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Capacitors: Voltage drop

  1. Dec 28, 2016 #1
    Where does the voltage drop of a capacitor happen? My answer would be that It produces an electric field the opposes the field of the battery along the wires. So it is continuous lose to the capacitor which stores this energy in it. Another question related to this, When a positive charge hits one terminal of a capacitor another fires away from the other. T I can see why this happens by saying that the current in and out must be equal because of KCL. But can this be related to Newton cradle? Where one only can go out to conserve momentum and energy? There is no voltage drop when a charge "move" from one side to another.
  2. jcsd
  3. Dec 28, 2016 #2


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    Staff: Mentor

    The charges on the plates establish an electric field between the plates. The greater the charge the larger the field. If you check the units for an electric field they are volts per meter (v/m). The plates are separated by some distance d. So the potential difference between the plates is V = d*E. So the answer is, the "voltage drop", or potential difference, occurs in the space between the plates.

    As to your second question, when we analyze electric circuits we treat charges as being massless, and so there's no mechanical momentum associated with them. So the Newton's cradle isn't a particularly good analogy if you're thinking of attributing momentum to charges. That said, there is an analog of momentum in electrical circuits that's due to inductance. Inductors (wire coils in particular) store energy in a magnetic field when current is flowing through them. This is analogous to the way a mass "stores" energy in the form of kinetic energy when it has a given velocity. In fact, just as a mass has inertia and resists sudden changes in velocity, an inductor resists sudden changes in current.
  4. Dec 28, 2016 #3
    How does the voltage drop happen between the plates even though charges dont pass to the other side? So they are not effected by the electric field between the plates.. Not sure tbh

    So I just have to stick to the KCL reason.. I just wanted a reason to satisfy why a each terminal has the same charge in the situation of two terminal and one between them to form two capacitors each of them has the same charge.
  5. Dec 28, 2016 #4


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    Voltage drop is a potential difference that occurs in an electric field. There's an overall electric field that is established in a circuit by some EMF that follows the path of the wiring and components. The "gradient" of the field is modified locally by various components. The field within the capacitor is just part of the overall "field circuit". Similarly, the potential difference across a resistor with current flowing through it is associated with an electric field within the resistor.
    KCL is an adequate explanation for understanding at the circuit analysis level. If you want to go deeper into the underlying physics then you need to return to the study of electrostatics and how charges rearrange themselves on conductive surfaces and objects. The concepts of induced charges, charge separation, electric potential, etc., come into play.
  6. Dec 28, 2016 #5
    It struck me how I forgot that. I was thinking more about charges rather than actually loops of "Field circuits" where the overall voltage is zero. Not sure why though :sorry:

    Yes, We have KCL and KVL this year only. But I read a book called Matter and interaction which was enlightening which discussed all of that. I have studied Drude model which explains all this in much intuitive. Thought it might help if I choose Electrical Engineering in the university. Two months without training made me forgot some concepts :/

    Thanks again gneil. Much appreciated
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