I hope you can help me.

Then considering a conductor traversed by an electric current that varies over time, it produces an electromagnetic field, under certain conditions the electromagnetic field moves further and further away from the conductor and electromagnetic waves are created.

Here are the conditions under which a conductor produces only an electromagnetic field in the near field such as EM induction or transcranial magnetic stimulation, and under which other conditions does it instead produce electromagnetic waves (far field)?

Last edited by a moderator:
Welcome to PF.

You have considered only a short piece of wire and the near field it radiates. That piece of wire is just one element of a closed circuit with current flowing.

The far field is the sum of all the fields radiated by the current in all the elements of that circuit. For big circuits and small wavelengths, the phase of the currents may vary about the circuit, which makes the summation more interesting.

The magnetic fields radiated by the circuit are accompanied in space by a perpendicular electric field.

Electric circuits with varying current, unless of shielded construction, will have both local fields, in which energy is stored, and radiated fields, where energy is lost from the circuit due to the creation of radiated EM waves. The radiated energy is small when the dimensions of the circuit are small compared to the wavelength.

Whenever a charged particle is accelerated or decelerated electromagnetic radiation will be produced. The only condition for generating an electromagnetic field in the near-field region and in the far-field region is a charged particle with changing its velocity.

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