Does the eletromagnetic field exists everywhere?

  • #1
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Okay, so we learn in basic physics that electric fields are created by charges and that magnetic fields are created by moving charges. After that, we learn that those two are just two faces of the same coin: the electromagnetic field. Also, we know that electromagnetic waves travel trough the electromagnetic field, and that those waves are just perturbations on this field.

What I'm having trouble to understand is: does the EM field exists everywhere in the space? Even where there are no charges? So it's not possible to exist some region on space where there's no EM field?
 

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  • #2
robphy
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The EM field exists everywhere [including places where its value is zero].

The EM field can exist in a region where there are no charges in that region.
Example: between the plates of a parallel-plate capacitor (with no dielectric between: a vacuum).
Example: a plane electromagnetic wave in vacuum.
Technically speaking... if there is a source of this wave, it could be at infinity
 
  • #3
Vanadium 50
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Indeed, all fields exist everywhere. That's part of the definition of a field. The value can be zero at a particular point, of course,
 
  • #4
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So, a EM field is something that exists apart from the charges? What's the role of charges and moving charges in a EM field, then? I mean: if a EM field can exist without those... Does that imply that a EM field is just one big entity, and not a collection of all the the electromagnetic fields generated by a bunch of charges everywhere in space?
 
  • #5
robphy
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When we are first introduced to the electric field via Coulomb's Law,
it is associated with a source charge... and it just seems like a mathematical convenience for calculations.
Later, when we are introduced to Faraday's Law, we see that a changing magnetic field can also be a source of the electric field (a curly one).
Eventually, we learn about the electromagnetic wave where we really see that electric field its own entity... not necessarily associated with a charge.
 
  • #6
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When we are first introduced to the electric field via Coulomb's Law,
it is associated with a source charge... and it just seems like a mathematical convenience for calculations.
Later, when we are introduced to Faraday's Law, we see that a changing magnetic field can also be a source of the electric field (a curly one).
Eventually, we learn about the electromagnetic wave where we really see that electric field its own entity... not necessarily associated with a charge.
Could we make the same analogy for gravity? Does the gravitational field is also a unique entity, not necessarily associated with a body (a planet, a star...)?
 
  • #7
robphy
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Could we make the same analogy for gravity? Does the gravitational field is also a unique entity, not necessarily associated with a body (a planet, a star...)?
Yes.... but things are more complicated with gravity (general relativity).
 
  • #8
anorlunda
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Indeed, all fields exist everywhere. That's part of the definition of a field. The value can be zero at a particular point, of course,

Is that true? I thought that Heisenberg uncertainty forbid E and B from both being zero at the same place and time. That quality is independent of the existence of any charges at any distance.
 
  • #9
Vanadium 50
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A field is an element that has a value at all positions and time. The definition of a field doesn't preclude the value from being zero. Or non-zero. Or having a complex relation with sources and/or other fields.
 

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