21'4 Electromagnetic Radiation

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21.4 Electromagnetic Radiation

• Maxwells prediction
• properties of electromagnetic waves

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James Clerk Maxwell Scottish Physicist
(1831-1879) Analyzed the theories of electricity
and magnetism and predicted electromagnetic
radiation. Energy contained in electric and
magnetic fields that propagate through space with
the speed of light. Applications radiowaves,
microwaves, television, cell phones, light,
x-rays
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Maxwell A changing magnetic field produces an
electric field why doesnt a changing electric
field produce a magnetic field?
It does A current is equivalent to a changing
electric field
B
B
capacitor
I
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Maxwells relations
Electric field lines originate on positive charge
and terminate on negative charge- Gauss
Law Magnetic lines form closed loops (no
magnetic monopoles) Gauss Law for
magnetism. Used the ideas the changing magnetic
fields produce electric fields. Faradays Law A
changing electric field is equivalent to a
current. Thus, a changing electric field can
produce a magnetic field. Amperes Law
Using the equations relating these ideas
(Maxwells equations) Maxwell predicted the
existence of electromagnetic radiation.
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Maxwell predicted the speed of light
Maxwells equations predicted the speed of light
to be determined by fundamental constants.
Experimental value
cexp. 2.99792458x108 m/s
exact agreement
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Electromagnetic plane wave- Transverse E and B
fields
Plane waves
y
x
z
Propagating in the x direction Electric field
polarized along y direction magnetic field
polarized along z direction
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T period
t
E
? wavelength
x
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Production of electromagnetic waves by electric
fields in a dipole antenna
dipole antenna
E propagating to right
The E field due to charges on the antenna
propagates at the speed of light.
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Production of magnetic fields by dipole antenna
-
E
B
B perpendicular to E

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Heinrich Hertz, German Physicist
1857-1894 Showed that electromagnetic waves
could be produced and detected.
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Some properties of electromagnetic radiation
Speed of light c2.99792x108 m/s Electric and
magnetic fields both present in fixed ratio
Energy is carried by both E field and B
field The average power / area carried by the E
field and B field are equal.
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A radio transmitter has a range of 50 km and a
power output of 1.0 kW. What is the max E field
at the edge of the range if the signal is
transmitted uniformly over a hemisphere?
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Momentum of electromagnetic waves
absorbed
reflected
U is the total energy absorbed or reflected
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Electromagnetic radiation
Find the wavelength of EM waves from a fm
radio station f100MHz
The size of the optimal antenna is about ¼
wavelength
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Electromagnetic radiation
Radio f100kHz-100 MHz ? 1km-1m
dipole antenna-detect electric field ferrite
core antenna- detect magnetic field
E
B
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Electromagnetic radiation
microwaves, cell phones f1-10 GHz ? 1-10 cm
dipole antenna
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Electromagnetic radiation
visible light f1015 Hz ? 400-700 nm
stained glass, gold particles with diameter 100
nm scatter specific colors of light
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Electromagnetic radiation
x-rays f1019 Hz ?0.1 nm
X-ray diffraction x-rays scatter from atoms
dimensions 0.1 nm
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Is cell phone radiation harmful? Cell phone
radiation is in the microwave range. ?1cm,
Low energy density Thermal effects are likely
to be negligible, unlike microwave oven. The
harmful effects of low doses of high
frequency radiation e.g. x-rays are due to
quantum effects. a single particle of x-ray
radiation can cause damage The quantum effects of
cell phone radiation is expected to be small.
There is no specific physical mechanism for
cell phone radiation damage.