Download Exam 1 - UNC Physics and Astronomy

Phys25-002, Spring 2005, Exam 1
Student Name: ___________________________ Seat Number: ________
Sequence Number: 101
Honor Pledge and signature:
I have neither given nor received unauthorized aid on this examination. _____________________
Instructions:
This exam is closed book, closed notes. However, you may use a calculator.
Mark your answers to the multiple-choice questions on a Scantron answer sheet.
Show all of your work on these test papers for full credit (no other scratch paper is allowed).
Questions 1-12 are worth 3 points each, while questions 13-22 are 4 points each.
Equations and conversion factors that may be useful:

 F
q
q
V
E
Ek 2
E
  EA cos   encl
qo
s
o
r
V 
U
qo
V 
kq
r
 A
Q
C o
U  12 QV  12 CV 2
u  12  o E 2
V
d
Q
L
I
V  IR
R
P  IV  I 2 R  V 2 R
t
A
Req  R1  R2  R3  
C eq  C1  C 2  C 3   q (t )  Q(1  e t /  )
F  qE
F k
q1 q 2
r2
C
1 Req 1 R1  1 R2  
e  1.6  10 19 C
k
q(t )  Qe t / 
1 C eq 1 C1  1 C 2  
1
4o
  RC
 o  8.85  10 12 C 2 /N  m 2
 8.99  10 9 N  m 2 /C 2
1. A good conductor of electricity is also a good conductor of heat.
1. True
2. False
2. Two charges, Q1 and Q2 are separated by a certain distance, d. If the magnitudes of the charges are doubled, and their
separation is also doubled, then what happens to the electric force between these two charges?
1. It increases by a factor of 2.
2. It increases by a factor of 4.
3. It decreases by a factor of 4.
4. It remains the same.
3. Four point charges are arranged on the corners of a square as shown (clockwise starting from upper right: +Q, -Q, +2Q, -Q).
What is the direction of the net force acting on the charge in the upper right corner of the square?
1. A (up and right)
A
2. B (down)
-Q
D
+Q
3. C (down and left)
4. D (left)
C
+2
Q
B
-Q
4. An electron and proton are released in space, far from any other objects. The particles move towards each other due to their
mutual electrical attraction. When the electron and proton meet, which one has more kinetic energy?
1. the electron
2. the proton
3. Their kinetic energies are the same since momentum and energy are conserved.
5. No work is required to move a football along a yard line of equal scoring potential, just as no work is done in moving an
electric charge along an
1. equipotential line
2. electric field line
3. electric line of charge
6. A proton is released between the plates of a parallel-plate capacitor. What can be said about the force acting on this charge as
it moves toward the negative plate?
1. The force increases.
2. The force decreases.
3. The force remains constant.
7. An electron is released between the plates of a parallel-plate capacitor. What can be said about the electrical potential of the
electron and its electrical potential energy as it moves between the plates?
1. Both its electrical potential and electrical potential energy decrease.
2. Both its electrical potential and electrical potential energy increase.
3. Its electrical potential increases while its electrical potential energy decreases.
4. Its electrical potential decreases while its electrical potential energy increases.
8. Since C = QV, then it follows that an uncharged capacitor has a capacitance of zero.
1. True
2. False
9. What does the unit A-h represent?
1. power
2. energy
3. charge
4. capacitance
10. If both the length and diameter of a certain wire could be doubled, what would happen to the resistance of this wire?
1. R would increase by a factor of 2.
2. R would increase by a factor of 4.
3. R would decrease by a factor of 2.
4. R would decrease by a factor of 4.
5. R would stay the same.
11. Four unequal resistors are connected in parallel. Which one of the following statements is correct?
1. The total resistance is less than the smallest resistor.
2. The total resistance is equal to the average of the individual resistances.
3. The total resistance is more than the largest resistor.
12. Kirchhoff's loop rule essentially states that charge is conserved in a circuit.
1. True
2. False
13. Three identical point charges q = -5.0 μC are separated from each other by a distance of 6.0 cm. What is the electric field at
the middle of one of the sides of the equilateral triangle formed by these charges?
1. 1.0 x 107 N/C away from the center
2. 1.0 x 103 N/C away from the center
3. 1.7 x 105 N/C toward the center
4. 1.7 x 105 N/C toward the center
14. A ball of mass 4.0 g hangs from a thread at an angle of 30º from vertical in a horizontal electric field of 150 N/C. What is
the charge on the ball?
1. 15 μC
2. 150 μC
3. 0.15 μC
4. 130 μC
2
15. A metal sphere of radius r = 5.0 cm has a charge of +12 μC. What is the force on a point charge of -6 μC located 15 cm
from the surface of the sphere?
1. 16 N toward the sphere
2. 29 N toward the sphere
3. 260 N toward the sphere
4. 260 N away from the sphere
16. How much work is required to bring four point charges, each with q = -8 pC, from infinity to a distance of 3 nm from each
other?
1. +0.8 mJ
2. -0.8 mJ
3. +1.2 mJ
4. -1.2 mJ
17. An electron is accelerated from rest through a potential difference of 400 V. What is the final kinetic energy of this
electron?
1. 6.4 x 10-17 J
2. 400 J
3. 640 J
4. 0 J
18. A parallel-plate capacitor has plates each with an area of 55 cm2 separated by 0.40 mm of mica (with dielectric constant 5.4).
What is the capacitance?
1. 0.66 nF
2. 6.6 nF
3. 66 nF
19. If the battery in the circuit below has an emf of 9.0 V, R = 100 Ω, and C = 12 μC, what is the current through the battery
immediately after the switch is closed and a long time after the switch is closed?
1. 0 A, 90 mA
2. 1.2 mA, 0 A
3. 180 mA, 0 A
4. 180 mA, 90 mA
20. The power rating of a 400-ohm resistor is 0.25 W. What is the maximum current that can flow through this resistor without
it overheating?
1. 25 mA
2. 40 mA
3. 0.63 mA
4. 100 mA
21. A 4.0 μF and 6.0 μF capacitor are connected in series across an 9.0-V battery. What is the voltage across the 4.0-μF
capacitor?
1. 2.4 V
2. 3.6 V
3. 4.8 V
4. 5.4 V
22. Bonus question (worth 3 points): Who won Superbowl XXXIX?
1. The New England Patriots
2. The Philadelphia Eagles
Answers: 1, 4, 3, 1, 1, 3, 3, 2, 3, 3, 1, 2, 4, 2, 1, 3, 1, 1, 4, 1, 4, 1
3