What is the difference between far and near electromagnetic field?

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  • #26
sophiecentaur
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Perhaps I am misunderstanding what far and near field means.

Situation 1:
If I have two charges in a vacuum, with the straight line between them called 'x direction', and I oscillate one of the charges in x direction, the other charge will be caused to oscillate in x direction.

Situation 2:
If I have two charges in a vacuum, with the straight line between them called 'x direction', and the line perpendicular to x direction called 'y direction', and I oscillate one of the charges in y direction, the other charge will be caused to oscillate in y direction.

Why is the second situation called EM radiation and the first situation is not?

Or perhaps a better question is: Why does the second situation create EM radiation and the first situation does not?

If you feel that 'Categorisation' is more important than the actual effect then you will easily be confused. The term 'Far Field' simply means that it is the remaining effect that you get as you increase the distance. Near Field is the situation in which the other terms have a relevant effect. Why should it be at all surprising that your two-charges models would behave differently? - that is, if you are prepared to do the sums.
The Far Field on the axis of motion of an oscillating charge tends to zero.
 
  • #27
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I have read this wikipedia article, but I still don't understand:
http://en.wikipedia.org/wiki/Near_and_far_field

Why can EM radiation only be transverse waves? Why can't it travel as a longitudinal wave?

If I have two charges in a vacuum, with the straight line between them called 'x direction', and I oscillate one of the charges in the x direction, the other charge will be caused to oscillate in the x direction.

Why is this situation not called EM radiation?

An electromagnetic wave is a transverse wave in that the electric field and the magnetic field at any point and time in the wave are perpendicular to each other as well as to the direction of propagation. Because such electric and magnetic fields generate each other, they occur jointly, and together they propagate as electromagnetic waves. In your example, there is only two charges. OK But where in Natur we can find only two charges ? Ιn one area there are many charges interacting (Gebühren Interaktion)
 
  • #28
Drakkith
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OK But where in Natur we can find only two charges ? Ιn one area there are many charges interacting (Gebühren Interaktion)

It doesn't matter. There's no problem with considering the case of two particles by themselves. It's just simplifying the problem and ignoring things that won't affect the final answer.
 
  • #29
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It doesn't matter. There's no problem with considering the case of two particles by themselves. It's just simplifying the problem and ignoring things that won't affect the final answer.
This is very truth. But we cannot use it in all cases. There are exceptions
 
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