1. - + 23.4 Dielectrics Action of a Dielectric
*The ability of a capacitor to store charge can be increased by inserting an
insulating material, called a dielectric, between the plates.
Molecules in a dielectric become
polarised by the electric field.
Positive nuclei are attracted
towards the negative plate
and the electrons towards
the positive plate.
The positive side of the dielectric
attracts more charges onto the
negative plate. ( similar – ve side )
( or we could say ‘ the positive potential
of the positive plate is reduced by the
negative charge at the surface of the dielectric.
Therefore the pd across the plates is lower
and more charge is needed for the capacitor
pd to become equal to the applied voltage) - +
E = W Q
Producing heat !

2. + 23.4 Dielectrics Polarised molecules
* Some molecules eg water, are already polarised and turn within the dielectric
in an electric field.
* Polarised molecules in a dielectric greatly increase the ability of a capacitor
to store charge. +
E = W Q
Producing heat !

3. Permittivity of material
Dielectrics
Area of plates
Capacitance of a parallel plate capacitor = ε A d
C = ε0 ε r A d
plate separation
Permittivity of material
between the plates
Dielectric constant ( air = wax = glass )
εr = charge stored with dielectric between the plates = Q = C V
charge stored with vacuum between the plates Q C0V
E = W Q
Producing heat !
εr = capacitance with dielectric = =
capacitance in a vacuum

4. Measurement of relative permittivity of the dielectric
Dielectrics
Measurement of relative permittivity of the dielectric
The Ammeter receives 400 charges
each second with the dielectric :
I = Q
I = CV
I = V
f = 400 Hz A
The Ammeter receives 400 charges
each second without the dielectric :
I0 = Q
I0 = CV
I0 = V
εr =
εr = I Io