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PH103 PRINCIPLES OF PHYSICS III SPRING 2010
EXAM II – RELATIVITY AND QUANTUM  A
All work on this exam must be your own. You may use a calculator and a 3"X5" card of equations. Clearly indicate
your final answer for each question.
Note that speed means the magnitude of velocity.
Section 1: Multiple choice. No partial credit for this section. Circle only one answer for each question. (2 pts each)
1.
For which of the following is the velocity of A relative to B is negative?
i A spaceship is traveling at 0.8 c in the positive direction away from the Earth (B). The spaceship
sends a shuttle (A) in the direction of the Earth at a speed of 0.5 c relative to the ship.
ii A silver car (A) is traveling in the positive direction relative to the road. A gold car (B) catches up
from behind and is about to pass the silver car.
a)
i only
b) ii only
c)
i and ii
d) neither i nor ii
2.
Which of the following is not a difference between an electron and a photon?
a)
One shows wave-particle duality, the other does not.
b) One has mass, the other does not.
c)
One has electric charge, the other does not.
d) One has a wavefunction described by an electromagnetic wave, the other does not.
3.
Our sun appears yellow, because it has its maximum power output in the yellow/green part of the
electromagnetic spectrum. If a star appears red, one could conclude that the star is
a)
made of a material that reflects mostly red light.
b) made of a material that emits only red light.
c)
hotter than our sun.
d) cooler than our sun.
4.
Compare a 3mW green laser to a 3 mW red laser. The red laser gives off ___ photons of ___ energy than the
green laser.
a)
more, higher
b) fewer, higher
c)
more, lower
d) fewer, lower
5.
The uncertainty principle implies that a particle can be created out of a vacuum, as long as
a)
it exists for a time t<h/(4mc2)
b) it exists for a time t>h/(4mc2)
c)
it exists for a time t<h/(4mv)
d) it exists for a time t>h/(4mv)
6.
When we describe an electron as a wave, what is it that is oscillating?
a)
an electric field only
b) electric and magnetic fields
7.
c)
a magnetic field only
d)
none of the above
High speed neutrinos are sent underground from Fermilab in Illinois to a mine shaft in Minnesota. Consider the
three observers: a worker at Fermilab, a worker at the Minnesota mine, and the neutrino (pretend a neutrino can
observe things). Two of these three observers measure the same time for the neutrino’s trip from Illinois to
Minnesota. The one that measure a different amount of time for the trip is ___, and this observer’s time is ___
than that measured by the other two.
a)
worker a Minnesota, less
b) worker at Minnesota, greater
c)
neutrino, less
d) neutrino, greater
8.
Jack shines a laser beam toward RJ, who is approaching Jack at 0.25 c. Take the direction of the laser beam
relative to Jack as the + direction. The velocity of the laser light relative to RJ is ___, and the velocity of Jack
relative to RJ is ___.
a)
0.75 c, -0.75c
b) -c, -0.75 c
c)
c, -0.25 c
d) c, -0.75 c
9.
Don's weight when at rest under Earth's gravity is 150 lbs. Don rides an elevator while standing on a scale. The
scale reads 145 lb. Which of the following might the elevator be doing?
a)
speeding up while traveling upward
b) speeding up while traveling downward
c)
either of the above
d) neither of the above
10. A particle’s rest mass is mo. The energy of this particle at rest is ___.
approaches the speed of light, the energy of a particle approaches ___.
a)
As the velocity of this particle
0, moc2.
b) 0, infinity.
c)
moc2, 2moc2
d) moc2, infinity
Section 2: Long-answer problems and questions. (20 pts each) Partial credit will be given for this section.
Show all work and justify all answers.
11. The photoelectric effect experiment can be fit by an equation of the form K max = hf - .
a)
In this equation. Kmax stand for maximum kinetic energy. Kmax is the maximum kinetic energy of what?
b) If the work function for a particular metal is 3 eV, find the cutoff frequency for this metal. What
frequencies of light will cause the photoelectric effect in this metal?
c)
What wavelength of light is needed to achieve a maximum kinetic energy of 6.4 eV?
12. A beam of protons traveling at 0.01 c passes through a slit that is 0.45 m wide. The protons are detected on
a screen 3 m away from the slits
a)
Find the wavelength of the protons.
b) Do you expect to see much diffraction of the proton beam thought the slit? Explain.
13. For the following questions, take East as positive, and if no reference frame is given, you may assume an
Earth-frame measurement.
a) A red car travels at 55 mph West on I-80. A blue car is traveling 65 mph East on I-80. Find the velocity
of the red car relative to the blue car (including sign).
b) A red spaceship travels at .55c West on the intergalactic highway. A blue spaceship travels at .65c West
on the intergalactic highway. Find the velocity of the red spaceship relative to the blue spaceship
(including sign).
(Don’t forget to check the sign and size of your answers to be sure they make sense).
14. A pion’s lifetime at rest is 2.60X10-8 s.
a)
Find the lifetime of a pion traveling at 0.37 c relative to a researcher.
b) How far does the pion go in the researcher’s frame?
c)
How far does the pion go in its own frame?
1 yr = 3.16X107 s
electron mass = 0.511 MeV/c2
= 9.11 X10-31 kg
1 ly = 9.46X1015
proton mass = 938 MeV/c2
= 1.67 X10-27 kg
s = 10-6 s, nm = 10-9 m
1 eV = 1.602 X10-19 J
h = 6.626 X10-34 Js
hc = 1240 eVnm