chapter
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P

The diagram shows a parallel plate capacitor with the distance between the plates exaggerated so that the field lines (electric) can be shown clearly.

The capacitance of the arrangement is defined in the equation: . _ charge stored on one plate

capacitance - p0 ten tja| difference between plates

All the lines of force go from the positive charge + Q on the upper plate directly to the negative charge - Qon the lower plate. The number of lines of force is Q / s 0 since there are l / s Q lines of electric force per unit charge. The electric field strength E is the

a+b

a

a+b

a b

b a

a-b

1111 Area A

number of lines of force per unit area. If the area of the plates is A, then:

QE =

But

Asn

X . - _Q_ d Aen

and since C= Q /V

C = EA. d

If the space between the plates is filled with insulating material, the capacitance of the arrangement changes by a numerical factor £r where £r is called the relative permittivity of the insulating substance. The equation for the capacitance becomes:

How this increase in the capacitance comes about is indicated in the diagram below.