1. science
2. physics

Review & Closure
Spot the Physics
Week 16: Exam #3
Exam time:
Tuesday, May 12
12:00 - 1:30 PM
Exam location:
• Surnames A - C! !
• Surnames D - H! !
• Surnames I - L! !
• Surnames M - P! !
• Surnames Q - Z!!
Clark A205
Clark A203
Clark A204
Clark A202
Clark A201
Details:
• Cumulative. 5 scenarios, pick 4.
• You may bring three study sheets and right hand rule devices.
Post-Exam Reception:
Thursday, May 14
6:00 - 10:00 PM
Brian & Carol’s
Note revised time
A sealed box is completely evacuated
(perfect vacuum), then 1,000,000
radioactive atoms are added. Their halflife is 2 days. After 4 days have passed,
how many atoms are in the box?
A.
B.
C.
D.
1,000,000
500,000
250,000
0
You see an upright, magnified image of
your face when you look into magnifying
“cosmetic mirror.” The image is located
A.
B.
C.
D.
In front of the mirror’s surface.
On the mirror’s surface.
Behind the mirror’s surface.
Only in your mind because it’s a virtual image.
9. Since the wires in the walls of your house carry current, you
might expect that you could use a compass to detect the positions of the wires. In fact, a compass will experience no deflection when brought near a current-carrying wire because the
current is AC (meaning “alternating current”—the current
switches direction 120 times each second). Explain why a compass doesn’t react to an AC current.
Two protons, one after the
other, are launched from point
1 with the same speed. They
follow the two trajectories
shown. The protons’ speeds at
points 2 and 3 are related by
A.
B.
C.
D.
v2 > v3.
v2 = v3.
v2 < v3.
Not enough information to compare their
speeds.
zontally—to reduce the glare from a horizontal surface such as
a road or a lake?
20. Old-fashioned roof-mounted
television
antennas
were
designed to pick up signals
across a broad frequency range.
Explain why these antennas had
metal bars of many different
lengths.
21. An AM radio detects the oscillating magnetic field of the radio
wave with an antenna consisting of a coil of wire wrapped
around a ferrite bar, as shown in Figure Q25.21. Ferrite is a
magnetic material that “amplifies” the magnetic field of the
wave. The radio antenna broadcasts waves with the electric
field vertical, the magnetic field horizontal. If a radio antenna
is located due north of you, how must the ferrite bar be oriented
for best reception?
Axis of ferrite bar
The current through the
Q25.21
3FIGURE
Ω resistor
is
A.
9 A.
22. B.
Three
laser beams have wavelengths l1 = 400 nm, l2 =
6 A.
600
nm,
and l3 = 800 nm. The power of each laser beam is 1 W.
C. 5 A.
a.
Rank
in order, from largest to smallest, the photon energies
D. 3 A.
E1 , E2 , and E3 in these three laser beams. Explain.
E. 1 A.
b. Rank in order, from largest to smallest, the number of photons per second N1 , N2 , and N3 delivered by the three laser
beams. Explain.
23. The intensity of a beam of light is increased but the light’s frequency is unchanged. As a result, which of the following (perhaps more than one) are true? Explain.
A. The photons travel faster.
B. Each photon has more energy.
C. The photons are larger.
D. There are more photons per second.
. The rapid vibration accompanying the swimming motions of
mayflies has been measured by gluing a small magnet to a
swimming mayfly and recording the emf in a small coil of wire
placed nearby. Explain how this technique works.
16/10/13
. Parts a through f of Figure Q25.3 show one or more metal wires
9:49 AM
The bar magnet is
pushed toward the
center of a wire loop.
Which is true?
A.
B.
C.
There is a clockwise
induced current in the
loop.
There is a
counterclockwise
induced current in the
loop.
There is no induced
current in the loop.
The current in the straight wire
is decreasing. Which is true?
There is a clockwise
induced current in the loop.
B. There is a counterclockwise
induced current in the loop.
C. There is no induced current
in the loop.
A.
8. Metal 1 has a larger work function than metal 2. Both are
illuminated with the same short-wavelength ultraviolet light.
Do electrons from metal 1 have a higher speed, a lower speed,
or the same speed as electrons from metal 2? Explain.
9. A gold cathode is illuminated with light of wavelength 250 nm.
The direction of the magnetic
force on the proton is
To the right.
B. To the left.
C. Into the screen.
D. Out of the
screen.
11. Suppose
the original nucleus is at rest in the fission reaction
E. The magnetic
force
is zero.
noted
above.
If we neglect the kinetic energy of the neutrons,
A.
after the two fragments fly apart,
A. The Br nucleus has more kinetic energy.
B. The La nucleus has more kinetic energy.
C. The kinetic energy of the Br nucleus equals that of the La
nucleus.
12. 200 MeV is a typical energy released in a fission reaction. To
get a sense for the scale of the energy, if we were to use this
energy to create electron-positron pairs, approximately how
many pairs could we create?
A. 50
B. 100
C. 200
D. 400
13. The two fragments of a fission reaction are isotopes that are
neutron-rich; each has more neutrons than the stable isotopes
for their nuclear species. They will quickly decay to more
stable isotopes. What is the most likely decay mode?
A. Alpha decay
B. Beta decay
C. Gamma decay
18.
19.
Detecting and Deciphering Radiation
A researcher has placed a sample of radioactive material in an
enclosure and blocked all emissions except those that travel in a
particular direction, creating a beam of radiation. The beam then
passes through a uniform magnetic field, as shown in Figure VII.2,
before reaching a bank of detectors. Only three of the detectors
record significant signals, so the researcher deduces that the particles
coming from the source have taken three different paths, illustrated
in the figure, and concludes that the sample is emitting three different kinds of radiation. Assume that the emitted particles move with
similar speeds.
u
B
Radioactive
sample
Lead
shielding
1
2
3
4
5
6
7
8
9
10
20.
Detectors
FIGURE VII.2
14. What type of radiation is detected by detector 4?
A. Alpha
B. Beta-minus C. Beta-plus
D. Gamma
15. What type of radiation is detected by detector 5?
A. Alpha
B. Beta-minus C. Beta-plus
D. Gamma
16. What type of radiation is detected by detector 10?
A. Alpha
B. Beta-minus C. Beta-plus
D. Gamma
21.
a. What is the speed of the emitted electron? (This spe
high enough that you’ll need to do a relativistic calcula
b. What is the power, in watts, provided by the radioa
decay process?
c. What will be the activity of the tritium in a watch
5 years, assuming none escapes?
An x-ray tube is powered by a high-voltage supply that del
700 W to the tube. The tube converts 1% of this power
x rays of wavelength 0.030 nm.
a. Approximately how many x-ray photons are emitte
second?
b. If a 75 kg technician is accidentally exposed to the
power of the x-ray beam for 1.0 s, what dose equiv
in Sv does he receive? Assume that the x-ray ener
distributed over the body, and that 80% of the ener
absorbed.
Many speculative plans for spaceships capable of inters
travel have been developed over the years. Nearly all are
ered by the fusion of light nuclei, one of a very few p
sources capable of providing the incredibly large ene
required. A typical design for a fusion-powered craft h
1.7 * 106 kg ship brought up to a speed of 0.12c usin
energy from the fusion of 21H and 32He. Each fusion rea
produces a daughter nucleus and one free proton with a
bined kinetic energy of 18 MeV; these high-speed par
are directed backward to create thrust.
a. What is the kinetic energy of the ship at the noted
speed? For the purposes of this problem you can do a
relativistic calculation.
b. If we assume that 50% of the energy of the fusion reac
goes into the kinetic energy of the ship (a very gene
assumption), how many fusion reactions are requir
get the ship up to speed?
c. How many kilograms of 21H and of 32He are required to
duce the required number of reactions?
A muon is a lepton that is a higher-mass (rest
105 MeV/c 2) sibling to the electron. Muons are produc
the upper atmosphere when incoming cosmic rays co
with the nuclei of gas molecules. As the muons travel to
the surface of the earth, they lose energy. A muon that tr
from the upper atmosphere to the surface of the earth typi
begins with kinetic energy 6.0 GeV and reaches the surfa
the earth with kinetic energy 4.0 GeV. The energy decr
by one-third of its initial value. By what fraction doe
speed of the muon decrease?
A muon is a lepton that is a higher-mass (rest
105 MeV/c 2) sibling to the electron. Muons are produc