- + 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 !

+ 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 !

Permittivity of material 23.4 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 = 1.0005 wax = 2 glass 5-10 ) εr = charge stored with dielectric between the plates = Q = C V charge stored with vacuum between the plates Q0 C0V E = W Q Producing heat ! εr = capacitance with dielectric = = capacitance in a vacuum

Measurement of relative permittivity of the dielectric 23.4 Dielectrics Measurement of relative permittivity of the dielectric The Ammeter receives 400 charges each second with the dielectric : I = 400 Q I = 400 CV I = 400 V f = 400 Hz A The Ammeter receives 400 charges each second without the dielectric : I0 = 400 Q I0 = 400 CV I0 = 400 V εr = . εr = . I Io