Transcript Intro to EMR and Wave Equation

 In 1864 James Maxwell developed four equations that
showed that electric and magnetic fields were linked and that
the fields can move through space as waves
1. Electric field lines begin and end on electric charges
E

dA


q
o
2. Magnetic field lines are continuous
B

dA

0

3. A changing electric field will produce a magnetic field
(this was difficult to detect because the magnetic fields produced by
changing electric fields are very weak)
B
 E  dl   dt
d E
 B  dl   0 I   0 0 dt
4. A changing magnetic field will
produce electric field
• The equations predicted the properties of the
electromagnetic waves (EMR):
a.EMR is produced by accelerating electric charges,
the accelerated charges lose energy, which is carried
away as a wave
b. if the charges are accelerated in simple harmonic
motion, the frequency of the wave = frequency of the
oscillating charge
SHM Animation
c. the changing fields will travel through space at the
speed of light (the equations predicted the speed of
light)
In a vacuum:
c = 3.00 x
8
10
m/s
d. the magnetic field is at right angles to the electric
field and both are at right angles to the direction of
travel (EMR is a transverse wave)
e. light and other EMR consist of oscillating electric
and magnetic fields moving together
All EMR shows wave properties like reflection,
refraction . . .
… interference…
…Polarization …
… Doppler effect.
The universal wave equation:
v  f
where:
v = speed of the wave (m/s)
 = wavelength (m)
f = frequency in (Hz = s-1)
•Frequency vs. period
1
T
f
where:
T = period (s)
Example:
A radio station transmits of 144.1 MHz.
Determine the wavelength and period.
 = 2.08 m
T = 6.94 x 10-9 s
•In a vacuum, all EMR travel at the speed of light regardless
of frequency
•19th century physics theory said all waves had to travel
through something, the medium that EMR moved was called
the ether (a transparent substance that filled all space)
•Experiments were done to measure the speed of light
through the ether, but they all failed
•All the experiments failed to locate the ether
•In 1905 Einstein proposed that the ether doesn’t exist and
that the speed of light is the same regardless of the speed of
the source or observer
Long wave
• Wavelengths of km (low frequency)
• Produced by accelerating electric currents such as AC
power lines
• Used in specialized communications
Radio waves
•metres long to hundreds of metres
•produced by accelerating electrons in conductors
•used in communication etc.
•In 1894 Hertz
produced the first
electromagnetic
waves in a lab
•He produced EMR
when an induction
coil created large
voltages that
accelerated
charges across a
spark gap
The EMR traveled
past a wire and a
current was
induced in the wire
which sparked
across a small gap
He showed that
the waves traveled
at the same speed
as light and had all
the properties of
transverse waves
•Radio waves are produced
when electrons are
accelerated back and forth
along a conductor
• The electric field is parallel to the conductor and the
magnetic field is perpendicular to the conductor
The radio wave is polarized in the direction of the conductor
Microwaves
•mm to cm length
•produced by special vacuum tubes and circuits
•used in radar, communication, heating, industry
Infrared
• 1 mm to the longest wavelength of visible light
• produced by vibrating molecules and atoms (heat) and by
special components and lasers
Visible light
red ~7.00 x 10-7 m (700 nm)
violet ~4.00 x 10-7 m (400 nm)
produced by electrons dropping from high energy levels to
low energy levels
Ultraviolet
•less than 4.00 x 10-7 m
down to 6 x 10-10 m
(400 nm to 0.6 nm)
•produced by electrons
dropping from very high
energy levels to low
energy levels
•used in sterilizing
material, tanning,
research
•“ionizing radiation”, can
knock electrons out of
atoms and break
chemical bonds
X-rays
• 10-8 m to 10-13 m
• produced by sudden deceleration of very high
speed electrons
• medical and industrial imaging
• “ionizing radiation”
Gamma rays
•10-10 m to less than 10-14 m
•produced by decelerating
atomic nuclei
•used in cancer treatment
•ionizing radiation