Parallel-Plate Capacitors

area A (

2

m

)

5

plate separation d (mm)

5

voltage V (V)

5

dielectric constant

κ

e

5

The capacitance of a parallel-plate capacitor (or condenser) is given in SI units by

C=

ϵ

0

κ

e

A/d

, where

A

is the area of each plate,

d

is the spacing between plates,

κ

e

is the dielectric constant (relative permittivity), and

ϵ

0

is the permittivity of free space,

8.85×

-12

10

farad/meter. If

A

is expressed in

2

m

and

d

in

mm

, then

C=8.85×

-3

10

κ

e

A/d

microfarads (

μF

). Capacitance determines the quantity of positive and negative charges

Q

that can be held on the plates by a voltage

V

, such that

C=Q/V

.

In an air-gap capacitor, in which there is no dielectric layer,

κ

e

≈1

.