Download Part III – Questions and Problems

General Physics – ph 213
Midterm Exam II (Ch 24 – 27)
November 14, 2005
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Name:____________
Type A
Exam is closed book and closed notes. Use only your note card.
Write all work and answers in the papers provided.
Show all your work and explain your reasoning (No credit will be given for an answer
that does not include the necessary solution or explanation, except for true/false or
multiple choice questions)
Partial credit may be awarded for a correct method of solution, even if the answer is
wrong.
Part I – True or False (2.5 points each): For questions 1 – 11, state whether each statement is true
or false.
1. The change in electric potential energy, Ub – Ua, is the work done on a charge by the electric
force as it moves from point a to point b.
2. The potential due to a spherically symmetric charge distribution is the same as that of a point
charge.
3. Equipotential surfaces are always flat and perpendicular to the electric field.
4. The units of , the permittivity of free space, may be expressed as F/m.
5. The fatter a given length of wire is the higher its resistance will be.
6. In an alternating current, the charge carriers switch between electrons and protons in a repeated
fashion.
7. A 1200-W toaster draws more current than a 1000-W toaster.
Part II – Multiple choice questions: For questions 8 – 16 select the only one choice that best
answers or completes each statement.
8. A circular ring of uniformly distributed charge of total charge Q and radius a penetrates a spherical
gaussian surface of radius b at 2 points so that only part of the ring is enclosed inside the surface
of the sphere. The net flux leaving the gaussian surface is Q/3. What is the angle made by the
lines from the center of the ring to the two points where the ring penetrates the spherical
surface?
A) 30 degrees
C) 120 degrees
B) 90 degrees
D) not enough information given
9. The units of electric potential are
A) J
B) J/kg
C) J/C
D) C2/m
10. The potential of a uniformly charged sphere is lowest at
A) the center of the sphere
B) the surface of the sphere
C) a distance from the sphere equal to its radius
D) infinity
11. The potential halfway between two equal charges Q separated by a distance d is
A) 0
C) Q2/4d
B) Q/8d
D) Q/d
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12. If an electric field of magnitude 25 V/m makes an angle of 30o with a path of length 10 m, then
 
the integral of E  d  over this path has a value
A) 250 V
B) -250 V
C) 125 V
D) 217 V
13. A circular disk of radius a has its central section of radius b removed forming an annulus. A
charge Q is uniformly distributed over the surface of the annulus. The potential at the center of
the annulus is
2
A) Q/2(a + b)
C) Q/2(a2 + b )1/2
2
2
B) Q/2(a - b)
D) Q/2(a – b )1/2
14. The minimum capacitance that can be made using five 10 nF capacitors is
A) 50 nF
B) 10 nF
C) 0.2 nF
D) 2 nF
15. A 1 mF, a 2 mF, and a 3 mF capacitor are connected in parallel, the combination being
connected across a 9 volt battery. The capacitor with the greatest charge on it is
A) the 1 mF
C) the 3 mF
B) the 2 mF
D) neither since they all have the same charge
16. If the voltage on a capacitor is doubled, then the energy stored by the capacitor
A) stays the same
C) quadruples
B) doubles
D) halves
17. A parallel plate capacitor has a capacitance of 10 mF and is charged with a 20 V power supply.
The power supply is then removed and a dielectric of dielectric constant 4 is used to fill the space
between the plates. The voltage now across the capacitor is
A) Zero
C) 20 V
B) 5 V
D) 80 V
18. If a 10 A current flows for two minutes, how many electrons have passed any point in the circuit?
C) 6.02 × 1022
A) 6.25 × 1018
D) electrons don't move in a circuit
B) 7.5 × 1021
19. A resistor is attached to a voltage Vo. As the current flows through the resistor the resistor heats
up, doubling it resistance. By what factor does the power dissipated by the resistor change as it
heats up?
A) 0.5
C) 1 (no change)
B) 0.707
D) 2
20. A metal bar of dimensions 2 by 4 by 10 units is to be used as a resistor. To get the smallest
resistance from this bar, you should attach leads to opposite sides that have the dimensions
A) 2 by 4 units.
D) All connections will give the same resistance.
B) 2 by 10 units.
E) None of the above is correct.
C) 4 by 10 units.
Ph 213 Midterm Exam II, f05
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Part III – Questions and Problems
Please explain or show your work clearly and completely for each of the following questions or
problem.
21. Five 12 mF capacitors are connected across a 12 V battery as indicated in the figure below.
A) What is the capacitance of the combination?
B) What is the charge on the bottom capacitor in the diagram?
22. The resistivity of the filament of a light bulb (tungsten material) increases approximately linearly
from 56 n·m at 293 K and 1.1 .m at 3500 K. Assuming the filament temperature is about
2500 K, and that 100-V DC supply is used to power a 40-W light bulb, and 0.5-cm length of the
filament remains constant, estimate
A) the resistance of the tungsten.
B) the diameter of the tungsten filament.
(temperature coefficient () of tungsten is 4.5 × 10-3 K-1.)
23. A thin rod of length 2L is centered on the x axis as shown. The rod carries a uniformly distributed
charge Q.
A) Determine the potential V for points along the y axis.
B) Determine the electric field due to the charged rod along the y-axis.
y
L
L
0
x
24. Paper has dielectric constant of K = 3.7 and a dielectric strength of 15 × 106 V/m. Suppose that a
typical sheet of paper has a thickness of 0.030 mm. You make a capacitor by placing a sheet of
8.5 × 11 inch paper between two aluminum foil sheets. The thickness of the aluminum foil is
0.040 mm.
A) What is the capacitance C0 of your device?
B) About how much charge could you store on your capacitor before it breaks down?
C) By overlaying sheets of paper and aluminum you can make a parallel combination. If you
make 100 such capacitors, and connect the ends of the sheets in parallel, so that you have
a single large capacitor of capacitance 100C0, how thick your new large capacitor be?
D) What is the maximum voltage you can apply to this 100C0 capacitor without breakdown?
Note: Your test on August 15, 2011 will have questions/problems from magnetism topics as well.
Ph 213 Midterm Exam II, f05
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