Electromagnetism PHYS20141

Paul.Campbell-3@manchester.ac.uk

Schuster 4-11

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Week 7

- Mechanisms for polarization

- Bound charge & surface charges

- Electrostatic equations in a dielectric

Aligned Polarization

- Intrinsic dipoles align to reduce energy

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Grant & Phillips pg 62-64

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Intrinsic dipole moment

Induced polarization

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External force

=

Attraction between +ve

and –ve charge distns

Electric susceptibility

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One can measure the

relative sizes of the 2

effects by measuring

χE as a function of T

1/kT

χE

Bound charge & polarization

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Induced polarization

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Surface charge

The overall charge in the dielectric = 0

We can deduce the

surface charge !

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+ve charge

on plate -ve charge

on plate

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Using D…

- Capacitance in a dielectric

- Connection back to relative permittivity

- Interfaces between dielectrics

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Addition of a dielectric

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What is happening in the

dielectric ?

Ways of understanding

� Reduction in the electric field

� Reduction in the charge on the plates

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Summary

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Interfaces between dielectrics

Consider the boundary between two regions

1 & 2 with relative permittivities εr(1) and εr(1) respectively with no free charge on the boundary.

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D⊥ is continuous

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E// is continuous

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At a boundary

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At a boundary