Lecture 22: TUE 13 APR Lecture 22: TUE 13 APR 2010 2010 Ch.32.1–5: Maxwell’s Ch.32.1–5: Maxwell’s equations equations Ch.33.1–3: Electromagnetic Ch.33.1–3: Electromagnetic Waves Waves James Clerk Maxwell (1831-1879) Physics 2102 Jonathan Dowling QuickTime™ and a decompressor are needed to see this picture. QuickTime™ and a decompressor are needed to see this picture. QuickTime™ and a decompressor are needed to see this picture.
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Is there an “ether” they ride on? Michelson and Morley looked and looked, and decided it wasn’t there. How do waves travel???Electricity and magnetism are
“relative”: Whether charges move or not depends on which frame we use…This was how Einstein began thinking about his “theory of special relativity”… We’ll leave that theory for later.
A solution to Maxwell’s equations in free space:
)sin( txkEE m ω−=
)sin( txkBB m ω−=n.propagatio of speed ,c
k=ω
c =Eμ
Bμ
=1μ0ε0
=299,462,954 μs = 187,163 μ ilεs/sεc
Visible light, infrared, ultraviolet,radio waves, X rays, Gammarays are all electromagnetic waves.
Electromagnetic WavesElectromagnetic Waves
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Radio waves are reflected by the layer of the Earth’s atmosphere called the ionosphere.
This allows for transmission between two points which are far from each other on the globe, despite the curvature of the earth.
Marconi’s experiment discovered the ionosphere! Experts thought he was crazy and this would never work.
Fig. 33-1
The wavelength/frequency range in which electromagnetic (EM) waves (light) are visible is only a tiny fraction of the entire electromagnetic spectrum.
Maxwell’s Rainbow
Fig. 33-2
(33-2)
An LC oscillator causes currents to flow sinusoidally, which in turn produces oscillating electric and magnetic fields, which then propagate through space as EM waves.
Fig. 33-3Oscillation Frequency:
1LC
ω =
Next slide
The Traveling Electromagnetic (EM) Wave, Qualitatively
(33-3)
c =Eμ
Bμ
=1μ0ε0
c =Eμ
Bμ
=1μ0ε0
Fig. 33-5
Mathematical Description of Traveling EM Waves
Electric Field: ( )sinmE E kx tω= −
Magnetic Field: ( )sinmB B kx tω= −
Wave Speed:0 0
1cμ ε
=
Wavenumber: 2k πl
=
Angular frequency:2πωt
=
Vacuum Permittivity:0ε
Vacuum Permeability: 0μ
All EM waves travel a c in vacuum
Amplitude Ratio: m
m
E cB
= Magnitude Ratio: ( )( )
E tc
B t=
EM Wave Simulation
(33-5)
Electromagnetic waves are able to transport energy from transmitterto receiver (example: from the Sun to our skin).
The power transported by the wave and itsdirection is quantified by the Poynting vector. John Henry Poynting (1852-1914)
211|| Ec
EBS00
==μμ
The Poynting Vector: The Poynting Vector: Points in Direction of Power FlowPoints in Direction of Power Flow
E
BS
Units: Watt/m2
For a wave, sinceE is perpendicular to B: BES
rrr×=
0μ1
In a wave, the fields change with time. Therefore the Poynting vector changes too!! The direction is constant, but the magnitude changes from 0 to a maximum value.
____________222
___)(sin11 tkxE
cEcSI m ω
μμ−===
00
The average of sin2 overone cycle is ½:
2
21
mEc
I0
=μ
21rmsE
cI
0=
μ
Both fields have the same energy density.
2 22 2
01 1 1 1( )2 2 2 2E B
B Bu E cB uε ε εε μ μ0 00 0 0
= = = = =
or,
EM Wave Intensity, Energy EM Wave Intensity, Energy DensityDensity
A better measure of the amount of energy in an EM wave is obtained by averaging the Poynting vector over one wave cycle. The resulting quantity is called intensity. Units are also Watts/m2.
The total EM energy density is then 022
0 / με BEu ==
Solar EnergySolar EnergyThe light from the sun has an intensity of about 1kW/m2. What would be the total power incident on a roof of dimensions 8m x 20m ?
I = 1kW/m2 is power per unit area.P=IA=(103 W/m2) x 8m x 20m=0.16 MegaWatt!!
The solar panel shown (BP-275) has dimensions 47in x 29in. The incident power is then 880 W. The actual solar panel delivers 75W (4.45A at 17V): less than 10% efficiency….
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The electric meter on a solar home runs backwards — Entergy Pays YOU!
The intensity of a wave is power per unit area. If one has a source that emits isotropically (equally in all directions) the power emitted by the source pierces a larger and larger sphere as the wave travels outwards: 1/r2 Law!
24 rPI s
π=
So the power per unit area decreases as the inverse of distance squared.
EM Spherical WavesEM Spherical Waves
ExampleExampleA radio station transmits a 10 kW signal at a frequency of 100 MHz. At a distance of 1km from the antenna, find the amplitude of the electric and magnetic field strengths, and the energy incident normally on a square plate of side 10cm in 5 minutes.