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B Doubts regarding Electromagnetic fields

  1. Jul 9, 2016 #1
    I read in a book that Electromagnetic fields propagate at speed of light and carry energy and momentum.
    Let's take electric field for example. Electric field is none other than a force per unit coulomb.So

    1.How could a force(here field) move as force has no property of moving as force causes motion and it is the object which moves not the force?

    2. How(Why) does a force carries energy because it is not something that contains energy?It is the matter that contains energy not the force.

    3. If I am wrong and force contains energy than when oscillating magnetic field induces another electric field from where does the excess energy of electric field comes?

    4. If an oscillating charge produces an EM wave then does the charge loses its kinetic energy?Does it means that if I "kick"(giving velocity) a charge at rest it is accelerated and radiates EM waves so does it comes to rest after radiating EM waves?

    5. Are EM wave just a sort of magnitude waves that is fields don't propagate but it is the change in magnitude of field which propagates?

    Please explain in good detail and don't expect me to understand higher level mathematics(hamiltonian etc) as I am just a High School Student.
    Try to answer all of my question please these doubts are eating my brain off :-p.
  2. jcsd
  3. Jul 9, 2016 #2


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    A field in itself does not move. By definition, a field is a physical quantity that takes a value at every point in space(-time). The electromagnetic field follows Maxwell's equations that, among other things, describe how disturbances in the electromagnetic field propagates.

    The 1/r^2 force belongs to a family of static solutions to Maxwell's equation and there is nothing in it that is propagating.

    1. The EM field is not a force. It can give a force on a charge, but it is not a force in itself.

    2. The question is based on the misconception in 1. The EM field is not a force, it is an EM field. Fields in general carry both energy and momentum.

    3. Normally we would speak of the energy in the EM field as a whole, which does not change unless you add or remove energy due to interactions with matter. If you really want to separate them, the energy gained by the electric field would be lost by the magnetic.

    4. An accelerating charge produces EM radiation. Once moving at constant velocity it will no longer do so and so will continue to move at that velocity unless acted upon by another force. The energy you have to use to accelerate the charge to that velocity is the kinetic energy of the particle plus the energy of any radiation generated during the acceleration phase.

    5. It is unclear what you mean by "magnitude" waves, but your description seems more or less fine, as described above.
  4. Jul 9, 2016 #3
    1.The force felt by one charge due to another situated some distance away is not instantaneous.It takes some time for one charge to influence another kept some distance away.And hence the reason we say EM waves travels at certain speed in a medium.
    2&3.When a charge produce EM waves it creates a potential all around it.Any charge nearby has a potential due to the EM wave.When you keep two electrons nearby they will attract each other.Due to the attraction they will accelerate towards themselves.You could ask from where they gained their new K.E.It is the EM wave that gave them the new K.E. Hence we say EM waves carry energy.
    Momentum is a direct result from the above.I dont understand what you mean by"where does the excess energy of electric field come from"
    4.It will lose its K.E if some other particle comes to interact with the EM wave.No it wont come to rest if there is nothing else to interact with it.
    5.Exactly,as my understanding goes its the change in the magnitude of the field that propagates
  5. Jul 9, 2016 #4
    What this magnitude(value) signifies?This magnitude can give us force but if it itself is not a force then what is the significance of this magnitude?Is this magnitude something very real/physical or just a man made concept?

    Does this means that if a field lies in a space energy also independently lies in the same space? Is this energy same as Zero Point Energy?What is the physical significance of momentum?like a force causes motion, energy is associated to field what is physical significance of momentum?

    It means that if i push a charge the energy of radiated EM wave is not equal to the kinetic energy achieved.Am I right?

    A wave is a moving disturbance. A sound wave is a wave of pressure which means pressure at a point changes repeatedly. By magnitude waves I mean that magnitude of field changes at a point repeatedly and the change of magnitude propagates not the field. (Due to lack of knowledge I was unable to explain it properly so I used the term magnitude wave)
  6. Jul 9, 2016 #5
    All concepts are "man made".
  7. Jul 9, 2016 #6


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    All concepts in every scientific theory are man made concepts. They are mental models that we use to describe the behavior of the world around us. We use these concepts and rely on them because when we perform real physical experiments, the results that we get match the theory very closely. So while they are man made they are compared very closely to real physical experiments.

    For the rest, you are in high school and should focus on simple models, like Newtonian mechanics and maybe circuits. You cannot truly understand a theory without the math, and deeper theories require more advanced math.
    Last edited: Jul 9, 2016
  8. Jul 9, 2016 #7


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    All physics concepts are man-made. The validity of using those concepts to describe nature is tested by performing experiments.

    Energy is not a substance, it is a property of fields and other types of matter.
  9. Jul 9, 2016 #8


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    This is a gross oversimplification. The ##1/r^2## behaviour of the Coulomb law is a static solution and for a static solution there is no such thing as travelling disturbance. In order to take such things into account, it is necessary to use the full Maxwell's equations.

    The EM potential is not only present when the charge is producing an EM wave. The electrostatic attraction between two electrons does not have so much to do with a travelling EM wave as it does with the static Coulomb solution to Maxwell's equations. These statements are misleading at best.

    This is outright wrong. The particle that creates an EM wave loses energy and momentum to the EM field. Once this has occurred, whatever happens with the produced EM wave does not change the state of the producing particle.
  10. Jul 9, 2016 #9


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    The energy provided by the push, is equal to the energy of radiation plus the kinetic energy achieved. Very little energy (from the push) will become radiation (less than 1%), most of the energy will become kinetic energy. This follows if you do the math.
  11. Jul 9, 2016 #10

    David Lewis

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    You're mixing a physical quantity with a unit of measure.
    You may have force divided by charge, or newton per coulomb.

    Two electrons will repel each other.
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