Leaky capacitor and movement of charges

  • Thread starter Papikoss
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  • #1
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Assume that we have a parallel plate capacitor filled with an imperfect dielectric. Then we apply a DC Voltage between the plates and expect some leakage current through the capacitor due to the dielectric having non-zero conductivity.

My question regards the movement of electric charges within the dielectric and through the electric circuit.

My best guess is that the (limited) charged particles, which are provided by the dielectric media, move under the influence of the electric field towards the oppositely charged plate, neutralizing some of the charges contained within the conductor. Now since the capacitor’s charge Q = CV should remain constant, some electrons have to move from the positively charged plate to the negatively one through the source of the voltage, thus closing the circuit.

If that’s correct, then the current flow should stop when the dielectric cannot provide any more charged particles, which should all be located on the plates of the capacitor at that time.

Is it realistic to assume that those particles are abundant, or maybe that there is some ‘hopping’ of electrons from the conducting plate to the dielectric media and vice versa, in a similar way to the electrons moving from a conductor of high conductivity to a conductor of lower conductivity?

Or do I have it all wrong? :confused: Can you provide me with some articles explaining the phenomenon?

Thank you in advance!
 

Answers and Replies

  • #2
kuruman
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The particles that move are electrons, nothing else, and yes they are abundant. If you understand how electrons move through a resistor connected to a DC voltage, you should also understand this. Just consider that the dielectric is a resistor.
 
  • #3
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Thanks a million for your reply! I’ll have to disagree though!

The particles that move could be ions, as in the case of salt water, and they can’t be abundant since the concentration of salt is limited.

Moreover, I think that there is a fundamental difference between a resistance and a capacitor connected to a DC Voltage. In the latter case I’m not sure if electrons leave behind the conducting plates and flow through the dielectric making complete loops around the circuit!

That is the point I’m trying to clarify: How does the circuit close, producing a steady leakage current?

Of course, as I said, I may have it all wrong! :smile:
 
  • #4
SammyS
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Thanks a million for your reply! I’ll have to disagree though!

The particles that move could be ions, as in the case of salt water, and they can’t be abundant since the concentration of salt is limited.

Moreover, I think that there is a fundamental difference between a resistance and a capacitor connected to a DC Voltage. In the latter case I’m not sure if electrons leave behind the conducting plates and flow through the dielectric making complete loops around the circuit!

That is the point I’m trying to clarify: How does the circuit close, producing a steady leakage current?

Of course, as I said, I may have it all wrong! :smile:
Generally, a leaky capacitor IS modeled as an ideal capacitor in parallel with a resistor. Look at your initial post. It suggests that the dielectric has non-zero conductivity - implying resistivity.
Assume that we have a parallel plate capacitor filled with an imperfect dielectric. Then we apply a DC Voltage between the plates and expect some leakage current through the capacitor due to the dielectric having non-zero conductivity.
...
 
  • #5
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Modeling the leaky capacitor as an ideal capacitor in parallel with a resistor is not convenient for me right now because I lose what really happens as far as the establishment of a closed circuit and current flow within the dielectric is concerned. :smile:
 

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