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I've read quite a lot of times about leakage of charge in capacitors, but what does it exactly mean?

Do we see sparks? Or is it some current flow? Or something else?

Say I have a charged sphere, so where will the charge go on charge leaking? (Assuming that it is has an electric field less than the breakdown strength of the medium)

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Leakage of charge in capacitors can have different reasons in practice. Charges may escape into an imperfect insulator, or flow through the insulator as a tiny current. Charges may also escape via surface conduction on the capacitor itself or on the PCB it is mounted on. The charged sphere in air may lose charge via surface conduction of its support, or even via charging air molecules or neutralizing ions present in the air. Very thin insulators may allow for small currents between capacitor plates due to quantum tunneling. Depending on the exact mechanism and measurement method, discharging could be seen as discrete events (e.g. of single charge escape) or as a steady continuous process.

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Excess charges in a metallic conductor always try to go as far as possible from each other or in other words, they try to lower their potential energy. The charges try to do this by moving to a region of low potential energy (electrically speaking).

For example, leaking in a capacitor means that the negative charges move from the negative to the positive plate. This way the charges reduce their potential energy and finally there is no net charge present in the cap plates. This would give rise to a current for a short time. And sparks are nothing but ionization of the surrounding material (air in this case). This happens when the electric field is strong enough to overcome the dielectric barrier. Sparks can be seen when you short a cap.

Now back to your question,

As per the details provided in your question, the metallic charged sphere will want to leak charges to a region of low potential energy

(say a negatively charged entity or a neutral entity or the ground).

But the charges have no way to do so if the material surrounding the sphere can ideally provide sufficient insulation and also you have to make sure the supporting entity on which the sphere sits can ideally isolate it from the ground.

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Capacitor leakage occurs in several ways. First, if the insulation material that coats the outside of the capacitor is slightly conductive, it is possible for the excess electrons on one plate of the capacitor to slowly migrate to the opposite plate and slowly cancel the charge being held by the capacitor. Second, if the dielectric separating the plates is slightly conductive, then the same slow process will occur, and the capacitor will eventually become completely discharged.

It is normal for capacitors to slowly "leak" in this way, but once a conductive path begins to develop in one particular spot either through the insulation or through the dielectric then the process can grow into a fast one, in which the power dissipated by the leakage current is sufficient to cause the capacitor to suddenly explode with a loud bang and sometimes to catch fire.

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