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Capacitors lose charge over time, even when they are disconnected. Why does it happen?

Is there a way to keep the charge longer, like for years.

If you cover the plates with better insulator, will it reduce the charge loss?

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That's because the material between the plate is a good, but not perfect insulator and it has a non-infinite resistance. Therefore it acts as a very high resistance across the terminals of the capacitor, leading to slow self-discharge.

Of course using better insulator materials could lead to lower self-discharge rate, as it is exploited in FLASH memory chips: in flash memory the bits are stored in tiny capacitors (formed by a floating gate inside a MOS structure) that can retain their charge for years.

Since self-discharge is due to dielectric resistance, you can improve it (i.e. make it bigger) by using a dielectric with higher resistivity and with higher thickness. Increasing the thickness of the dielectric between the plates means decreasing the capacitor capacitance, though. Moreover, also using a dielectric with better insulating properties can lead to a decrease in capacitance since some good insulating materials have poor dielectric constants (i.e. low relative permittivity).

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