Exam 2

1
Exam 2 cutoffs A 80 B 70 C 60 D 50
42 /- 4
72.3 /- 9
2
Bonus points
What can I do to help my grade?
Special WEBQUIZ like last time one time
only 30 minute limit noon-midnight, this
Thursday ONLY for those scoring below 70
points on Exam 2 Weighted to close the gap
between the dual distribution Exam Score
Possible gain in points 40
30 50
20 60 10
70 0
This weeks regular WEBQUIZ cancelled!
3
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.
4
Electromagnetic Waves
EM Waves are possible!
What produces a magnetic field?
current
What produces a changing magnetic field?
changing current
Accelerating or oscillating charge ? changing
current
5
Recall speed of mechanical waves along an
elastic medium
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
while the mutually induced fields guaranteed E?B
Maxwell concluded disturbances in E, B field
lines ripple out at constant speed
6
In 1849 (Maxwells time)
1

311,000,000 m/sec
v
m
e
0
0
and Fizeaus best measurement c315,000,000 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.
7
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 3) 100 GHz
1 GHz 109 Hz
8
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
9
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