When a charge is accelerated its said that it produces electromagnetic waves. They also say that an accelerating charges produces time changing electromagnetic field and which when passes through a conductor induces voltage in it. I wonder are these two terms same ? Electromagnetic waves is the concept used in communication while electromagnetic field/induction is the concept mostly used in motors,transformers and generators. Please explain. I am confused on it.

  • \$\begingroup\$ Recently i found this also. "Thus, EMR is sometimes referred to as the far field. In this language, the near field refers to EM fields near the charges and current that directly produced them, specifically, electromagnetic induction and electrostatic induction phenomena." Will anyone explain that more. \$\endgroup\$ – Alex Jan 19 '17 at 17:30

The basic distinction here is between "field" and "wave". An EM wave is a radiation of power. That radiation contains an electric and a magnetic field content. Both fields are in phase and, together they have a ratio of 377 in a vacuum. Without the phase alignment or the correct amplitude ratio they don't travel very far but can (if the circumstances are favourable) form a coherent EM wave such as at about one wavelength distant from a correctly made radio antenna.

Why 377 ohms?

Thats the impedance of free-space governed by the square root of the ratio of magnetic permeability to electric permeability: -

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The term "electromagnetic field" is looser and could be used to mean or imply: -

  • EM wave (due to it time changing as per the question title)
  • Near fields around an antenna
  • The magnetic field caused by an electric field
  • \$\begingroup\$ When an ac voltage/current is applied to a conductor it will give both time changing electromagnetic field and electromagnetic waves and Electromagnetic waves will travel a longer distance than the electromagnetic field ? @andy aka \$\endgroup\$ – Alex Jan 19 '17 at 20:11
  • \$\begingroup\$ Once an em wave is formed the fields individually reduce with distance. When not formed into an em wave the reduction rapidly becomes an inverse cube law. For this absolutely important reason, this is why radio is so important! \$\endgroup\$ – Andy aka Jan 19 '17 at 20:29
  • \$\begingroup\$ "Once an em wave is formed the fields individually reduce with distance. When not formed into an em wave the reduction rapidly becomes an inverse cube law." Is it not true that both EM field and EMwaves are formed every time the charge is accelerated ? \$\endgroup\$ – Alex Jan 19 '17 at 20:39
  • \$\begingroup\$ My definition for an em wave as given in my answer means that unless the antenna impedance matching criteria is largely met then an em field might be somewhere between virtually zero and useful. I know that is a generalization but you have no example stated so that's realistically my best response. \$\endgroup\$ – Andy aka Jan 19 '17 at 21:23
  • \$\begingroup\$ For instance, you can use a ferrite rod antenna to pick up long wave radio transmissions because it captures the mag field of an incident em wave. However, try and use that rod as an antenna to produce an em transmission and you might as well try pushing a puddle of water up a 1 mile high hill. \$\endgroup\$ – Andy aka Jan 19 '17 at 21:29

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