# unit 4 section 3 capacitance

## capacitance

the capacitance of an object is the amount of charge it is able to store per unit of potential difference across it.

C=Q/V

C in farads, f        Q in coulombs, C          V in volts, V

Capacitor: an electrical component that can store charge, made up of two conducting plates seperated by a dielectric.

- when a capacitor is connected to D.C., charge builds up on its plates.

- one plate becomes +ve and one becomes -ve

- plates are seperated by electrical insulation so no charge moves between them.

-this means a potential difference builds up between the plates.

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## capacitance part 2

uses of capacitors:

- camera flash, battery chargs the capacitor then all the charge is instantly used as the flash

-'ultracapacitors' are really big capacitors that can be used in back-up power supplies to provide reliable power for short periods of time.

-to smooth out variations in D.C. voltage supplies - a capacitor absorbs the peaks and fills in the peaks.

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## Charging and discharging

Q = Qo e -t/RC

Q = charge of capacitor at the time

Qo = charge of capacitor when fully charged

e = constant on calculator and formula sheet

-t = time since discharging began

-R = resistance of fixed resistor

-C = capacitance of capacitor

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