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Electricity and Magnetism RC Circuit Conceptual Questions

  1. Jul 28, 2015 #1
    Edit: sorry i realized i posted this in the wrong section. Can some move it to hw? Thank you.

    Hey i was studying intro E&M recently and i started learning about RC circuits. Up until this point everything, concept wise, has made sense for me. Then I learned how RC circuits work, and the math makes complete sense to me, but a lot of it seems to defy what i already know. I can't find any answers anywhere, and even textbooks seem to ignore what i am having a hard time understanding. I'm hoping it's just a simple misconception that i'm having, because i don't think anyone else has these questions.

    1. First i'm curious why an uncharged capacitor does not contribute to the resistance of a circuit. I under stand that since it's uncharged the voltage drop across it is zero, but why doesn't the same apply to an open circuit. Isn't it harder for charge to move from one plate to the other through the space.

    2. Second, if positive charge is moving from the positive terminal of the batter to the negative, why does charge build up on either plate. Why doesn't the charge just continue through the whole circuit to the other end of the battery. And what makes the charge build up on the specific plate that it does. I can't really find an explanation. I don't think i really need to understand this because mathematically it makes complete sense, but it's really bugging me.

    I think i might have more questions but i'll stick to these for now. Thanks in advance.
     
    Last edited: Jul 28, 2015
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  3. Jul 28, 2015 #2

    sophiecentaur

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    Hello and welcome to PF.
    Resistance involves dissipation of energy (heating etc) and there is no fundamental loss of energy involved in putting a charge on one plate and an equal and opposite charge on the other. The energy to do this is stored energy and not dissipated. In reality, when AC is applied to a capacitor, the material used in the dielectric will actually dissipate a small amount of power so a real capacitor has a loss factor.

    Remember, there is no extra 'net' charge involved. It is always an imbalance on charge. The capacitor and its dielectric become Polarised.
    The battery maintains a PD across its terminals (that's its job) and will keep supplying charge to the capacitor (+ to the + terminal and - to the - terminal) until the PD across the capacitor equals the battery PD.
     
  4. Jul 28, 2015 #3
    Okay thank you. But why does the net charge stay stationary, what makes it not complete the loop all the way to the battery? Why does it stay on the capacitor?

    Also what's the difference between a capacitor in a circuit and an open circuit? Isn't an rc circuit an open circuit? so why does it behave differently?

    Thanks again.
     
    Last edited: Jul 28, 2015
  5. Jul 28, 2015 #4

    sophiecentaur

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    The charges (electrons) do not move very far or very fast on average. Drift speed is mms/second. Charge flows all around the circuit but there is no 'local imbalance' within the wire because you couldn't tell whether a group of electrons in a given place were there 1 second ago -could you?
    If you want to think of it as an "open circuit" you could - eventually - because no more current would be flowing through the resistor. A Capacitor can accumulate a big charge (imbalance) with a small voltage but a bare end of wire could only collect a small charge imbalance. The Capacitances are very different and Q = CV. It is possible to lose a lot of sleep trying to impose classification on situations where it is not necessary.
     
  6. Jul 28, 2015 #5
    Are these DC circuits or AC circuits?
     
  7. Jul 28, 2015 #6
    A capacitor is a temporary charge storage device. You know what a real capacitor is don't you. essentially two large sheets of metal foil separated by a thin insulating material. That metal foil can accommodate a certain amount of additional charge depending on the voltage between the two foils. That charge is given by the relation Q=CV. A true capacitance has an infinite resistance because it is an open circuit, BUT since it can accommodate a certain amount of charge depending on the available voltage applied say from a battery electron produce by the chemical reactions of the battery will move into the capacitor until the voltage given by V = Q/C is reached. How fast it flows depend on the resistance between the battery and the capacitor which can be on either side of the capacitor. If the resistance is zero the flow time is zero. But the electrons flowing into the capacitor are block by the insulator so they never reach the other terminal of the battery. They just pile up in the capacitor. these electrons that have fowed into the capacitor repel an equal number of electrons on the opposite foil leaving a net excess of positive charges equal to that of the negative charges on the opposite foil. you disconnect the battery then the capacitor has an excess charge Q =CV will be stored theoretically indefinitely in the capacitor.
     
  8. Jul 28, 2015 #7
    Omg thank you so much that makes sense now. I though the charge was making a complete loop. I don't know why and videos I saw didn't talk about this. I knew it was something simple.
     
  9. Jul 28, 2015 #8

    sophiecentaur

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    But there is a loop. From one plate, right round the charging circuit to the other plate. But don't think of charge actually racing around like on a track. The charge carriers travel very slowly. It is the effect of the small scale repulsion between all the electrons, all the way along a conductor that causes n electrons to enter one end of a wire and n to exit the other.
     
  10. Jul 28, 2015 #9
    I thought a loop means a closed path. Maybe in British English can be open too. :smile:
     
  11. Jul 28, 2015 #10

    sophiecentaur

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    It depends how bothered you are about applying elementary school ideas to non-school circuits and components. The current will flow 'through' all series resistive components and connecting wires - right up to the actual plate of the capacitor. the same current carries on on the other side of the gap. Whether you want to say it stops and magically appears the other side or you think in terms of charge build up on the plates, is up to you. Is the phrase "closed loop" really relevant?
    If in doubt - do the sums, when considering what goes on in electric circuits.
     
  12. Jul 28, 2015 #11
    Closed loop will be redundant. All loops are closed.
    I thought you explained pretty well the first time. The charge moves from one plate to the other, going through the battery and all circuit elements.
    I just had an issue with calling this path a loop, sory if you understood that i did not like your explanation. I actually found it to be the clearest.
     
  13. Jul 29, 2015 #12
    Yeah the current is the same through the whole circuit. But just to make sure an individual electron would not make a complete loop because it cant jump across the plate, right? I just like to know the details of things, i hate it when my teachers dumb down explanations to make them easier to understand. They always end up over simplifying the concepts.
     
  14. Jul 29, 2015 #13
    There is a net movement of charge but the electrons on one side of the cap remain on that side, they just pile up.
     
  15. Jul 29, 2015 #14

    sophiecentaur

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    That's a refreshing attitude. PF is full of posts from people who want a dumb explanation of things and expect it to give them a non-dumb understanding.
    No pain no gain.
     
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