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Exam 4, Thursday, August 9, 2012
PH204
For Dr. Harold Williams
Your Name:
1) Worth 8 points. 25.2 second half of Induced Current in a Circuit. A metal truck is
driving East at 80 km/hr in a region where the earth’s magnetic field is 4.5 × 10-5T,
north. a) what is the potential difference between the top and the bottom of the truck.
b) Will the top be positive or negative relative to the bottom.
2) Worth 15 points. 24.4 and 25.3 Magnetic Flux: A solenoid 50 meters long and a
diameter of 4 meters has 10,000 turns with a 2 A current is surrounded by a loop that is 5
km in diameter centered perpendicular to the solenoid. a) What is the magnetic flux in
the loop?
b) The loop is rotated 30 degrees. What is the magnetic flux now?
c) 25.4 Faraday’s Law. The loop is rotated the 30 degrees in 5 minutes. This is some sort
of science fiction movie device. What is the voltage induced in the 5 km loop by the 30
degree rotation in 5 minutes.
3) Worth 3 points. 25.5 Induced Fields and EM Waves. A an electromagnetic wave of
frequency f and wavelength λ travels with a speed in a vacuum in term of the permittivity
and permeability constants from our expression for electric and magnetic fields is what?
4) Worth 8 points. 25.6 Properties of Electromagnetic Waves. If the maximum leakage
radiation from a microwave oven is 5.0 mW/m2 . What would be the maximum leakage
amplitude of the electric fields? What would be the maximum leakage amplitude of the
magnetic field?
5) Worth 8 points. 25.6 Properties of Electromagnetic Waves (polarization). Given
unpolarized light and perfect polarizers, after the first polaroid filter what is the intensity
of light? If the second polarizer is at a 30 degree angle with respect to the axis of the
first polaroid filter what is the intensity after the second polaroid filter? The third
polaroid filter is oriented such that without the second polaroid filter the angle is 90
degrees and no intensity gets through with the second filter removed from beyond the
third filter. How much gets beyond the third polaroid filter with all three polaroid filters
in and the orientation as stated? We saw this demonstrated in class! This is actually a
two state quantum mechanical system with the wave function regenerated being
demonstrated! This is actually deep physics! Draw a diagram showing your answer in
terms of I coming into the first polaroid filter unpolarized and then after the first filter,
then the second filter, and then the after the third filter. Don’t panic just do it!
6) Worth 8 points. 25.7 The Photon Model of EM Waves. What is the energy in joules
and in electron volts of a single photon in the ultraviolet with a wavelength of 400nm?
This is two questions!
7) Worth 8 points. 25.8 The EM Spectrum. The spectrum of an incandescent light has a
wavelength peak of 1000nm, what is the temperature of the filament if it is a perfect
blackbody radiator? At this temperature how much energy/area in Joules/ m2 comes off
of the filament?
8) Worth 10 points. 26.2 AC Electricity and Transformers
What is the ratio of the primary to the secondary windings (turns) for each of the three
transformers in this diagram.
a) Step-up transformer between the power plant and the high-voltage transmission
lines?
b) Step-down transformer (substation) between the high-voltage transmission lines
and after the substation?
c) Step-down transformer from after substation voltage to your house at 120 volts?
9) Worth 10 points. 26.4 Biological Effects and Electrical Safety
If you place both hands on an electrified hot wire and it flows through to your feet
both on the ground what is your resistance using the numbers above? Draw the
resistance circuit, label it and calculate!
If you place only one hand on the electrified hot wire and stand on one leg what is
your equivalent resistance? Draw the resistance circuit, label it and calculate!
If you foolish try to craw under a electrified hot wire on all fours, arms and legs, and
your for head touches the hot wire with your feet and legs grounded what is your
resistance using the number above and the added fact that you head has a resistance
of 15 ohms? Draw the resistance circuit, label it and calculate!
10) Worth 10 points. 26.5 Capacitor Circuits. A 30 micro Farad capacitor is connected
across and AC generator that produces a peak voltage of 12 volts and a peak current of 3
amps. What is the oscillation frequency?
11) Worth 8 points. 26.6 Inductors and Inductor Circuits. An RLC tuning circuit is used
to bring in a signal with a wavelength of 21 centimeters. The inductor used in the circuit
is 2mH. What is the value of the capacitor used in this tuning circuit? 21 centimeter
radiation is used to study the density of atomic hydrogen in the cosmos. This is
important in astronomy, big time!
12) Worth 4 points. Relativity. What are the two postulates or principles of special
relativity?