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Electromagnetic (EM) Theory
Recall: Oersted, Ampere, Henry and Faraday linked electricity and
magnetism by finding that current creates magnetic fields which exert
forces and that changing magnetic fields will generate a current in nearby
conductors.
The linking of these two forces (electric and magnetic) allowed physicists to
move closer to the ultimate goal of linking all the forces into a Grand
Unified Theory (G.U.T.)
The nuclear forces (weak and strong) are explained by an interaction of EM
waves
Gravity is the elusive force that has not been nailed down into the G.U.T.
In 1846, Faraday theorized that light could be a vibration of electric and
magnetic fields (drops the ether model)
James Maxwell proved Faraday’s theory mathematically, 10 years later.
All electromagnetic waves are generated by accelerated charges.
As a charge is accelerated, it will continually emit excess energy as an EM
wave (to limit the particle’s velocity to less than “c”).
The accelerated charge generates a changing magnetic field which creates
a changing electric field, which generates a changing magnetic field, etc
The EM wave is a 3D transverse wave
A (m)
|E|
λ
x (m)
B
A (m)
EM Wave Direction
 Note that the |E| component lies in one plane; this means it’s plane
polarized (oscillates in 2D)
 The intensity of EMR is proportional to the square of the amplitude of the
electric (or magnetic) component.
 The intensity of the EMR is also proportional to the square of the
frequency of the EMR
 An electron forced to oscillate at 102.1 MHz will produce an FM radio wave
 An electron accelerated by a Mega-Volt and fired into a metal target will
produce X-rays (rapid deceleration of electrons)
 A magnetron in a microwave oven accelerates charges to produce an EM
wave that is forced to oscillate at about 2.4 x 109 Hz (the electric field
spins polar water molecules to create heat).
 In a vacuum (or air), the electric and magnetic components carry equal
amounts of energy per unit volume of space.
 In 1865, Maxwell proved that all EMR travels at the speed of light where:
c
1
 0 o
 The permittivity of free space is
εo = 8.85x10-12 C2/(N●m2) and the permeability of free space is
μo = 4π x 10-7 T●m/A
 The speed of light is defined today as
c = 299 792 458 m/s
 EM waves carry linear and angular momentum (exerts pressure as
Lebedev found). A radiometer proves this.
 EM waves are produced using the following steps:
1) Energy is supplied to a source of EM radiation
2) The source does work on charges making them accelerate
3) The charges continually emit EMR radiating away excess kinetic energy
 The atomic model proved a challenge as accelerated electrons (in the Bohrorbit model) did not radiate EMR continuously (the atom would collapse)
and Bohr solved this by stating this didn’t occur in the atom.
See: http://abyss.uoregon.edu/~js/ast122/lectures/lec04.html
The EM Spectrum
* Wavelengths provided for information only; memorize the order of EMR.
Know the range of visible light’s wavelengths.
Regions may be defined but overlap occurs.
1) Low Frequency AC waves (103  105 m)
 electrons vibrate in household circuits (usually at 60 Hz) which
generates very long wavelength EMR
2) Radio AM Waves (102  103 m)
 generated by electric oscillator circuits
 reflected by charged upper atmosphere
 diffract (scatter) easily around small objects of approximate equal
size (bridges, not mountains)
3) TV, Radar, FM Radio Waves (100  103 m)
4)
5)
6)
7)
8)
9)
 not reflected by upper atmosphere so satellites needed to bounce
signals
 not diffracted easily
 TV transmission will be phased out soon to allow more wireless
frequencies to be used
Microwaves, IR (Infrared) (10-1  10-7 m)
 IR radiation is emitted by hot objects (vibration and rotation of
molecules)
 Skin molecules vibrate creating warmth
 Penetrates dense haze (little diffraction)
 Microwaves (high frequency radio waves) generated by klystrons
(electron tubes)
Visible Light (400-750 nm)
 ROYGBIV with red of longest wavelength
 eyes sensitive most to yellow and green
 emitted by very hot objects (burning log, sun, incandescent light
bulb)
UV (Ultraviolet) (< 400 nm  10-8 m)
 Causes photochemical reactions
 used in ozone production (UVA/B/C)
 Skin produces melanin with UV on it
X-Rays (10-8  10-11 m)
 produced by firing accelerated (by voltage) energetic electrons at
tungsten
 more energetic X-rays result from higher accelerating potentials
 Used to determine crystal layer separation using interference
patterns
Gamma Rays (10-11  10-16 m)
 emitted EMR from unstable nuclei
 high frequency means high penetration
 used to treat cancer cells by destroying them with focussed gamma
beams
Cosmic Rays (<10-16 m)
 emitted radiation left over from Big Bang and stars (makes up
cosmic wind)
 neutrinos (3 types), anti-neutrinos and possibly gravitons (HiggsBoson?) and dark matter particles.
 Residual radiation from Big Bang is radio waves  2.7K of space