PHY2054 - Final - Fall 1997 - Selman Hershfield
1. As shown in Fig. 1(a) and (b), when a rod is moved closer to an electroscope, the leaves of the electroscope move further apart. Which of the following is true?

1. The electroscope must be charged, i.e, the net charge is not zero.
2. The rod can not be overall neutral and metallic.
3. The electroscope can have an overall positive charge, if the rod is a positively charged insulator.
4. The electroscope can have an overall positive charge, if the rod is a negatively charged insulator.
5. None of the other answers is correct.

2. Four charges are arranged on a rectangle as shown in Fig. 2. What is the magnitude of the force on Q3 ?

1. 1340 N
2. 1490 N
3. 1560 N
4. 1730 N
5. 1900 N

3. In the above problem, what is the direction of the force on Q3 ?

1. 112 degrees
2. 145 degrees
3. 176 degrees
4. 184 degrees
5. 248 degrees

4. The large parallel planes shown in Fig. 3 have charge per unit area of sigma 1 = -8x10-9 C/m2 , sigma 2 = -8x10-9 C/m2 , and sigma 3 = 2x10-9 C/m2 . What is the electric field at point P2? Use the convention that positive is in the +x direction, and negative is in the -x direction.

1. -1017 N/C
2. -791 N/C
3. -113 N/C
4. +113 N/C
5. +1017 N/C

5. A spherical conductor is surrounded by two conducting shells as shown in Fig. 4. The radius of the sphere is 1m. The inner radius of the middle shell is 1.5 m, and the outer radius of the same shell is 2m. Finally, the inner radius of the outside shell is 2.5 m, and the outer radius of the outer shell is 3m. Given that the electric field at r=1.2m is 6.24x103 N/C, the charge on the inside of the outer shell is 7.00x10-6 C, and the net charge on the outer shell is 6.00x10-6 C, what is the electric field at r=2.3m in N/C? (We are using the convention that the electric field is positive going away from the center of the sphere and negative going towards the center.)

1. -12000 N/C
2. -10200 N/C
3. +1700 N/C
4. +10200 N/C
5. +12000 N/C

6. In Fig. 5, which point, P2 or P3, has (i) the higher potential energy for an electron and (ii) the higher electrical potential for an electron? (The charge densities are the same as for Fig. 3: sigma 1 = -8x10-9 C/m2 , sigma 2 = -8x10-9 C/m2 , and sigma 3 = 2x10-9 C/m2 .)

1. (i) P2, (ii) P2
2. (i) P2, (ii) P3
3. (i) P3, (ii) P2
4. (i) P3, (ii) P3
5. None of the other answers is correct.

7. If the current through R4 in Fig. 6 is 4A, what is the net current, I, through the whole network?

1. 15 A
2. 16 A
3. 17 A
4. 18 A
5. 19 A

8. In the circuit shown in Fig. 7, what is the voltage difference VB - VD ?

1. -24 V
2. -4 V
3. +4 V
4. +12 V
5. +24 V

9. In the circuit shown in Fig. 8, each of the resistors are light bulbs with identical resistance. List the light bulbs from dimmest to brightest. Group light bulbs with the same brightness in parentheses.

1. dimmest (B),(AD),(CEF) brightest
2. dimmest (B),(AD),(EF),(C) brightest
3. dimmest (B),(CEF),(AD) brightest
4. dimmest (B),(EF),(ACD) brightest
5. dimmest (B),(EF),(AD),(C) brightest

10. What is the effective capacitance of the circuit shown in Fig. 9?

1. 4.6 micro-F
2. 5.3 micro-F
3. 6.5 micro-F
4. 7.4 micro-F
5. 27 micro-F

11. In the RC circuit shown in Fig. 10, what is the current through R2 (a) immediately after the switch is closed, (b) a long time after the switch is closed?

1. (a) 0 mA, (b) 0 mA
2. (a) 0 mA, (b) 6 mA
3. (a) 0 mA, (b) 12 mA
4. (a) 6 mA, (b) 0 mA
5. (a) 12 mA, (b) 0 mA

12. An electron travels in the x-y plane as shown in Fig. 11. It experiences a force in the z direction due to a uniform magnetic field. Which of the following are possible directions for the magnetic field?

1. The magnetic field can be in the x direction or the y direction.
2. The magnetic field can be in the x direction or the -y direction.
3. The magnetic field can be in the -x direction or the y direction.
4. The magnetic field can be in the -x direction or the -y direction.
5. The magnetic field can be in the x direction or the z direction.

13. A proton moves perpendicular to an magnetic field of 3 Tesla. How long does it take the proton to complete one orbit?

1. 3.5 x 10-12 sec
2. 2 x 10-11 sec
3. 2 x 10-9 sec
4. 3.5 x 10-9 sec
5. 2 x 10-8 sec

14. Two wires with current flowing perpendicular to the plane of the page produce the magnetic field lines shown in Fig. 12. Which direction is the current moving in each wire?

1. into the page for wire A, into the page for wire B
2. into the page for wire A, out of the page for wire B
3. out of the page for wire A, into the page for wire B
4. out of the page for wire A, out of the page for wire B
5. None of the other answers is correct.

15. In Fig. 13 (a) the current in the outer loop is going clockwise and decreasing. (a) What is the direction of the induced current in the inner loop? In Fig. 13 (b) the current in the wire is moving down and increasing. (b) What is the direction of the induced current in the loop to the left of the wire?

1. (a) clockwise, (b) clockwise
2. (a) clockwise, (b) counterclockwise
3. (a) counterclockwise, (b) clockwise
4. (a) counterclockwise, (b) counterclockwise
5. None of the other answers is correct.

16. Light is incident at angle A from a media with n = 1.5 to one with n = 2.0 as shown in Fig. 14. What is the minimum value for A so that there is total internal reflection at point P, which is at the interface between the n = 2 and n = 1 materials? ?

1. 25 degrees
2. 30 degrees
3. 35 degrees
4. 42 degrees
5. 49 degrees

17. An object of height 5 cm is placed 30 cm in front of spherical concave mirror. If the image is real and 10 cm high, what is the radius of curvature of the mirror?

1. 10 cm
2. 20 cm
3. 40 cm
4. 60 cm
5. 120 cm

18. Two lenses are placed on the x-axis. The first lens is a diverging lens with focal length 4cm placed at x = -2cm. The second lens is a converging lens with focal length 7cm placed at x = 8cm. If an object is placed at x = -18cm, what is the position of the image on the x-axis? (Choose the closest answer.)

1. x = 11 cm
2. x = 15 cm
3. x = 19 cm
4. x = 21 cm
5. x = 23 cm

19. In the above problem, which of the following characterizes the final image when compared to the original object?

1. real, upright, and larger
2. real, upright, and smaller
3. real, inverted, and larger
4. real, inverted, and smaller
5. virtual, upright, and larger

20. Place your name, social security number, and exam number on your bubble sheet -- NOW. Also, print and sign your name on the exam printout.

1. Done correctly.
2. Done incorrectly.
3. Done incorrectly.
4. Done incorrectly.
5. Done incorrectly.

1. Answer = (3) The electroscope can have an overall positive charge, if the rod is a positively charged insulator.
2. Answer = (2) 1490 N
3. Answer = (3) 176 degrees
4. Answer = (3) -113 N/C
5. Answer = (1) -12000 N/C
6. Answer = (2) (i) P2, (ii) P3
7. Answer = (4) 18 A
8. Answer = (2) -4 V
9. Answer = (5) dimmest (B),(EF),(AD),(C) brightest
10. Answer = (1) 4.6 micro-F
11. Answer = (4) (a) 6 mA, (b) 0 mA
12. Answer = (2) The magnetic field can be in the x direction or the -y direction.
13. Answer = (5) 2 x 10-8 sec
14. Answer = (3) out of the page for wire A, into the page for wire B
15. Answer = (2) (a) clockwise, (b) counterclockwise
16. Answer = (4) 42 degrees
17. Answer = (3) 40 cm
18. Answer = (5) x = 23 cm
19. Answer = (4) real, inverted, and smaller
20. Answer = (1) Done correctly.