Differences Between Magnetic Field and Magnetic Wave?

  1. [​IMG]

    Hi

    I have some confusions about Electric and Magnetic fields and waves basics.

    First of all, Is that diagram fully correct?

    Are electric field and electric wave the same thing?

    And are magnetic wave and magnetic field the same thing? I mean every magnetic field is a part of a wave?

    If not what are the differences?

    I have read lots of thing, one says, dc generates electric and magnetic field, but ac generates electric and magnetic wave, is that right?

    And one more confusion, Is the electric current( I mean while DC current is flowing) itself generates magnetic field/wave or the voltage differences(I mean the distance of the positive and negative poles)?

    Thanks...
     
  2. jcsd
  3. sophiecentaur

    sophiecentaur 13,695
    Science Advisor
    Gold Member

    People do make a distinction between an oscillating / changing magnetic field and the field due to DC currents. However, no DC current has been flowing around a circuit for ever so it is arguable that switch-on generated a radiated (electro) magnetic field which is still spreading out over the Universe in the 50 years since the circuit was activated.

    When a magnetic field varies, then there is also an electric field, due to that change so you have an em wave.

    In practice, however, we tend to treat the two situations differently. It will make the sums easier when em radiation does not need to be accounted for. Dammit, the simple circuit laws are hard enough!
     
  4. DaleSpam

    Staff: Mentor

    It is a correct depiction of the electric and magnetic fields along a line perpendicular to a linearly polarized plane wave.

    No, they are not the same thing. First, there is no such thing as an electric wave or a magnetic wave, there are electromagnetic waves. I.e. all EM waves consist of both an E field and a B field. Furthermore, an EM wave is an EM field which behaves according to the wave equation. So, all EM waves are EM fields, but not all EM fields are EM waves.

    Usually when you have an arbitrary source and you are looking at the fields you would speak of the "near field" and the "far field", with the far field being a wave, and the "near field" being the part that doesn't propagate like a wave. A completely DC current has no far field. An AC current generates both near and far fields.

    I didn't understand your question, can you please re-word it more carefully?
     
  5. Thank you sophiecentaur and DaleSpam great explanations...

    What makes them(EM Fields) EM waves?

    I was just trying to ask, Does a "current" generate EM Field/Wave or "Voltage difference"? even current can't find a way to flow(like in parallel plates of capacitance).

    I think both of them generates EM field/wave. So even DC current generates EM field/wave. I still can't figure out what makes an EM field an EM wave if not all EM fields are EM waves

    Thanks for your help...
     
  6. sophiecentaur

    sophiecentaur 13,695
    Science Advisor
    Gold Member

    A 'Wave' involves quantities changing in time and over distance. A field that is not changing does not constitute a wave.

    It might be a good idea to think of waves other than EM waves (waves on strings. sound etc) which are a bit more 'physical' and get their general properties first. Then move on to EM waves, which are harder to grasp.
     
  7. A quick question on the same topic: on the x axis there is usually time or distance, but what is on the y-axis? Is it the force a charged particle would feel at that spot/time? Is it true that any time a charged particle moves it creates a EM wave? So basically an EM wave is nothing more than the information of a changed EM field spreading around, because if the particle moves closer or further away, the EM force also changes? And if the wave oscillates back and forth, it meas somewhere a charged particle is oscillating?

    When you have DC on, you don't have an EM wave, because there is a constant EM field and an EM wave is just a representation of a changing em field, but where the information of the changed EM field has not yet arrived an EM wavefront is heading, is this correct?
     
  8. sophiecentaur

    sophiecentaur 13,695
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    The y (and z) axis usually depict the 'displacement' (non-specific) for all waves.

    This is precisely what I wrote about 100 years back on this thread. every DC circuit was switched on at some time and that information is propagating outwards into space for ever, as a wave front. But is that relevant to basic electrical theory?
     
  9. Why do all the diagrams show electric field (and magnetic field) as a 2 dimensional vector ?

    Doesn't really electric field (and magnetic field) have "width" as seen in the diagram?
     
  10. sophiecentaur

    sophiecentaur 13,695
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    Gold Member

    You have to remember that diagrams are like graphs. They show a relationship and are not a 'picture' of what something looks like. Correct interpretation is essential.
     
  11. In case of an EM wave, displacement of what? Is it not the strength of the EM field at that point?
     
  12. sophiecentaur

    sophiecentaur 13,695
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    You guys don't seem to do much reading around when you want to learn something. If you did then you would realise that the term 'displacement' refers to WHATEVER happens to be varying.
    For EM waves it refers to the values of the two fields at a given position and time.
     
  13. Thanks sophiecentaur;

    I have one last question about that topic, if someone can verify it, I will be very glad.

    I don't know what "field" is, I think it's solid definition is related with quantum physics. I have been searching differences between electric "field" and magnetic "field". I expected to read something like that:

    " Electric and magnetic fields are the same thing. The only difference between them is, one of the field is created by static electric charges but the same field that we call magnetic field's field is generated by moving electric charges. But they are same thing."

    But still I haven't read such a thing on websites that google showed me.

    Is my statement fully correct?

    Thanks...
     
  14. sophiecentaur

    sophiecentaur 13,695
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    Did you ever look up the word "field"?
    There are many ways of stating the definition. One way of defining it is to sat that a field is a region in which a force acts on an object with appropriate properties. E.g. mass charge, current.
    Bringing QM into it at this stage can't particularly help.
     
  15. DaleSpam

    Staff: Mentor

    Here is a good place to start:
    http://en.wikipedia.org/wiki/Electromagnetic_field

    I agree with sophiecentaur, stick with classical EM at this point. QM will just add confusion.
     
  16. OK. Thank you.

    But can you please give me one sample about that information:
    "So, all EM waves are EM fields, but not all EM fields are EM waves"

    How can not an EM field be an EM wave?

    DC voltage is related with Electric field,
    DC current is related with Magnetic field,
    I am assuming DC can't generate EM field, either Electric field or Magnetic field but not both of them.

    AC current changes Magnetic field that means it generates Electric field. Ac always generates EM field. So now we have an EM field. So, now, at this point how can not an EM field be an EM wave?
     
  17. DaleSpam

    Staff: Mentor

    That is a bad assumption, if DC can do one or the other then you can just combine circuits to do both.

    You can easily build a DC circuit that has an electric field due to a steady state voltage (e.g. across a capacitor) and a magnetic field due to a steady state current (e.g. through a resistive inductor). You can even mount the capacitor inside the inductor so that the E and B fields overlap.

    There are lots of ways that you can make non-propagating EM fields.
     
  18. sophiecentaur

    sophiecentaur 13,695
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    Gold Member


    I think you may be a bit preoccupied with what to 'call' things rather than in understanding what is going on here.
    If , as already suggested, you read around more, you will see the way the terms are used and resolve any uncertainties you have at this point.
     
  19. I think maybe no one answered the question. The question is clear and should be answered at the root "what differs a wave from a field"?
    You know the development of Quantum Mechanics (QM) to Quantum Field Theory (QFT) answered this question. A wave has the property to propagate: Means we need a moving/vibrating source at a fixed position (for example) and the medium will propagate the source perturbation through the medium without motion of the source. Any point of the medium (in the direction of the propagation) will do the same thing as the source but after a delay time (time that the source perturbation will reach the point). So a virating point that does not propagate has the equation y(t) = A sin(ω*t) but if there is a propagation (means wave) we need to enter the position of propagation and the wave equation is y(x,t) = A sin(k*x - ω*t) with k = 2*pi/Lambda .

    QM mechanics is a wave theory as per Louis De Broglie so we use Wave function in QM. But QM did not solve the particles scattering issue and in particular creation of particles, so physicists introduced special relaticity, second quantification (creation and destruction of a particle), and espcially the field concept. A field is like a wave but it generates itself at each vibrating point reached by the wave, so we need to enter creation and annihilation of particle (means each point becomes a source) operators (a+ and a-) so the field eqaution is: y(x,t) = A ∫ a+ exp(kx- ωt) + a- exp(kx+ωt) dtd3k
    The field equation as it is mentioned above solved all particles interaction and theories like Quantum ElectroDynamics (QED) based on QFT had a tremendous success to check all experimental results. Physicits like Feynman and Schwinger ... had been awarded the Nobel Prize in physics beginning of the sixtees for development QED.
     
  20. jtbell

    Staff: Mentor

    We do not need to bring QM and QFT into this! There is such a thing as classical electromagnetic field theory, as described by Maxwell's equations long before QM came along.
     
  21. UltrafastPED

    UltrafastPED 1,919
    Science Advisor
    Gold Member

    An analogy from back home on the farm:

    "Field" = field of grain ... there is something growing everywhere in the field; some are taller, some are closer together. But they are all part of the field. Anywhere you go you can "measure" the field at that spot.

    "Wave" = when the wind blows you see waves in the field - just like waves on the water. Always something is causing the waves.

    For light there is a coupling of electric and magnetic waves (=electromagnetic); the fatm analogy doesn't help here.
     
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