EM waves 3'2

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EM waves3.2

• Producing EM waves
• Electromagnetic Spectrum
• Energy in EM waves

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EM radiation is produced by accelerating charges.
example Radio waves
at t0
Evolution of the E and B fields
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Dipole Antenna
q
Oscillating Dipole
d(t)
-q
Electric Field lines
radiation ? to the dipole axis
Electric field lines
No radiation ?? to the dipole axis (Why?)
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Dipole antenna
?/4
?/2
Quarter wave antenna
Half-wave antenna
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Question

• A cell phone uses a frequency of 2.0 GHz. Find
  the length of the quarter wave antenna.
• 1) 3.8mm
• 2) 0.38 cm
• 3) 3.8 cm
• 4) 38 cm

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Question

• A cell phone uses a frequency of 2.0 GHz. Find
  the length of the quarter wave antenna.
• 1) 3.8mm
• 2) 0.38 cm
• 3) 3.8 cmv
• 4) 38 cm

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Wavelength and Frequency of EM waves

• The frequency and wavelength of EM waves are
  related
• The higher the frequency the lower the
  wavelength.
• The optimum antenna to detect an EM wave has a
  length of the order of a wavelength.
• Interference effects occur which scattering from
  structures having spacing of the order of the
  wavelength.

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Spectrum of EM waves
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Radio Waves
Fm radio, TV
f 100 MHz ? 1 m
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Microwaves
f 1-10 GHz ? 1-10 cm
dipole antenna
length ?/4
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Visible Light
f1015 Hz ? 400-700 nm
stained glass, gold particles with diameter 100
nm scatter specific colors of light
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Silver nano-particles
Electron micrograph
light micrograph
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X-rays
f 1016-1020 Hz ? 10-8 -10-12 M
Atomic dimensions
x-ray diffraction pattern
3-dimensional structure
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X-rays
X-rays penetrate soft tissue but are absorbed by
heavy atoms, such as Calcium in bones.
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Energy carried by an EM wave
Energy density of E and B fields (from capacitor
and inductor)
Joules/m3
Average energy density (average of sin2?1/2)
Use
The average energy density for E and B are the
same
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Intensity
VA?
A
Average power
c?/T
?
Average Intensity
Use
gt
gt
gt
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Poynting Vector

• In general case

Direction of S is perpendicular to E and
B according to a right hand rule.
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34.4 Light intensity
The light intensity at noon on a sunny day is
about 1kW/m2. What is the maximum E field due to
the light? What is maximum B field?
Electric Field
Magnetic Field
In terms of normal laboratory fields the E field
is more significant than the B field. (i.e. for
interactions with matter.
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Microwave ovens and cell phones.
Frequency 2.45GHz Power 1kW compare to cell
phone power 1W.

• Microwaves are reflected from the walls
• of the cooking chamber and the energy is confined
  to cook the food.
• Microwaves form standing waves in the chamber.

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Microwaves are absorbed by water molecules
reoriented by the E field
E field
electric dipole orients with the fluctuating field