Chapter 34 Electromagnetic waves

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Chapter 34 Electromagnetic waves November 3
Electromagnetic waves 34.2 Maxwells equations
and Hertzs discoveries Maxwells equations

• Electromagnetic waves are a natural consequence
  of Maxwells equations.
• The speed at which electromagnetic waves travel
  is the speed of light. ?Maxwell predicted that
  light waves were electromagnetic waves.
• Hertz first generated and detected
  electromagnetic waves (1887).

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34.3 Plane electromagnetic waves
Plane, linearly polarized EM wave
Simplest solution
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Properties of EM waves 1) Maxwells equations
predict that E and B satisfy the wave
equation. 2) Electromagnetic waves travel at the
speed of light. 3) Electromagnetic waves are
transverse waves. E and B and the direction of
light are perpendicular to each other. 4) The
magnitudes of E and B are related by E/B c. 5)
E and B are in phase.
Example 34.2
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Read Ch341-3 Homework Ch34 (1-14) 9,11 Due
November 14
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November 5 Energy of electromagnetic waves 34.4
Energy carried by electromagnetic waves
Energy density the energy per unit volume of the
E or B field. For E field,
. For B field, . Since for
an EM wave B E/c, we have
The instantaneous energy density of the magnetic
field equals that of the electric field.
Total instantaneous energy density of an EM
wave Total average energy density of an EM wave
Intensity of an EM wave
Remember one.
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Poynting vector A vector that describes the
energy flow rate (instantaneous intensity) of an
EM wave.
The magnitude of S represents the rate at which
energy flows through a unit area perpendicular to
the direction of the wave propagation. Its unit
is W/m2.
Compared with intensity, we have
.
Quiz 34.3 Example 34.3
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Read Ch343 Homework Ch34 (15-27) 17, 21 Due
November 14
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November 7 Momentum and radiation pressure 34.5
Momentum and radiation pressure Momentum and
pressure Electromagnetic waves transport
momentum as well as energy. As this momentum is
absorbed by a surface, pressure is exerted on the
surface. 1) For complete absorption. If light
transports energy U to a surface, the momentum
transferred is p U / c (from E hf, p
h/l). Pressure exerted on the surface
(S is the Poynting vector).
2) For a perfectly reflecting surface, p 2U/c
and P 2S/c. 3) For a surface with a
reflectivity between a perfect reflector and a
perfect absorber, U/clt p lt 2U/c Example For
direct sunlight, the radiation pressure is about
5 10-6 N/m2.
An apparatus for measuring radiation
pressure The system is kept in a high vacuum.
The pressure is determined by the angle through
which the horizontal rod rotates. Quiz 34.4
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34.6 Production of electromagnetic waves by
antenna Fundamental mechanism for EM wave
radiation the acceleration of a charged particle.
Half-wave antenna Two conducting rods (l/4 in
length) are connected to an alternating voltage
(oscillating electric dipole). The B fields form
concentric circles (latitude lines). The E fields
are on longitude lines. 1) Near the dipole,
there is no net energy flow. 2) Far from the
dipole, the radiation flows outward at all times.
The E and B fields are in phase with each other
and vary as 1/r.
Angular dependence of the radiation intensity
The intensity is maximum in a plane that is
perpendicular to the antenna and passing through
its midpoint.
Quiz 34.5
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34.7 The spectrum of electromagnetic
waves Various types of electromagnetic waves,
distinguished by frequency or wavelength, make
up the EM spectrum.

• Radio waves (104 m to 0.1 m )
• Radio and television communication
• Microwaves (0.3 m to 10-4 m)
• Radar systems, microwave ovens
• Infrared waves (10-3 m to 7 10-7 m)
• Produced by hot objects and molecules
• Visible (700 nm to 400 nm)
• Different wavelengths different colors
• Ultraviolet (410-7 m to 6 10-10 m)
• X-rays (10-8 m to 10-12 m)
• Gamma rays (10-10 m to 10-14 m)
• Emitted by radioactive nuclei

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Read Ch345-7 Homework Ch34 (28-)
29,31,37,60,61 Due November 14