Dielectrics Flashcards

1
Q

What is a dielectric?

A

A fixed lattice with the electrons bound to the nuclei
There are no free charges

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2
Q

What happens when we apply an electric field across an atom?

A

The centre of the electric charge distribution will move in the opposite direction of the electric field whilst remaining bound

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3
Q

Where are the charges on a dielectric?

A

On the surface

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4
Q

What is the polarisation density?

A

The total local atomic electric dipole moment per unit volume
P(r)=1/𝒱 ∑p_i

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5
Q

What is the electric displacement vector?

A

The combination of the external electric field and the polarisation density
D=ϵ0E+P=ϵ_r ϵ_0 E

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6
Q

What is the relative permittivity?

A

A reflection of the response of the dielectric
Denoted by ϵ_r and ϵ_r is ≥ 1 always

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7
Q

What is the permittivity of a material?

A

ϵ_r ϵ_0

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8
Q

What is the electric suscpetibiility?

A

χ_e= ϵ_r − 1

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9
Q

What is the capacitance of a dielectric?

A

C= ϵ_r C_0
With C_0 the capacitance when the plates are separated by a vacuum

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10
Q

What is the capacitance of a parallel plate capacitor with a dielectric?

A

C_0=ϵ_r ϵ_0 A/d

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11
Q

How does a dielectric affect electrostatic energy?

A

U=U_0/ϵ_r
As ϵ_r is ≥ 1 the energy decreases

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12
Q

What is the dielectric breakdown?

A

When under a sufficiently strong electric field the electrons in a dielectric become unbound and it becomes a conductor

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13
Q

What is the maximum electric field that a dielectric can withstand?

A

The dielectric strength denoted by E_m

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