PHY3054 - Exam 1 - Fall 1995 - Selman Hershfield
1. Two point charges are located on the x-axis as follows: charge Q1 = +2x10-6C is at x = 0 m and charge Q2 = -1x10-6C is at x = 1 m. What is the net force in the x direction on a third charge, Q3 = -3x10-6C, placed at x = 2 m ?

1. -0.03 N
2. 0.01 N
3. 0.03 N
4. 0.08 N
5. 0.04 N

2. In a charging by induction experiment a positive rod is brought close to an electroscope, which was initially neutral (Fig. 1.1). The electroscope is then grounded (Fig. 1.2). Finally the ground is disconnected and the charged rod is removed (Fig. 1.3). Which of the following is true?

1. The overall charge on electroscope is positive, and the foil is positively charged.
2. The overall charge on the electroscope is zero, and the foil is positively charged.
3. The overall charge on the electroscope is zero, and the foil is negatively charged.
4. The overall charge on electroscope is negative, and the foil is positively charged.
5. The overall charge on the electroscope is negative, and the foil is negatively charged.

3. Four charges are placed on the corners of a diamond (rotated square) as shown in Figure 2. What is the direction of the net force on a -1 micro Coulomb charge placed at the center of the diamond?

1. 45 degrees
2. 135 degrees
3. 180 degrees
4. 225 degrees
5. 315 degrees

4. Four charges are placed on the corners of a diamond as shown in Figure 2. What is the magnitude of the net force on a -1 micro Coulomb charge placed at the center of the diamond?

1. 0.025 N
2. 0.035 N
3. 0.050 N
4. 0.072 N
5. 0.108 N

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

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

6. In Figs. 3.1 - 3.3, one of the planes is positively charged (Q1 > 0), and the other is negatively charged (Q2 < 0); however, the magnitudes of Q1 and Q2 are not specified. From the information given, which of Figs. 3.1 - 3.3 is/are definitely incorrect?

1. All can be correct.
2. Fig. 3.1 is incorrect.
3. Fig. 3.2 is incorrect.
4. Fig. 3.3 is incorrect.
5. Figs. 3.2 and 3.3 are incorrect.

7. Using the superposition principle determine the electric field at point P in Fig. 4.

1. 0 V/m
2. 56 V/m
3. 113 V/m
4. 282 V/m
5. 339 V/m

8. A conducting sphere is enclosed by a conducting shell as shown in Fig. 5. The electric field at point Q, which is 2m from the center of the sphere, is 2,000 V/m pointing away from the sphere. The electric field at point P, which is 1m from the center of the sphere, is 1,000 V/m pointing away from the sphere. What is the net charge on the conducting shell?

1. 1 micro Coulomb
2. 2 micro Coulomb
3. 4 micro Coulomb
4. 8 micro Coulomb
5. 9 micro Coulomb

9. A 1 Coulomb charge is located at the origin. Another charge, Q, is located at x = 3 m. If the electric field is zero at x = 1 m, what is Q?

1. -4 C
2. -2 C
3. 1.4 C
4. 2 C
5. 4 C

10. Five charges are arranged in a pentagon as shown in Fig. 6. What is the direction of the net electric field at the center of the pentagon?

1. 54 degrees
2. 126 degrees
3. 198 degrees
4. 270 degrees
5. 342 degrees

11. Five charges are arranged in a pentagon as shown in Fig. 6. What is the electrical potential (voltage) at the center of the pentagon?

1. -90kV
2. -18 kV
3. 0 kV
4. 18 kV
5. 90 kV

12. An electron initially has velocity 5 x 105 m/s. It is accelerated through a potential of 2 V. What is its final velocity?

1. 6 x 105 m/s
2. 7 x 105 m/s
3. 8 x 105 m/s
4. 9 x 105 m/s
5. 10 x 105 m/s

13. In a parallel plate capacitor an electron moves from the positively charged plate to the negatively charged plate. Which of the following are true for the electron?

1. The potential energy increases, and the electrical potential increases.
2. The potential energy increases, and the electrical potential decreases.
3. The potential energy decreases, and the electrical potential increases.
4. The potential energy decreases, and the electrical potential decreases.
5. The potential energy of the electron is unchanged.

14. Initially a proton travels with velocity 5 x 105 m/s perpendicular to a uniform electric field. After 2 milliseconds, the magnitude of the proton's velocity is 7.6 x 105 m/s. What is the magnitude of the electric field?

1. 0.4 V/m
2. 1 V/m
3. 2 V/m
4. 3 V/m
5. 4 V/m

15. Four 1 micro Coulomb charges are initially arranged in a line as shown in Fig. 7.1. The charges are then reconfigured as shown in Fig. 7.2. What is the change in the potential energy, Uf - Ui?

1. -3.0 mJ
2. -1.5 mJ
3. 0 mJ
4. 1.5 mJ
5. 3.0 mJ

16. In Fig. 8 all capacitors are 1 micro Farad except those marked otherwise. What is the effective capacitance of this circuit?

17. Which of the circuits in Fig. 9 are different? All capacitors have the same capacitance in Fig. 9.

1. All are the same.
2. Fig. 9.1 is different. Figs. 9.2 and 9.3 are the same.
3. Fig. 9.2 is different. Figs. 9.1 and 9.3 are the same.
4. Fig. 9.3 is different. Figs. 9.1 and 9.2 are the same.
5. All of them are different.

18. What is the ratio of the energy stored in the capacitors of Figs. 10.1 and 10.2: (U for Fig. 10.1)/(U for Fig. 10.2)? All capacitors have the same capacitance in Figs. 10.

1. 1/9
2. 1/3
3. 1
4. 3
5. 9

19. In Fig. 11 the charge on the 3 micro Farad capacitor is 1 micro Coulomb. What is the voltage across the 2 micro Farad capacitor?

1. 1/6 V
2. 1/3 V
3. 1/2 V
4. 2/3 V
5. 4/3 V

20. A parallel plate capacitor is held at constant voltage. Initially there is only air between the plates. If a dielectric with a dielectric constant of 2 is inserted into the capacitor, what happens to the energy stored in the capacitor?

1. The energy will go up by a factor of 4.
2. The energy will go up by a factor of 2.
3. The energy will be unchanged.
4. The energy will go down by a factor of 2.
5. The energy will go down by a factor of 4.

1. Answer = (2) 0.01 N
2. Answer = (5) The overall charge on the electroscope is negative, and the foil is negatively charged.
3. Answer = (1) 45 degrees
4. Answer = (3) 0.050 N
5. Answer = (1) Done correctly
6. Answer = (1) All can be correct.
7. Answer = (1) 0 V/m
8. Answer = (1) 1 micro Coulomb
9. Answer = (5) 4 C
10. Answer = (2) 126 degrees
11. Answer = (4) 18 kV
12. Answer = (5) 10 x 105 m/s
13. Answer = (2) The potential energy increases, and the electrical potential decreases.
14. Answer = (4) 3 V/m
15. Answer = (2) -1.5 mJ