21111
21111
PHYSICS DEPARTMENT
PHY 2054
D. Reitze
Exam
Name (print):
September 29, 1999
Signature:
On my honor, I have neither given nor received unauthorized aid on this examination.
YOUR TEST NUMBER IS THE 5-DIGIT NUMBER AT THE TOP OF EACH PAGE.
DIRECTIONS
(1) Code your test number on your green answer sheet (use 76{80 for the 5-digit number). Code your name
on your answer sheet. Darken circles completely (errors can occur if too light). Code your student number
on your answer sheet.
(2) Print your name on this sheet and sign it also.
(3) Do all scratch work anywhere on this exam that you like. At the end of the test, this exam printout is to be turned
in. No credit will be given without both answer sheet and printout with scratch work most questions demand.
(4) Work the questions in any order. Incorrect answers are not taken into account in any way; you may guess at answers
you don't know if you feel that a correct answer is listed. Guessing on all questions will most likely result in failure.
(5) If none of the answers is correct, please leave the answer sheet blank. It is not our intention to omit the right answer,
but in case of a mistake, please leave the answer sheet blank.
(6) Blacken the circle of your intended answer completely, using a number 2 pencil. Do not make any stray
marks or the answer sheet may not read properly.
(7) As an aid to the examiner (and yourself), in case of poorly marked answer sheets, please circle your selected answer
on the examination sheet.
(8) Good luck!!!
>>>>>>>>WHEN YOU FINISH <<<<<<<<
Hand in the green answer sheet separately.
CONSTANTS
k = 1=(4¼²o ) = 9:0 £ 109 N m2 / C2 ²0 = 8:85 £ 10¡12 C2 / (N m2 ) e = 1:6 £ 10¡19 C
g = 9:8 m/s2 electron mass: mp = 9:11 £ 10¡31 kg proton mass: mp = 1:67 £ 10¡27 kg
p = 10¡12
n = 10¡9
¹ = 10¡6
m = 10¡3
K = 103
1. Two point charges are located on the x-axis as follows: charge q1 = ¡4:0¹C is at x = ¡2 cm, charge q2 = +8:0¹C is
at x = 3 cm. Where can a third charge be placed on the x axis such that the net force on it is identically 0 N?
(1) ¡14:1 cm
(2) 10.1 cm
(3) ¡3:2 cm
(4) 2.1 cm
(5) ¡8:2 cm
2. A +2.5 nC charge with mass 10¡4 kg is suspended using a thin insulating wire with negligible mass as shown in
Figure 1. A second ¡2:5 nC charge is located 1 cm to the left of the suspended charge. Find the angle (in degrees).
(1) 29.9
(2) 16.2
(3) 4.5
(4) 20.1
(5) 24.2
3. Two point charges q1 = +4:0¹C and q2 are located at (x, y) = (-a,0) and (0,0) respectively (see Figure 2). If the
electric ¯eld at (x, y) = (0,a) is pointed to the right (parallel to the x-axis), what is the value of q2 (in ¹C)?
(1) ¡1:4
(2) ¡2:8
(3) ¡5:6
(4) 5.6
(5) 2.8
21111
21111
4. What is the electric ¯eld (in V/m) at point P located 25 cm from the center of the sphere in Figure 3? Assume the
sphere has a charge of ¡15¹C and the shell has a charge of 200¹C.
(1) 2:2 £ 106 , inward (2) 1:0 £ 107 , inward (3) 1:4 £ 107 , outward (4) 6:7 £ 106 , outward (5) 9:7 £ 10¡8 , outward
5. Which of the electric ¯eld line con¯gurations shown in Figure 4 is CORRECT?
(1) B, E
(2) A,B, D
(3) A, D
(4) B
(5) E
6. Four 2 C charges are initially arranged as shown in Figure 5a. The charges are then rearranged to the ¯nal con¯guration
shown in Figure 5b. Compute the change in potential energy (in J).
(1) ¡1:0 £ 10¡10
(2) ¡2:4 £ 109
(3) ¡4:6 £ 107
(4) 4:6 £ 107
(5) 9:0 £ 108
7. A positively charged insulating rod is brought close to a neutral electroscope but they do not touch (see Figure 6).
The electroscope is then grounded. After the ground is disconnected, the rod is moved away. Which of the following
statements is true?
(1)
(2)
(3)
(4)
(5)
The
The
The
The
The
overall
overall
overall
overall
overall
charge
charge
charge
charge
charge
on
on
on
on
on
the
the
the
the
the
electroscope
electroscope
electroscope
electroscope
electroscope
is
is
is
is
is
negative, and the foil is negatively charged.
negative, and the foil is positively charged.
zero, and the foil is neutral.
positive, and the foil is positively charged.
zero, and the foil is negatively charged.
8. A reitzon (m = 2:6 £ 10¡26 kg, q = +3:5 £ 10¡18 C) is launched with an initial velocity of 1:2 £ 107 m/s toward an
in¯nite conducting sheet of charge with charge density +5:4¹C/m2 ? How long (in s) does it take to stop?
(1) 2:9 £ 10¡7
(2) 4:3 £ 10¡4
(3) 7:8 £ 10¡12
(4) 9:6 £ 10¡15
(5) 2:2 £ 10¡9
9. The electric ¯eld at a point P in space is identically zero. Which of the following statements must be true?
(1)
(2)
(3)
(4)
(5)
Any charge placed at P will experience zero net force.
The absolute electric potential at P is identically zero.
A negative charge placed at P will accelerate in a direction opposite to that of a positive charge placed at P
The electric ¯eld very near P is identically zero.
There is no charge in the vicinity of P.
10. A parallel plate capacitor is connected to a battery and charged. The battery is then disconnected from the capacitor.
The plates are then moved closer together. Which of the following statements is CORRECT?
(1)
(2)
(3)
(4)
(5)
The
The
The
The
The
voltage across the plates decreases
electric ¯eld between the plates increases
capacitance decreases
stored electrical energy increases
charge on plates increases
21111
21111
11. Four thin, °at parallel charged conducting sheets are arranged as shown in Figure 7. The plates have uniform charge
densities of +3¹C/m2 , +6¹C/m2 , ¡4¹C/m2 , and +7¹C/m2 respectively. What is the magnitude of force (in N) on
a ¡1¹C charge placed at point P?
(1) 3:4 £ 10¡1
(2) 8:0 £ 10¡1
(3) 8:9 £ 10¡3
(4) 6:7 £ 10¡1
(5) 1:1 £ 105
12. In the above problem, which way does the charge move?
(1) {y
(2) +y
(3) +x
(4) {x
(5) +z
13. Two protons initially separated by 10¡6 m are traveling toward each other with velocities of 1:7 £ 104 m/s. How close
(in m) will they come to each other?
(1) 4:8 £ 10¡10
(2) 3:2 £ 10¡9
(3) 9:8 £ 10¡12
(4) 2:1 £ 10¡7
(5) 5:4 £ 10¡7
14. An insulating sphere has a uniform charge density ½ and radius R (See Figure 8). Using Gauss's Law, which expression
below correctly expresses the magnitude of the electric ¯eld at point P inside the sphere located r away from the center?
The volume of a sphere of radius r = (4=3)¼r3 and the surface area of a sphere is 4¼r2 . Hint: Q = (½)£ (Volume)
(1) ½r=3²0
(2) 4¼½r=3²0
(3) ½r3 =4¼²0
(4) 4¼½r3 =²0
(5) Something else
15. Six point charges are arranged in a hexagon as shown in Figure 9. What is the direction (in degrees) of the electric
¯eld at the center of the hexagon?
(1) 60
(2) 300
(3) 120
(4) 210
(5) 0
16. How much charge (in C) must be put on a 2.0 mF capacitor to have enough energy to operate a 25 W light bulb for
10 seconds?
(1) 1.0
(2) 1.4
(3) 3.3
(4) 0.7
(5) 5.4
17. The graph in Figure 10 shows the potential (in V) as a function of distance. In what region is the magnitude of the
electric ¯eld the smallest?
(1) 2{3 m
(2) 1{2 m
(3) 0{1 m
(4) 3{4 m
(5) 4{5 m
18. A gatoron particle (m = 6:5 kg, q = ¡9:6 C) moves in a circular around a massive moron particle (q = +3:5 C)
assumed to be ¯xed in space. If the speed of the gatoron is 200 m/s, ¯nd the radius of orbit (in m)
(1) 1:2 £ 106
(2) 4:7 £ 106
(3) 3:2 £ 102
(4) 2:9 £ 108
(5) 7:6 £ 105
21111
21111
19. An experiment is performed with three rods of unknown charge states. We ¯nd that rod A is repelled from rod B.
Rod B is attracted to rod C. Rod A is attracted to rod C. Which of the following CAN be true?
(1)
(2)
(3)
(4)
(5)
Rod
Rod
Rod
Rod
Rod
A
A
A
A
A
is
is
is
is
is
positively charged, rod B is positively charged, rod C is negatively charged
positively charged, rod B is negatively charged, rod C is negatively charged
positively charged, rod B is a neutral conductor, rod C is negatively charged
negatively charged, rod B is positively charged, rod C is positively charged
a neutral conductor, rod B is positively charged, rod C is negatively charged
20. Make sure that your name, social security number, and EXAM CODE are correctly entered on your bubble sheet.
Which of the following statements is correct? (This problem is graded, so answer it correctly.)
(1) I SWEAR THAT I HAVE CORRECTLY ENTERED THE ABOVE INFORMATION. IF I HAVENT, THIS
QUESTION WILL BE MARKED WRONG!!! (No kidding.)
(2) The device has yet to be invented that can measure the magnitude of my hatred for this class.
(3) Physics? I thought this class was \Basketweaving 101"!
(4) Einstein proved that the laws of physics are relative; therefore, even though I bubbled the information in incorrectly
in this universe, it is correct in some other universe. I deserve credit regardless.
(5) I'm changing my major to philosophy, so physics is the last thing on my mind.
E
(0,a)
1 cm
- 2.5 nC
25 cm
+200 C
4 C
+ 2.5 nC
10-4 kg
-15 C
q2
(0,0)
(-a,0)
Figure 3
Figure 2
Figure 1
a) Initial
+ 3q
- 3q
2C
B
A
1m
C
b) Final
2C
D
2C
1m
1m
2C
2C
1m 2C 1m
2C
1m
E
Figure 4
Figure 5
+6 C/m2
+3 C/m2
+
+
2C
-4 C/m2
R
r
P
P
+7 C/m
+z
2
+y
Figure 6
-q
a
-q
120
90
150
a
-q
P
+q
210
240
Figure 8
V
60
30
180
+q
-q
Figure 7
+x
Figure 10
0
330
270 300
Figure 9
0
1
2
3
4
5
x (m)