Voltage in a capacitor

  1. Hi there
    What does it mean when there is a viltage across the plates of the capacitor please make the explanation easy and I would be so grateful.
    Thanks in advance.
  2. jcsd
  3. Simple explanation: A voltage across a capacitor means one plate has excess positive charges while the other has excess negative.

    More involved explanation: If there's a voltage across a parallel plate capacitor it means that there are more of one kind of charge on one plate than the other; whether an excess of positive charges on one side or negative depends on how the power source is applied (This is assuming you're dealing with a DC source). Capacitors act like temporary batteries- they hold charge; and that, essentially, is what it means to have voltage across a capacitor.
    If it's DC the capacitor will charge up and current will stop flowing in the circuit. When the battery is removed, yet the circuit is still not complete, the charge inside the capacitor will remain. If the circuit is completed with a resistor, the charge stored in the capacitor will flow and the voltage across the capacitor will decrease over time. If the circuit is completed with an inductor the current and voltage will oscillate between the two. All the energy of the circuit, at some instance, is stored as voltage in the capacitor; then, at some other instant, all of the energy of the circuit is stored as current in the inductor.
  4. Thank you very much for this explanation but what it reall means when we say the potential difference between the plates of the capacitor is 12 volts
  5. And why the capacitor stops charging when the potential difference equals the emf source ?????
  6. nsaspook

    nsaspook 1,381
    Science Advisor

    I would say that capacitors and batteries store energy not charge if you mean electric charges. The total amount of charge remains the same in both. As we apply a voltage across the capacitor (that implies a circuit of some sort with a path to both ends of the device) a excess of charge is on one side but a equal amount moves from the other plate in the circuit so charges are separated. This separation of charge creates a electric field that increases as more charge is separated until it equals the applied voltage and is in equilibrium (no net potential around the circuit to move charges) with balanced charges on each plate. If we then disconnect the capacitor from the circuit there is not an excess of charge in the capacitor as a whole but there is a charge imbalance and a electric field (voltage) between the plates that stores potential energy.
  7. Thank you very much indeed
  8. nsaspook

    nsaspook 1,381
    Science Advisor

    It's easy to get confused with the common usage of the term 'charged' when dealing with the physics meaning of charge. 'Energized' might have been a better word as the net charge remains the same at zero volts or a thousand volts. :biggrin:
  9. Yes, I'm sorry. That's my mistake. Thanks for clearing that up for those who might not know. :D
    Last edited: Aug 22, 2014
  10. nsaspook

    nsaspook 1,381
    Science Advisor

    I've made the same mistake in the past in a casual remark.
    Another problem is when you look-up the definition of capacitance it usually comes back with something like this: http://hyperphysics.phy-astr.gsu.edu/hbase/electric/capac.html

    If you notice closely it actually says:
    That's only part of the story as it's really defined by the geometry of the capacitor's physical structure and the dielectric constant of the material medium in the electric field. The formula is a relationship of charge and voltage to a capacitor's capacitance that's always the same if the physical structure doesn't change.
    Last edited: Aug 22, 2014
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