Electromagnetism PHYS20141

Paul.Campbell-3@manchester.ac.ukSchuster 4-11

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Lecture 19

- E, B and k perpendicular – general case- Poynting vector- Intensity- Radiation pressure

Summary of last time

with = c|k| NB. |k| = wavenumber = 2 = angular frequency = 2f

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Helpful formulae

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Electromagnetic waves

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E

Bk

Wave travelling through a box

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ct

A

Flux in Flux out

Poynting vector

Defnition

Units

Physical interpretation- energy flux through a surface- N is in the direction of wave motion

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Radiation pressure

Momentum of photon is p = U/c

At a boundary the radiation creates a force, Fr

All radiation absorbed : Fr = A<|N|>/c

All radiation reflected : Fr = 2A<|N|>/c

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Lecture 20

- EM waves in the presence of current- Good conductors and skin depth- Plasma- Reflection of EM waves at a perfect conductor

Maxwell’s equations

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Maxwell’s equations in the presence of current only

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Final solution

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Skin depth :

Skin depth

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d

Intensity, |E0|2 Intensity, |E0|2exp[-2z/Conductor,

Plasma

State of matter which is partially ionized and electrons move under the influence of the electric field – ignore magnetic field.

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Hence :

Reflection (infinite thickness)

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y

x y=0

Perfect conductor, E = 0

I R

Incident wave, EIReflected wave, ER

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Reflection of EM waves