Difference between electricity and electromagnetic waves

  1. Hello,

    Is there any difference between 50/60 AC current and a 50/60 Hz photon stream/electromagnetic wave propagating through a medium ??

  2. jcsd
  3. mfb

    Staff: Mentor

    Where is the similarity between those two, apart from the common frequency?
  4. My confusion started because of the following lines in a text book i am reading (skilling - fundamentals of electric waves) where there is quote along the lines of
    "circuit theory may be considered as a special case of more general theory of electromagnetic fields" from page numbered 1.
  5. mfb

    Staff: Mentor

    Circuits work with electromagnetic fields and electric currents and charges - but they require a completely different analysis compared to electromagnetic waves in vacuum or a medium.
  6. nsaspook

    nsaspook 1,159
    Science Advisor

    The special case is mainly non-radiation of EM energy in free-space, the energy from point A to point B remains in the near-field and it's field components can be computed accurately using point measured voltage/current magnitudes and phases. It's mainly a matter of circuit size and signal wavelength that determines when "circuit theory" stops being accurate not a absolute frequency.

    At the level of EM energy there is not a difference because in both cases the actually energy is carried in fields (near-field with the wire creating a reactive impedance space around it or the far-field free-space resistive impedance for the Em wave).
    Last edited: Jan 3, 2013
  7. @mfb and @nsaspook : thanks for the reply.
    Quoting from http://www.capturedlightning.com/frames/Non-Herzian_Waves.html
    "but one particularly useful consequence is the tendency of long thin conductors to guide an EM wave along themselves. When an EM field is guided in this way, we call it electricity flowing along a wire,"
    It is from the section titled "How do charges behave in a conductor?"
    This confuses me even more.
  8. nsaspook

    nsaspook 1,159
    Science Advisor

    You will be confused if you think that "electricity" is different from EM energy.
    From your link:
  9. Sorry for bringing it up again, but could you please clear this confusion

    So "electricity" is not electromagnetic wave but it is electromagnetic energy ??
  10. electricity is moving charges(mainly electrons in circuitry) and electromagnetic waves are produced by charges in motion(not same as charges,but say some photons!)
  11. nsaspook

    nsaspook 1,159
    Science Advisor

    My standard "what is electricity" link: http://amasci.com/miscon/whatis.html

    Last edited: Jan 4, 2013
  12. BruceW

    BruceW 3,598
    Homework Helper

    "electricity" is a very vague term that doesn't mean anything specific. I would say it is mostly associated with how electromagnetism is useful to us humans. For example, it is used in terms like "electrical device". Also, it is strongly associated with electromagnetic energy because one of the important uses of electromagnetism for us humans is to turn EM energy into other forms (such as light).
  13. Hello i got another doubt,

    If electricity is electromagnetic energy, what is the need for photovoltaic ??
    Cant sunlight for example be used directly as electricity ??
  14. mfb

    Staff: Mentor

    You cannot connect a cable to sunlight.
    It has been proposed to directly use the oscillating electromagnetic field from sunlight, but that is an engineering challenge.
  15. A given electrical device works on a fairly narrow range of currents and potentials as well as a narrow range of frequencies. It cannot utilize energy that doesn't match its operating specifications quite closely. Sunlight typically has frequencies much too high, and voltages and currents much too low.
  16. nsaspook

    nsaspook 1,159
    Science Advisor

    The main problem is not the size and structure of the "wiring" to contain the EM fields at light frequencies but the fabrication of diodes to convert the energy into a DC voltage like is done with utility frequency energy. Nanoscale EM structures can also be used to focus light energy to increase the conversion efficiency of some types of solarcells.

  17. anorlunda

    anorlunda 745
    Gold Member

    If you put a pulse in one end of a 1000 mile power line, how long does it take to come out the other end? A wire is not free space so the answer is not 1000/c, but it is still very fast close to 800/c. One needs to solve the wave equations to calculate that.

    But the wave effects die away quickly, leaving other equations to better describe what happens on the scale of milliseconds or seconds. Ohm's law, rather than the wave equation is a better tool for those time scales.

    The point is that for engineering analysis we almost always work with equations that are simplified forms of more general equations. That doesn't make the general equations invalid, or suggest any conflict. The wave equations for electromagnetic propagation and Ohm's law are both valid and each is more practical for studying a subset of the problem.

    As a power engineer, I've had to deal with electrical phenomena with time scales of nanoseconds up to and including decades. Every analysis chooses to neglect faster effects (assume them to be instantaneous) and slower effects (assume them to be constant) and to deal with what remains in the middle. In other words, we partition analysis by time duration, often coming up with completely different sets of equations for each duration. But there is no conflict between these equations, each is a specialized subset of the whole.

    I suspect that is part of the source of your confusion. Both electromagnetic waves and Ohm's law are valid but useful in different circumstances.
  18. That is interesting. What kind of electrical phenomena has a time scale of decades in power engineering?
  19. RonL

    RonL 907
    Gold Member

  20. anorlunda

    anorlunda 745
    Gold Member

    Generation planning: making sure there is adequate generation of the right type at the right place to serve future demands.

    I'm sure physicists will sneer at the idea, but a power engineer has to deal with physics and human behavior at the same time. We may approach quanta at the fast end of the time spectrum and demographics/politics at the slow end. The point is that it is a continuum. In some regions of the time domain behavioral and physical models overlap and interact. Engineers, but not scientists, are forced to deal with both.

    For example, in the region of days-months, the dominant equations are the economics of the energy/capacity markets but everything bought or sold must conform to what the grid and the power plants are physically able to do. An operating plan must also be prepared for any weather nature might deliver, plus a spectrum of equipment failures and repairs.

    These things can not be divided up and solved separately; they interact. We do however, break up the problem into different time domain regions. We partition by duration not by discipline.

    Roughly speaking, anything less than 15 minutes is almost purely the comfortable domains of physics/chemistry/nuclear.
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