Help understanding electromagnetism (fields and radiation)?

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I am struggling to understand the relationship between electromagnetism, electromagnetic fields and electromagnetic radiation. I understand that a moving electric charge creates a magnetic field and a moving magnetic field creates an electric charge and that this is the foundation of electromagnetism. Does this make an electromagnetic 'field' or are there only electric and magnetic fields separately? Also, how does electromagnetic radiation fit in with this, fields seem to exist around the source while radiation is propagated in a straight line from it. How do they relate to each other?
Thanks.
 

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  • #2
ZapperZ
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You need to back up a bit. Whenever you ask something like this (and this applies to everyone on this forum as well), you need to clearly describe what you already know, i.e. what your level of understanding is. This is because, when we see a question like this, the first thing that comes to our mind (or at least to MY mind) is, does this person already know about Maxwell Equations? Has he/she derived the EM wave equation using them?

We need to know in what "language" and at what level we can offer our answers. This is because we don't want to give you something beyond what you can understand, and we don't want to keep backtracking for each answer that we give (which can be annoying).

Zz.
 
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Alright, thats a good point. I'm sorry, I should have gone into that detail. Really, I am a complete beginner, I know of maxwell's equations although I can't say I've encountered them in an class. Really my only understanding is of the law I stated above about induced electric and magnetic fields and that electric field lines radiate outwards from the source. I also have some understanding that there has to be second order motion of the electric charge for EMR to be induced.
Sorry if this makes it difficult to explain to me.
Thanks.
 
  • #4
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The electric and magnetic fields are two separate, distinct fields created by stationary and moving charges. One can set up only electric fields or only magnetic fields in static cases where these fields do not change in time. When time is considered, Maxwell's equations must be used and they show how the electric and magnetic fields are interdependent on each other.
An 'electromagnetic field' is a characterization of both fields simultaneously through those relationships.

Electromagnetic radiation comes into play when considering how the fields change with time and space. When a new or additional sources of electromagnetic fields is introduced, the fields do not change instantaneously throughout all space. Instead, the field first starts to change at the source and propagates away. Think of dropping a rock into a pond and watching the first wave crest and move away. This propagation carries with it some of the energy intrinsic to the fields and their sources. Electromagnetic radiation is this flow of energy as the fields change in space.

As for the second order of motion required for EMR, I'm not exactly sure what you mean. Any single charge that is accelerated will emit some electromagnetic radiation.

Hope this helps.
 

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