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Question about AC circuits

  1. May 27, 2014 #1
    Hey, I don't if this is the right place to ask ( it is not a homework question but just a curious one about what i read in my textbook)..

    How can AC circuits even work? If the drift speed of an electron in a normal household wiring is something in the micro m/s and it reverse every 1/120 per second, how can it even get anywhere?

    Plus if the current is reversed and another one is induced by self induction, wouldn't it get in the way of the current too ( I mean if the Emf is in the opposite direction... ) so it would slow it down even more?

    AC currents kinda get me confuse :/

    Edit: if you explain, try to keep it within my realm of understanding, I only have DC circuits and a bit of AC circuits understanding. I don't really understand the fact that all the expression of an RLC circuit are derived from one equation of induced emf... I can somewhat apply them but blindly.
  2. jcsd
  3. May 27, 2014 #2
    talking first about conduction in DC ,we can understand it by considering a straight metal wire being kept in a uniform electric field. Clearly almost every electron in the conductor experiences this electric field simultaneously and starts to accelerate . But presence of ions only allow them to drift with some finite average velocity.
    Same thing happens to this conductor when it is connected to a battery. A battery gives a Potential difference which in turn generates Electric field across the conductor.
    As before almost every electron experience this force and drift. In this way it can make a round trip.

    It can be easily generalised in the case of AC . But if its frequency goes too high then Yes its possible for electrons to rarely make a round trip across circuit. But That doesnot stop it to "light a Bulb" (which is caused by the resistance of the filament ).

    Also I agree u with your point that current reduces when AC is passed though a inductor by self inductance...this is back EMF that offers a resistance along with thermal one.
  4. May 27, 2014 #3
    well wouldn't the use of an induce emf be counter active then??
  5. May 27, 2014 #4
    If u hav taken an undergraduate course on elecrodynamics then u may hav come across an expression of the current flowing through a circuit containg an inductor , resistor and an AC source in series as
    I0 = V0/sqrt(R2+(ω L)2)
    ...ω is the freq. of the source...L the inductance
    And phase difference between the current and source
    I=I0sin(ω t +ø), V=V0sin(ω t)
    tan(ø)= [itex]\frac{ω L}{R}[/itex]
    I hope these eqs. will help u to clear ur doubt.
  6. May 27, 2014 #5
    I suspect your confusion with AC has to do with the common analogy of water through a hose, one designed to give us a frame of reference with which we feel comfortable. However it is a limited analogy since up here in "our world" water must actually flow (and preferably in a unidirectional manner) to do work like a river over a water wheel or through a turbine. From our point of view it seems that a molecule of water in the river which once was "here" is now "there" and later, further along that path, seemingly never to return to it's original position, however briefly.

    "Seemingly" is the key word because on Earth water is also a largely closed system. Given enough time any given molecule of water will indeed likely return to it's once observed location. It's just not within our eyesight or lifetimes. If you "stand back" from our limited normal perspective and see that it is "the flowing" that has energy and can do work and that it really doesn't matter if it is unidirectional or an oscillation as long as the device harnessing that energy is non-directional, like a resistance.
  7. May 27, 2014 #6


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    Energy is transferred from the battery to the resistor in a way, analogous to how a bicycle chain transfers energy from the crank to the wheel. You can use a high gear or a low gear; the chain can move at various speeds but, if you are going up a hill at a given speed then the power throughput in all gears will be the same. So the chain speed is not what counts (i.e. not the only thing).
    AC could be likened to having a fixed wheel and doing alternate backward and forward strokes on the pedals. The wheel will alternate in direction (not getting anywhere) but you could warm up the brakes - transferring power via the chain. So the charges don't actually have to go anywhere (no net flow) for energy to be transferred.
  8. May 29, 2014 #7
    Electric information is transmitted as a guided electromagnetic wave at the speed of light. So every electron along the wire 'learns' that a source of emf has been applied across the circuit almost instantaneously. But you know that the electron drifts at a very tiny speed ( compared to it's average thermal velocity ). What's crucial is that a whole bunch of these electrons drift at this very negligible velocity. So an appreciable amount of charge is transferred across any cross section of the conducting material ( or device ) in the interval between the reversal of the direction of the emf. This leads to an appreciable current. This produces the effects that you see hundreds of miles away from the source of the applied emf.

    So what's probably confusing you is, why use AC at all ? There are a number of reasons, most notably, AC allows you to use induction effects to step up the voltage and cut down on transmission losses. Also, you can step it down once it gets to households. And it's pretty easy to convert AC to DC.
  9. May 29, 2014 #8
    To answer your second question, it is counterproductive if you don't want your energy stored in magnetic fields. Inductors are a crucial part of oscillator circuits for example.
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