PHY3054 - Exam 1 - Fall 1996 - Selman Hershfield
1. Three point charges are located on the x-axis as follows: charge Q1 = +1x10-6C is at x = 0 m, charge Q2 = -1x10-6C is at x = 1 m, and charge Q3 = +4x10-6C at x = 2 m. What is the net force in the x direction on charge Q1 ?

1. -0.054 N
2. -0.018 N
3. 0 N
4. 0.018 N
5. 0.054 N

2. A charging experiment involves three rods. Rod A is attracted to rod B. Rod B is repelled from rod C. Rod A is attracted to rod C. Which of the following CAN be true?

1. Rod A is positive, rod B is positive, and rod C is negative.
2. Rod A is positive, rod B is negative, and rod C is negative.
3. Rod A is negative, rod B is positive, and rod C is a neutral conductor.
4. Rod A is negative, rod B is positive, and rod C is negative.
5. Rod A is a neutral conductor, rod B is negative, and rod C is positive.

3. As shown in Fig. 1(a), two conducting spheres rest on insulating stands and are touching. They are initially neutral. A positively charged rod is then brought close to one of the spheres as shown in Fig. 1(b). The spheres are then separated by pulling apart the insulating bases (see Fig. 1(c)). What is the charged state of the spheres?

1. Both sphere A and sphere B are neutral.
2. Both sphere A and sphere B are positively charged.
3. Both sphere A and sphere B are negatively charged.
4. Sphere A is positively charged and sphere B is negatively charged.
5. Sphere A is negatively charged and sphere B is positively charged.

4. Three charges are placed on the corners of an equilateral triangle as shown in Fig. 2. What is the direction of the net force on the 2 micro-Coulomb charge?

1. 20 degrees
2. 70 degrees
3. 110 degrees
4. 160 degrees
5. 340 degrees

5. Three charges are placed on the corners of an equilateral triangle as shown in Fig. 2. What is the magnitude of the net force on the 2 micro-Coulomb charge?

1. 1.2 N
2. 1.35 N
3. 1.5 N
4. 1.65 N
5. 1.8 N

6. 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

7. Three point charges are arranged at the corners of a rectangle as shown in Fig. 3. If the net electric field at point P is in the y-direction, what is the relation between q1 and q2?

1. q2 = -0.8 q1
2. q2 = -0.7 q1
3. q2 = -0.5 q1
4. q2 = 0.7 q1
5. q2 = 0.8 q1

8. Four large charged planes each have charge per unit area of 1 micro-Coulomb/m2 as shown in Fig. 4. What is the magnitude of the electric field at point P?

1. 0 N/C
2. 0.5x105 N/C
3. 1.0x105 N/C
4. 1.5x105 N/C
5. 2.0x105 N/C

9. A conducting shell encloses a conducting sphere as shown in Fig. 5. If the charge on the sphere is 3C and the charge on the outer surface of the conducting shell is -2C, what is the charge on the inner surface of the conducting shell?

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

10. A charge of -1x10-8C weighs 1 g. It is released at rest from point P and moves to point Q. It's velocity at point Q is 1 cm/s. What is the potential difference, VP - VQ?

1. -5000V
2. -5V
3. 5V
4. 500V
5. 5000V

11. Which of the following is true?

1. The electric field and electric force act in the same direction.
2. A higher electrical potential means a higher potential energy.
3. If the electric field is zero in a region, the electrical potential is also zero.
4. If the electrical potential is zero in a region, the electric field is also zero.
5. None of the above are true.

12. Three charges are arranged on the corners of a 1cm square as shown in Fig. 6. One of the charges is moved from an initial configuration to a final configuration. What is the change in the potential energy, Uf - Ui, of the charge configuration?

1. -1 J
2. -0.5 J
3. 0 J
4. 0.5 J
5. 1.0 J

13. Figure 7 (a) consists of two identical positive charges, and Figure 7 (b) consists of a positive and a negative charge with the same magnitude. In both figures the dashed line bisects the line between the two charges. Which of the following is true of the magnitude of the electric field at points P, Q, R, S?

1. |EP| is bigger than |EQ|, and |ER| is bigger than |ES|.
2. |EP| is bigger than |EQ|, and |ER| is smaller than |ES|.
3. |EP| is smaller than |EQ|, and |ER| is bigger than |ES|.
4. |EP| is smaller than |EQ|, and |ER| is smaller than |ES|.
5. None of the above.

14. Consider the large charged sheets shown in Fig. 8. Using Gauss's Law determine the charge on the right sheet. (Hint: Use the cylinder shown in the figure for your Gaussian surface.)

1. -2.7 micro-Coulomb/m2
2. -0.9 micro-Coulomb/m2
3. 0.9 micro-Coulomb/m2
4. 1.8 micro-Coulomb/m2
5. 2.7 micro-Coulomb/m2

15. A thin spherical insulating shell has charge 1 C spread uniformly over its surface. A -2 C point charge is placed outside the shell. Which of the following is true for an imaginary Gaussian surface drawn inside spherical shell?

1. The flux through the surface is positive.
2. The flux through the surface is negative.
3. The flux through the surface is zero, but the electric field is not zero on the Gaussian surface.
4. The flux through the surface is zero, and the electric field is zero on the Gaussian surface.
5. Not enough information given.

16. Two electrons are initially 10-10 meters apart. They are released from rest, and they both fly away. What is the speed of the electrons when they are very very far away from each other?

1. 1.6x106m/s
2. 2.0x106m/s
3. 2.26x106m/s
4. 2.6x106m/s
5. 3.2x106m/s

17. A -1 C charge and a +2 C charge are placed on the x-axis as shown in Fig. 9. In which of the three labeled regions can the electric field be zero?

1. Region I
2. Region II
3. Region III
4. Regions II and III
5. Regions I, II, and III

18. Four charges are arranged on the corners of a square as shown in Fig. 10 (a), (b), and (c). In which case(s) is the electric field equal to zero at the center of the square? Assume that all charges have the same magnitude and the only difference is the sign.

1. a
2. a and b
3. b and c
4. a and c
5. a, b, and c

19. Four charges are arranged on the corners of a square as shown in Fig. 10 (a), (b), and (c). In which case(s) is the electrical potential equal to zero at the center of the square? Assume that all charges have the same magnitude and the only difference is the sign.

1. a
2. a and b
3. b and c
4. a and c
5. a, b, and c

20. Which of the following is not true?

1. A positive charge moves from high potential energy to low potential energy.
2. A negative charge moves from high potential energy to low potential energy.
3. A positive charge moves from high electrical potential (voltage) to low electrical potential.
4. A negative charge moves from high electrical potential (voltage) to low electrical potential.
5. They are all true.

1. Answer = (3) 0 N
2. Answer = (2) Rod A is positive, rod B is negative, and rod C is negative.
3. Answer = (5) Sphere A is negatively charged and sphere B is positively charged.
4. Answer = (4) 160 degrees
5. Answer = (1) 1.2 N
6. Answer = (1) Done correctly
7. Answer = (2) q2 = -0.7 q1
8. Answer = (3) 1.0x105 N/C
9. Answer = (1) -3 C