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How does alternating current travel anywhere?

  1. Jun 5, 2013 #1
    I'm just learning the basics of electromagnetics and am a bit confused as to how ac works.

    From my understanding, a rotating conductive wire in a magnetic field create a current that changes direction from positive to negative flow, modelled in accordance with the sine function.

    But i was under the impression that to actually power something, it needs to flow from one terminal to the other.

    How does the powering of my home appliances work if the current in a power line "ebbs and flows" so to speak, and doesnt actually travel a distance?
     
  2. jcsd
  3. Jun 5, 2013 #2

    CompuChip

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    The common story about electrons travelling in the wire is not exactly true. Electrons do tend to move in a single direction over time, the so-called drift, but they do so very slowly. Electrons carrying the energy from one terminal to another is therefore incorrect, it would be better to view it as the energy being passed from one electron to the next, like people passing a water bucket. This transfer of energy happens at nearly the speed of light, on timescales which are much shorter than the alternation time of the current.
     
  4. Jun 5, 2013 #3

    CWatters

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    Imagine a pipe full of hot water. The molecules are moving about at some speed dependant on their temperature. If you inject a small amount of extra water in one end then a pressure wave travels down the pipe very fast and the same amount of water comes out the other end. The energy went through the water very quickly (speed of sound in water perhaps) but the water itself didn't move very far. So averaged over time the velocity (drift velocity) of the water is low but the speed of the energy wave can be quite high. For electrons in a wire the drift velocity can be of the order of mm per second. At that rate they take quite a long time to travel down a transatlantic telephone cable but clearly the signal travels a lot faster.

    For AC imagine doing that in alternate directions.

    http://en.wikipedia.org/wiki/Newton's_cradle
     
  5. Jun 5, 2013 #4

    vanhees71

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    In reality it's not the electrons that transport energy from the generator to your appliance but the electromagnetic field, no matter whether you look at DC or AC! Just calculate the (stationary) fields of a current conducting wire (e.g., a coax cable or some other easy to calculate configuration) and evaluate the Poynting vector. You'll see that the electromagnetic field transports the energy!
     
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