Download B - UNL CSE

The electric field in an EM wave
traveling northward oscillates up
and down. In what plane does the
magnetic field oscillate?
1) In the north - south plane.
2) In the up - down plane.
3) In the northeast - southwest plane.
4) In the northwest - southeast plane.
5) In the east - west plane.
Recall:
speed of mechanical waves along an elastic medium
~
T
m
Tension
linear density
E directly pushes charges (connecting distance charges),
conveying the disturbance on one charge to others
B induces back emfs to resist the changing E-fields
v=
E
B
while the mutually induced fields guaranteed EB
Maxwell concluded disturbances in E, B
field lines ripple out at constant speed
1
v=
m 0e 0
In 1849 (Maxwell’s time)
v=
1
m 0e 0
= 311,000,000 m/sec
and Fizeau’s best measurement: c=315,000,000 m/sec
Maxwell: “The velocity of the transverse undulations calculated from
the electromagnetic measurements of MMs. Kohlrausch
and Weber, agrees so exactly with the velocity of light
calculated from the optical experiments of M. Fizeau, that
we can scarcely avoid the inference that light consists in the
transverse undulations of electric & magnetic phenomena.”
The wavelength of microwaves
generated by a microwave oven is
about 3 cm. At what frequency do
these waves cause the water
molecules in your breakfast
burrito to vibrate ?
1) 1 GHz
2) 10 GHz
1 GHz = 109 Hz
3) 100 GHz
The magnetrons of microwave ovens are tuned to 2.54 GHz
(the natural vibrational energy that sets water molecules
spinning/flipping and heating up any surrounding food).
2.54  109 Hz
  0.125 meter = 12.5 cm
Infrared Photography
Infrared photography is used to check
heat insulation of houses in winter. If
you look at an infrared picture of a
house which areas would need better
insulation?
1) the bright areas
2) the dark areas
3) need more information
Oxygen
O8
1s
2s
2p
Iron
3s
26
Fe 3p
4s
3d
4p
5s
4d
Energy Levels for Hydrogen
Atomic Spectra
Blackbody spectrum is continuous:
But spectrum of a heated rarefied gas shows lines:
Emission
spectrum
And when white light shines through a rarefied gas,
dark lines appear in the continuous spectrum:
Absorption
spectrum
Explaining these phenomena requires and understanding of the atom
Some answers to previous slides:
5) In the east - west plane.
In an electromagnetic wave, the E and B fields are perpendicular
to each other and to the direction the wave propagates.
2) 10 GHz
f = v /  = 3  108 m/s / 0.03 m = 1010 Hz = 10 GHz
1) the bright areas