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if I make a circuit with a battery and a capacitor (with a dielectric inside), how it is possible to get a current in the circuit? If electrons go from one pole of the battery and they arrive to one plate of the capacitor they cannot flow to the other plate of the capacitor because the plates are separated by a dielectric and this dielectric doesn't have free charges, they are bound charges.

How can I explain a net flow of electrons in this circuit?

Thanks.

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How can I explain a net flow of electrons in this circuit?

The crucial fact to keep in mind is that each plate of the capacitor has a vast amount of mobile electrons.

To charge a capacitor, mobile electrons are 'pumped', via the external circuit, from one plate of the capacitor to the other plate in order to charge the capacitor.

This leaves the plate that supplies the mobile electrons positively charged and the plate that receives the electrons negatively charged.

A capacitor discharges when the external circuit allows electrons from the negatively charged plate to travel 'back' to the positively charged plate.

To the external circuit, the capacitor is not an open circuit even though there is no conduction current through the dielectric.

From a circuit theoretical point of view, there is an electric current through the capacitor and that current is proportional the time rate of change of the voltage across.

From an electromagnetic point of view, there is a displacement current (changing electric flux) through the dielectric and this current produces a magnetic field in the same way that a conduction current does. So, on this view, there is a current through the dielectric.

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  • $\begingroup$ I just want to explain my students what will happen with one electron coming from one end of the battery. Would it be stacked in one of the plates? Then how could I explain easily the current flow? Thanks for your answer. $\endgroup$ – armando Jan 24 '14 at 21:56
  • $\begingroup$ @armando, have I not already addressed the questions in your comment? $\endgroup$ – Alfred Centauri Jan 24 '14 at 22:14
  • $\begingroup$ I got confused with the words in quotation marks such as "pumped", "back", do you mean that the electrons are not really moving? $\endgroup$ – armando Jan 24 '14 at 22:28
  • $\begingroup$ @armando, no, electrons are moving through the external circuit. The scare quotes are there to indicate that the terms are not to be taken to literally. Electrons, for example, are indistinguishable. So, we can't meaningfully claim that a particular electron left one plate and moved to the other. $\endgroup$ – Alfred Centauri Jan 24 '14 at 23:36
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The current lasts only for a while necessary to charge the capacitor to voltage that will oppose the electromotive force of the battery.

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You are correct; electrons can't flow across the dielectric (unless there's dielectric breakdown, but that's another story).

The usual interpretation of what's going on is that the electrons arriving at one of the plates just bunch up there, without jumping across, while at the other plate electrons are leaving.

If one looks at the circuit as a whole, it does appear that there's a continuous current, as you've indicated, especially since if one measures the current next to each plate you'll find the same magnitude. But one must remember this isn't actually the case.

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