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AC question -- How does AC current "reach" its load?

  1. Jan 2, 2015 #1
    How does AC current "reach" its load?
    If the electrons/fields/charges are oscillating (reversing direction 50 or 60 times per second) instead of traveling in one direction like DC does, how does current flow and reach the load in a circuit? I have read some articles but couldn't find an answers I could understand
    Thank you
     
  2. jcsd
  3. Jan 2, 2015 #2

    Nugatory

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    Don't mix up the electrons and the fields here. Even with direct current, the drift velocity of the electrons in the wire is surprisingly low (millimeters per second), yet if I apply a high DC voltage to one end of a long wire and you're grounded and holding the other end you'll notice pretty much immediately because the change in voltage level propagates at significant fraction of lightspeed.

    Now for alternating current: Imagine that you pass a rope around a pulley, hold one end of the rope in each hand, and work your hands back and forth to make the pulley turn one direction and then the other. Clearly you're transferring energy to the pulley; equally clearly the atoms in the rope are just moving back and forth without going anywhere. The work is being done by the changes in the tension of the rope, which do flow from one end of the rope to the other. This changing tension is analogous to the voltage level in a wire carrying alternating current.
     
  4. Jan 2, 2015 #3

    rcgldr

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    A simple example is an incandescent light bulb. It resists the cycling flow of electrons, slightly reducing the voltage, and converting consumed energy into light and heat. Back at the generator, as the overall load consumes energy, the generator senses a greater load and has to perform more work in order to maintain the voltage.
     
    Last edited: Jan 3, 2015
  5. Jan 2, 2015 #4

    phinds

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    As an alternate analogy to Nugatory's rope, think of current as being like a bicycle chain. If you are using DC, the chain moves around the loop the way it would if you were riding a bike, but if you are using AC, the chain just moves back and forth (which, of course, wouldn't get you far on a bike :smile:)
     
  6. Jan 3, 2015 #5
    Thank you for your answers, so current is more than a flow of electrons, the pulley and bicycle chain analogies cleared it, that energy flows and doesn't get stuck in the wire as I though when I read about AC current
    PD sorry for the bad thread title
     
  7. Jan 3, 2015 #6

    phinds

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    Actually not all that bad and unfortunately one of the very few major annoyances in this forum is that you cannot edit your thread titles, even immediately after you post, when the post itself is still open for editing. You CAN ask a moderator to change it for you, but they are busy folks who do this for free so try not to overburden them unless you feel you've really goofed seriously in a thread title.
     
  8. Jan 3, 2015 #7
    Thank you I was referring to my previous title "AC question" but it was edited by Berkeman I received a Pm from him "scolding" me for that title and he /she was right :)
     
  9. Jan 3, 2015 #8

    CWatters

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    At room temperatures electrons wiz about at over 1,500,000 meters per second. So you might expect them to move down a wire very fast when you turn on the power to a circuit. However that's not the case at all. This Wikipedia article calculates that a current of 3A in a wire of 1mm diameter gives a velocity of just 0.00029 m/s ! You can run about 10,000 times faster than that. They call this the drift velocity which seems appropriate given the slow speed with which they move...

    http://en.wikipedia.org/wiki/Drift_velocity

    Given that slow speed it's perhaps wrong to think of electrons as flowing around and around a circuit. It's perhaps better to imagine them like a long queue of people. If someone at the front of the queue steps forward one place the next person can also step forward one place. Then the person behind him can also step forward and so on all the way down the queue. That "shuffle forwards" can propagate along the queue much faster than any one person moves along the length of the queue.
     
  10. Jan 4, 2015 #9
    Interesting I didn't know that electrons traveled THAT slow :) so they should teach that electricity is more than that because it would take a lot of time to turn on a light lol.
     
  11. Jan 4, 2015 #10

    sophiecentaur

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    Absolutely. Particle theory has its place on occasions but electrons and photons are best not considered in most of life's little mysteries. I only wish educationists could take that on board.
     
  12. Jan 5, 2015 #11

    Drakkith

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    Yes, and even this "drift velocity" isn't an accurate description of what's going on, as the electrons in a wire are not simply standing still in the absence of an applied voltage. They are moving around in random directions, and when you apply a voltage you get a slight imbalance in the number/momentum of the electrons moving in one direction compared to the opposite direction. The "drift velocity" is just a simplification of a more complicated scenario.

    While you do have current flow throughout the circuit, it may help to think of the propagation of the voltage down the line as the main reason the circuit works as it does. When the electrical length of the AC signal is much longer than the physical length of the circuit's conductors, we can say that the change in voltage happens everywhere in the circuit at the same time, which is the reason the current alternates magnitude and direction over time as well. (Ignoring reactance of course)
     
  13. Jan 5, 2015 #12

    CWatters

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    It's a bit like turning on a tap. The water comes out immediately. The water co pumps a little in at their end and the same volume comes out of your tap, but you don't have to wait for it to travel all the way from the local reservoir.

    Meanwhile water molecules wiz about at 1300mph...
    http://www.verticallearning.org/curriculum/science/gr7/student/unit01/page05.html

     
  14. Jan 5, 2015 #13

    sophiecentaur

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    Frankly, the idea of including electrons in the appreciation of the workings of Electricity is counter productive. Electricity has enough conceptual hurdles to cope with, electrons aside. There is an expression "you can't see the wood for the trees", which applies here, I think.
    I was lucky enough to get my School Physics over with before 'particles' took over. These days, newbies even seem to want to introduce gravitons into their learning of gravity. Educationists have a lot to answer for.
     
  15. Jan 5, 2015 #14

    rcgldr

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    Note that each person can start to move forwards just a fraction of a second after the person in front of them. One issue with this analogy is that the wave of moving people propagates backwards as the people step forwards.

    Another analogy would be a series of balls connected by stiff springs, an impulse (push or pull) is imparted onto the ball at the trailing end of the series, and the impulse propagates through the balls at a much faster rate than the actual movment of any ball, depending on the stiffness of the springs. Or take the case of a very long solid rod, if one end experiences some form of impulse, the impulse moves through the rod at the speed of sound in the rod (much faster than the speed of sound in air).

    Getting back to drift velocity, despite slow drift velocity, the propagation speed of the voltage is some fraction of the speed of light, like about 1/3 c, depending on what the wire is made of (copper, steel, ... ), and the insulator (could be air in the case of power grid), and this is continously happening with an AC circuit.
     
    Last edited: Jan 5, 2015
  16. Jan 12, 2015 #15

    anorlunda

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    I agree. There seems to be a surge of posts similar to OP on PF lately. It makes me wonder if there has been some change in secondary school education that teaches these misconceptions. (common core???) I have long been suspicious of science teachers, textbook authors, and curriculum designers who don't themselves properly understand the science or the math.
     
  17. Jan 12, 2015 #16

    sophiecentaur

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    Education is a political issue. Politicians like to be able to claim that they are responsible for a well educated populace. They are, in general, very limited in their appreciation of Science and rely on what the Ivory Tower Scientists say - or what they think they can hear them saying. The boffins said "Particles" so now everyone is taught particles. But the particles that the boffins talk of (Feynman, particularly) are not the particles that kids and teachers know and love (bullets and peas). So between them, they end up exposing a false idea of quantum particles to the kids. This idea propagates through the generations. The small minority of 'informed' Science teachers are not influential enough to put the rest of them right.Frustrating or what?
    Feynman (only a little lower than the angels) is responsible for this problem because he insisted that his particles are particles - not realising that his ideas were more sophisticated than most people (kids and students in particular) could take on board. His teaching style was such that people who heard him, thought that they understood what he said. A radar assisted collision.
     
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