Download Exam II

EXAM II
Physics 100
March 29, 2017
Professor Menningen
Name:_____________________________
You have 50 minutes to complete the exam. The only allowed tool is a calculator. Please indicate the color of your exam
(white, purple, gold, or green) in the “Special Codes” section of the answer sheet. Put your student ID number in the
space marked “Identification Number” and fill in the circles. It’s a good idea to mark your answers on your exam sheet as
well as the bubble sheet just in case there’s a dispute about what you meant to answer.
Thermal Energy
Nuclear Energy
Heat engines: e 
T
W
actual, 1  C max
QH
TH
QH  QC  W
E = mc2 Energy, mass, c2 = 9.0×1016 m2/s2
Radioactive Decay
Power P = W / t  Power, Work or energy, time
Heat = Q = m c T mass, specific heat, Temperature
Electrical Energy
V = I R V volts, I current (A), R resistance (Ω)
x
N
1
 e  t    Number of nuclei, x  # half-lives
N0
2
ln 2
half-life  1/2 
, where   decay constant

P  V I  I 2R  V 2 R
power (Watts)
V
N
transformers s  s and Ps  Pp (ideal)
Vp N p
Section I. (35 pts) Show all of your work, including the symbolic formula you are using, when solving the
following problems. Include the appropriate units in your answer. All of the necessary formulas are
displayed above.
A. 150 identical Christmas tree lights are connected in series and plugged into a 120-V wall outlet.
a) (5 pts) If each bulb consumes 0.75 Watt of power, how much current will the entire string
of lights require?
P  150  0.75 W   112.5 W
I
P 112.5 W

 0.9375 A  938 mA
V
120 V
b) (5 pts) What is the resistance of each bulb?
V
120 V
R

 128  so that Rbulb  total  0.853 
I
0.9375 A
150
P
0.75 W
or use P  I 2 R  R  2 
 0.853 
2
I
 0.9375 A 
Rtotal 
B. A power company wishes to deliver 100 MW = 100×106 W of power to its customers with only
5.0% transmission loss (or, in other words, a 95% transmission efficiency).
a) (4 pts) How much power (in MW) must the power plant generate at the beginning of the
transmission line?
Pdelivered
 100%  5%  95%  0.95
Pgenerated
Pgenerated 
Pdelivered 100 MW

 105.26 MW
0.95
0.95
[continued from previous page]
B. b) (6 pts) If the resistance of the transmission line is 24 Ω, what is the maximum current that
should flow in the transmission line if the transmission power loss is 5.0%?
Ploss  Pgenerated  Pdelivered  105.26  100 MW  5.26 MW
Ploss  I R  I 
2
Ploss
5.26  106 W

 468 A
R
24 
C. (7 pts) An ancient sample of rock contains some 210
83 Bi , which decays by
6
-decay with a half-life of 3.0×10 years. 210
206
4
83
Bi 
Tl  2 α
81
(i) What is the daughter isotope? _______________________
1
210
(ii) If the amount of 83 Bi in the rock is only
of the amount that was present in the rock
128
when it was formed, how many years old is the rock?
7
1 1
   so that 7.0 half-lives have transpired.
128  2 
7.0   3.0  106 y   2.1  107 y  21 million years
D. (8 pts) One ton of 235U fuel converts only 0.0283 kg of mass into energy during the fission
process. If a nuclear power plant were 32% efficient at producing electricity, for how much time
(in years) could you use the one ton of nuclear fuel to continuously operate a single grocery store
that consumes 115,000 W of electricity? {1 year = 3.156×107 s}
E  mc 2   0.0283 kg   9.0  1016 m2 /s 2   2.547  1015 J from the fuel
Welectrical  0.32  E  8.15  1014 J of electricity
t
Welectrical 8.15  1014 J
1 year

 7.09  109 s 
 225 years
P
115,000 J/s
3.156  107 s
Section II. (20 pts). Please answer True (T) or False (F) by filling in the corresponding circle on
your answer sheet.
1.
Radioactive nuclear waste should be permanently stored in rocks that are mixed with water to
provide a radiation-absorbing barrier very dry to prevent corrosion and leaking of radiation.
2.
Iron-56 has the largest binding energy per nucleon of any known isotope.
3.
The core of a nuclear fission reactor contains the uranium fuel together with a neutron absorber
material such as cadmium.
4.
During a nuclear fission process the mass of the products is greater less than the mass of the
reactants.
5.
Doubling the current passing through a constant resistance will quadruple the power dissipated in
the resistor.
6.
Magnetic fields come out of the south north pole end of a bar magnet and enter the north south
pole end.
7.
Parallel circuits have the same voltage throughout and series circuits have the same current
throughout.
8.
Faraday’s law explains how mechanical energy can be converted into electrical energy.
9.
A step-down step-up transformer is used to decrease the current flowing from an electric power
plant.
10. The electric potential of charges is commonly measured in watts volts.
Section III. (45 pts). Answer the following questions by filling in the appropriate letter on your
answer sheet. Choose the BEST answer.
11. Nuclear power plants generate electricity by
a. using uranium fission to boil water and run a steam turbine.
b. using uranium fission to generate neutrons that spin the turbine of a generator.
c. using hydrogen fusion to boil water and run a steam turbine.
d. using hydrogen fusion to generate neutrons that spin the turbine of a generator.
12. The fissioning of 235U can produce a chain reaction because it
a. releases two or three neutrons.
b. is unstable to radioactive decay.
c. converts mass into energy.
d. emits photons
13. The potential difference across the terminals of a battery that is connected to a circuit is a
measure of the
a. energy supplied by the battery each second.
b. work per charge done by the battery.
c. amount of charge flowing through a wire each second.
d. resistance to the flow of charge through the circuit.
14. If the steady current in a wire is going directly away from you, the magnetic field lines
a. circle the wire in the clockwise direction
b. circle the wire in the counterclockwise direction
c. point radially outward
d. point radially inward
15. A radioactive substance decays at the rate of 0.70% per minute. What is the half-life?
[Hint: remember the video on exponential growth!]
70
70



 100 min
1
2
a. 1.4 min
b. 70 min
rate 0.70% /min
c. 100 min
d. 693 min
16. Current is produced in a loop when a magnet is moved into the loop. Where does the electrical
energy come from?
a. Energy in the magnet's motion.
b. Energy stored in the wire atoms.
c. Energy stored in the magnet.
d. Energy in electromagnetic photons.
17. To get the most light from two batteries and two bulbs you would connect the batteries in _____
and the bulbs in _____.
a. parallel ... series
b. parallel ... parallel
c. series ... series
d. series ... parallel
18. Which of the following particles is NOT a spontaneous radioactive decay product?
a. alpha (helium nucleus)
b. beta (electron)
c. gamma (photon)
d. delta (neutron)
19. Which costs the most to operate?
a. A 100-W light bulb for 10 hours
c. A 1500-W heater for 1 hour
b. A 250-W floodlight for 4 hours
d. A 40-W refrigerator for 20 hours
20. What is a disadvantage of using an inexpensive extension cord (with a small-diameter wire) to
operate a high power air conditioner?
a. A large voltage drop across the wire will supply too much voltage to the air
conditioner and damage the unit.
b. The large current required by the air conditioner will heat the extension cord
wire and cause a fire hazard.
c. The thin wire will generate large magnetic fields that can be a health hazard.
d. The thin wire draws extra current from the outlet and will increase the cost of
operating the unit.
21. The nucleus of an atom contains
a. isotopes.
c. alpha and beta particles.
b. neutrons and protons.
d. gamma photons and electrons.
22. In which of the following situations is an electric current NOT produced?
a. The magnetic field through a loop of wire is increasing.
b. The magnetic field through a loop of wire is decreasing.
c. A loop of wire is stationary in a steady magnetic field.
d. A loop of wire rotates in a steady magnetic field.
23. An ideal step-down transformer will______ the voltage and ______ the power transmitted to the
secondary circuit.
a. decrease … decrease
b. increase … increase
c. decrease … not affect
d. increase … not affect
24. How would nuclear energy be best derived from plutonium-239?
a. It would be impossible
b. Spontaneous radioactive decay
c. Nuclear fusion
d. Nuclear fission
25. In order to double the amount of light emitted by a light bulb you need to
double the current that passes through it. This can be accomplished by
a. doubling the resistance in the circuit.
b. doubling the voltage difference across the bulb.
c. increasing the resistance in the circuit by a factor of four.
d. cutting the voltage difference across the bulb in half.