Definition & Determination of Capacitance

• Definition of Capacitance

• Uses for Capacitors

• Capacitor Symbology

• Properties of a Capacitor

• Determination of Capacitance

Capacitors

• A capacitor is a device which

stores electric charge on two

conducting plates, separated

by a finite gap.

• Electric potential energy is stored in a uniform electric field, generated by the charge distribution on the two plates

Uses for Capacitors• Capacitors are used to dissipate electric

energy to run a camera flash

• A capacitor can provide energy to back-up computer power supplies

• Capacitors can block low frequency AC signals, and act as power supply filters

Capacitor Symbology• Capacitor schematic symbols

• vs. Battery schematic symbol

Properties of a Capacitor

• When a capacitor is connected to a source of potential difference (battery), the electrons are drawn to the (+) terminal of the battery & flow to the other plate, until equilibrium has been reached

Properties of a Capacitor• The electron debt on the LH plate leaves the

plate with a net (+) charge. The excess electrons on the RH plate leaves the plate with a net (-) charge.

• +Q -Q • The establishment of this charge

distribution leads to the development

of a uniform electric field between the

plates

Capacitance Parameters

• The amount of potential difference (ΔV) that can be applied across the plates is limited by the amount of charge that can be accumulated on the plates & is given by the relationship:

• SI Units: Farads (1μF = 1.0 x 10-6 F)

V

QC

Determination of Capacitance• The electric field between the plates

Is mostly uniform

• The capacitance does not depend on Q or ΔV but only on the geometry of the device

area

echiswhere

A

QE

arg1

00

A

QddEV

0

d

A

V

QC 0

The Coaxial Capacitor

b

a

R

R

lC

ln

2 0

Capacitance of Parallel Conducting Wires

• 2 conducting wires, of radius R, and length l

R

dl

Cln

0