Electromagnetic waves

  • Thread starter nobahar
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  • #1
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Hello!
Not sure if I've posted here before... (in the intorductory Physics section, that is!)
Anyway, I don't have a particularly good understanding of physics, and I have just started looking at waves recently. However, I have a quick question regarding electromagnetic waves. I have read that the electric field is perpendicular to the magnetic field, but if a wave emanates out in to space in all directions, how can the waves be perpendicular? If that makes sense.
Any explanation in layman's terms would be appreciated. If the question isn't clear, I can try and restate it.
Many thanks,
Nobahar.
 

Answers and Replies

  • #2
tiny-tim
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hello nobahar! :wink:
I have read that the electric field is perpendicular to the magnetic field, but if a wave emanates out in to space in all directions, how can the waves be perpendicular?

at every point, a wave has a direction of movement …

the electric and magnetic fields are perpendicular to each other and to that that direction

so for example if the waves are all radiating from a point, then the electric and magnetic fields are tangent to the spheres centred on that point :smile:
 
  • #3
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Okay. So my understanding of waves is wrong.
For example. I thought that, for phenomena such as electromagnetic waves, the 'mathematical' description supplied information about the 'properties' of a wave at some particular point in space. For example, at some time t, the amplitude of the wave may be given by Y sin (t), where Y is the maximum amplitude. So at some specific point in space, the 'property' that the function describes has the value Y sin (t), and the value of the property changes in time at this point, described by the function. But that it refers to a specific point, not that the wave is physically what it looks like as it moves trhough space. This must be wrong, otherwise the electric and magnetic waves couldn't be perpendicular. I hope that makes sense.
Any help appreciated.
 
  • #4
tiny-tim
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… So at some specific point in space, the 'property' that the function describes has the value Y sin (t), and the value of the property changes in time at this point, described by the function. But that it refers to a specific point, not that the wave is physically what it looks like as it moves trhough space. This must be wrong, otherwise the electric and magnetic waves couldn't be perpendicular.

yes, the 'property' (which is a vector in this case) changes at each point, but it also changes from point to point …

you're concentrating on the wave as experienced by a stationary observer, but it'll behave similarly as seen by any observer

(but i don't follow what you're saying about being perpendicular)
 
  • #5
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you're concentrating on the wave as experienced by a stationary observer, but it'll behave similarly as seen by any observer

I don't know if I follow what you mean. I have only a basic understanding of reference frames. Sorry!

(but i don't follow what you're saying about being perpendicular)

Again, sorry, its difficult to articulate what I mean when I don't really understand myself.
I'll try and explain. If the function identifies the 'magnitude' of the property at a specific point in space, as a function of time, the the wave as a function itself doesn't really represent a physical 'thing'. At some time, t, the function identifies a magnitude, dictated by the amplitude at time t; and this is the value of that property at a specific point. The magnitude oscillates, as described by the function, but there isn't a physical orientation to the magnitude. So what does it mean to be perpendicular? The electric wave function at time t for a specific point will have some amplitude, and the magnetic wave function at the same time t will have an amplitude. Both of which identify the two magnitudes of two properties at a specific point. But the two waves being perpendicular would suggest that there is a region in space where the electric component of the wave doesn't reside (namely everywhere except perpendicular to the magnetic component of the wave!), and the same for the magnetic component of the wave. In which case they can't both occupy the same specific region in space.
I'm sorry if this is convulted and doesn't make sense. I can try to make it a little clearer if needs be.
Thanks for all the help so far.
 
  • #6
tiny-tim
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The magnitude oscillates, as described by the function, but there isn't a physical orientation to the magnitude.

Yes there is!

The amplitude is times a vector, the vector of the electric field.
 
  • #7
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Oh okay, so the electric field exerts a force, and the direction of that force is dictated bya vector, and the amplitude of the function at some time will identify the magnitude of the vector, and the original vector, is a unit vector in the appropriate direction?
This means my concept of waves is wrong.
Thanks for all the help!
 
  • #8
how electric field give rise to magnetic field?plz reply in simple wording
 
  • #9
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how electric field give rise to magnetic field?plz reply in simple wording

Bebachsheed/ana asif Khalood. I do not know, I had to divert my attention elsewhere. Hopefully, someone else will be of assistance to you.
 
  • #10
nomadreid
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khaloodsarwar, it has been a month since you posted your question. Since then, no knowledgeable physicist has given you an answer, perhaps because it was posted after the question which started the post was answered, or possibly because the answer is hard to explain in simple wording. I have a solution, if you are still interested in obtaining the answer: I will give you a clumsy explanation, in the hope that someone will be outraged enough at the simplification to step in with a better explanation.

So, the first thing is that a static electric field would not give rise to the type of magnetic field you are talking about (there is another kind, which is intrinsic to the electron, from its spin), but rather a changing electric field, or more precisely, the changing of an electric field. Actually, "gives rise" is misleading: the electric and magnetic fields are not really separate entities, but two different ways of looking at the same thing. There is a field associated with the moving electron, and the way it is measured determines whether you get measurements which one associates with magnetism, or measurements which one associates with electric fields.

So, anyone care to improve on this, without bringing in complications like the four-potential, relativity, and so forth?
 

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