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Voltage & Capacitance

  1. Feb 9, 2014 #1
    As everybody already knows, voltage is defined as the work done/energy per unit charge. I see voltage as being the idea of giving the charge carriers a 'want' to move: when a closed circuit is attached to a battery, the voltage makes the charges want to move, and they can. Yet, in an open circuit they want to move but can't. Eventually if you were to increase the voltage between the gap in the circuit to the breakdown voltage of the air, you'd get a spark and a quick burst of current - is this all correct so far?

    However, I've stumbled across something when revising capacitance that's confused me:

    We know that current is the flow of charge, and when a circuit is broken, the current(flow of charge) stops. In a capacitor, there are two metal plates, separated by an insulating material, thus meaning the circuit is broken, therefore no flow, therefore no movement of charge carriers; no dispute.

    The book then goes on to say that the electrons spread out across the plate attached to the negative terminal of a battery. Now I thought there would need to be a current for the electrons to move and "spread out" along the surface. Evidently, there is a voltage, therefore a 'want' to move, but as I understand, because the circuit is broken, they shouldn't be able to do so?

    Thanks for any help given in advance,
    LanguageNerd
     
  2. jcsd
  3. Feb 9, 2014 #2
    There is very rapid current flow for a very short amount of time. During this time, negative charges build up on one plate of the capacitor, and deplete from the other plate. This goes on until the negative charge on the negative plate and the positive charge on the positive plate just balance the voltage across the capacitor. Then the flow of current stops. This all takes place very rapidly, and involves very little flow of charge.
     
  4. Feb 9, 2014 #3
    How, exactly, can this current flow when the circuit is not connected? Throughout all my studies, they've drawn the conclusion that when the circuit breaks, or in this case never was connected, the current immediately stops , or never flows to begin with.
     
  5. Feb 9, 2014 #4

    sophiecentaur

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    Your studies have always considered the 'steady state' after things have settled out. When you 'charge something up', current flows for a short time.
     
  6. Feb 9, 2014 #5
    To elaborate on what SC said, this isn't quite like two wires with a big open space between them. The capacitor plates are very close together, and the short burst of current that occurs allows charge to build up on the negative plate and get depleted from the positive plate. They call it a capacitor because it has the capacity to build up charge.
     
  7. Feb 9, 2014 #6

    sophiecentaur

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    That's a point well worth making.
    The capacitance between two random wires near each other will be a few picoFarads, whereas the capacitance of a 'big' capacitor, available these days, can be 1 Farad (10^12 times bigger!!!)
     
  8. Feb 9, 2014 #7

    Drakkith

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  9. Feb 10, 2014 #8
    Keep in mind that there is something called displacement current which propagates across the gap of a capacitor as it charges and discharges. This confusion about capacitors is very common based on many questions on these boards. It also confused Maxwell who discovered the displacement current in an effort to explain it.

    The displacement current occurs when the electric field changes in time. It has the same units as current but does not require mobile charges to propagate.
     
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