Dielectrics Flashcards
What is a dielectric?
A fixed lattice with the electrons bound to the nuclei
There are no free charges
What happens when we apply an electric field across an atom?
The centre of the electric charge distribution will move in the opposite direction of the electric field whilst remaining bound
Where are the charges on a dielectric?
On the surface
What is the polarisation density?
The total local atomic electric dipole moment per unit volume
P(r)=1/𝒱 ∑p_i
What is the electric displacement vector?
The combination of the external electric field and the polarisation density
D=ϵ0E+P=ϵ_r ϵ_0 E
What is the relative permittivity?
A reflection of the response of the dielectric
Denoted by ϵ_r and ϵ_r is ≥ 1 always
What is the permittivity of a material?
ϵ_r ϵ_0
What is the electric suscpetibiility?
χ_e= ϵ_r − 1
What is the capacitance of a dielectric?
C= ϵ_r C_0
With C_0 the capacitance when the plates are separated by a vacuum
What is the capacitance of a parallel plate capacitor with a dielectric?
C_0=ϵ_r ϵ_0 A/d
How does a dielectric affect electrostatic energy?
U=U_0/ϵ_r
As ϵ_r is ≥ 1 the energy decreases
What is the dielectric breakdown?
When under a sufficiently strong electric field the electrons in a dielectric become unbound and it becomes a conductor
What is the maximum electric field that a dielectric can withstand?
The dielectric strength denoted by E_m
