The Physics
Hypertextbook
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# Capacitors

## Summary

• A capacitor is…
• a device for storing separated electric charges.
• a pair of oppositely charged conductors (called plates even if they aren't flat) separated by an insulator (called a dielectric).
• The capacitance (C) of an electrostatic system is, by definition, the ratio of the quantity of charge separated (Q) to the potential difference applied (V). C = Q V
• The SI unit of capacitance is the farad [F], which is equivalent to the coulomb per volt [C/V].
• One farad is generally considered a large capacitance.
• The energy stored in a capacitor can be calculated using one of the following equations… Uc = ½QV = ½CV2 = ½ Q2 C
• The capacitance of a parallel plate capacitor is.
• directly proportional to the area (A) of one plate
• inversely proportional to the separation (d) between the plates
• directly proportional to the dielectric constant (κ the Greek letter kappa) of the material between the plates
 C = κε0A d
• The capacitance of a cylindrical capacitor is given by C = 2πκε0ℓ ln(r2/r1)
where  ℓ = length b = outer radius a = inner radius
• The capacitance of a spherical capacitor is given by C = 4πκε0 (1/r1) − (1/r2)