Download Electromagnetic Waves

Electromagnetic Waves
ISAT 241
Fall 2003
David J. Lawrence
Waves (review)
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A wave is the propagation of a disturbance.
A wave can be transverse or longitudinal (or
mixed).
Waves propagate (travel) with some speed v.
Mechanical waves require a medium.
Electromagnetic waves do not require a
medium and can travel through empty space (a
vacuum).
Examples include radio, television, cell
phones, radar, light, x-rays, and gamma rays.
Electromagnetic Waves
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Electromagnetic waves are the propagation of
disturbances of electric and magnetic fields.
A time-varying electric field produces a
magnetic field.
A time-varying magnetic field produces an
electric field.
Electromagnetic waves travel through vacuum
at “the speed of light”:
c = 3  108 m/s
Electromagnetic Waves
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The simplest electromagnetic waves are plane
(not plain!) electromagnetic waves.
Plane electromagnetic waves consist of
oscillating electric and magnetic fields.
The electric and magnetic fields are in the
same plane.
The electric and magnetic fields are
perpendicular to each other.
The electric and magnetic fields are
perpendicular to the direction of propagation.
Plane Electromagnetic Waves
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See Figure 24.3 on page 902:
 
EB
y
E
 
Ec
 
Bc
c
B
z
x
Note that E and B are not constant; they vary
with x and t.
Serway & Jewett, Principles of Physics
Figure 24.3
Plane Electromagnetic Waves
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Also see Figure 24.6 on page 904:
 
EB
y
E
 
Ec
 
Bc
c
B
z
x
• Note that E and B are not constant; they vary
with x and t.
• Note that the wave is traveling in the
direction of the cross product E x B
Serway & Jewett, Principles of Physics
Figure 24.6
Plane Electromagnetic Waves
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The speed of an electromagnetic wave in
vacuum is given by
1
c
moeo
where mo = 4p  10-7 N/A2
= permeability of vacuum
and
eo = 8.854  10-12 C2/N-m2
= permittivity of vacuum
c is “the speed of light” (in vacuum)
Plane Electromagnetic Waves
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The wave equations for a plane electromagnetic
wave are
E  E max cos(kx - wt )
B  Bmax cos(kx - wt )
Recall the definitions of k, w, f, and l .
Also
w
cfl
k
Plane Electromagnetic Waves
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The magnitudes of E and B in for a plane
electromagnetic wave in vacuum are related by
E E max
c 
B Bmax
S is called the Poynting vector
 1  
S
EB
μo
The magnitude of the Poynting
vector represents the power
per unit area (W/m2) being
transported by the wave at any
instant of time.
Serway & Jewett, Principles of Physics
Figure 24.13