PHY3054 - Exam 1 - Fall 1994 - Selman Hershfield
1. Two point charges are located on the x-axis as follows: charge Q1 = +8x10-9C is at x = 0 cm and charge Q2 = -1x10-9C is at x = 1 cm. What is the net force in the x direction on a third charge, Q3 = +2x10-9C, placed at x = 2 cm ?

1. 2x10-8 N
2. 1.8x10-4 N
3. -1.8x10-4 N
4. 5.4 x 10-4 N
5. -5.4 x 10-4 N

2. Three charges are placed on corners of an equilateral triangle as shown in Figure 1. An electron is placed at the center of the triangle. What is the direction of the net force on the electron?

1. 30 degrees
2. 60 degrees
3. 180 degrees
4. 210 degrees
5. 330 degrees

3. Three charges are placed on corners of an equilateral triangle as shown in Figure 1. An electron is placed at the center of the triangle. What is the magnitude of the net force on the electron?

1. 0
2. 2.9x10-9 N
3. 4.3x10-9 N
4. 8.6x10-9 N
5. 1.74x10-8 N

4. In a hydrogen atom an electron circles a proton. Since the mass of the proton is much greater than the mass an electron, assume that the proton stays fixed in space and the electron rotates around it with radius 0.529x10-10 m. What is the velocity of the electron?

1. 1.1x106 m/s
2. 2.2x106 m/s
3. 4.1x106 m/s
4. 1.1x1012 m/s
5. 4.1x1012 m/s

5. In a charging by induction experiment a negative rod is brought close to an electroscope as shown in Figure 2. The electroscope was initially neutral. Which of the following is true?

1. The overall charge on electroscope is zero, and the foil is neutral.
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 positive, and the foil is positively charged.
5. The overall charge on electroscope is positive, and the foil is negatively charged.

6. A charge of 1 micro Coulomb experiences a force of 2N upwards in a uniform electric field. Another charge placed in the same electric field experiences a force of 1N downwards. What is its charge?

1. -2 micro Coulomb
2. -1 micro Coulomb
3. -0.5 micro Coulomb
4. 0.5 micro Coulomb
5. 2 micro Coulomb

7. Which of the drawings of electric field lines in Figures 3.1 - 3.3 is/are definitely incorrect?

1. Fig. 3.1
2. Fig. 3.2
3. Fig. 3.3
4. Figs. 3.1 and 3.3
5. none of them

8. A uniform electric field exists in the region between two oppositely charged plane parallel plates. An electron is released from rest at the surface of the negatively charged plate and strikes the surface of the opposite plate 2x10-8 s later. If the magnitude of the electric field is 4x103 N/C, what is the separation between the plates?

1. 3 cm
2. 6 cm
3. 7 cm
4. 12 cm
5. 14 cm

9. Two charged particles are placed on the x-axis as follows: Q1 = 1x10-9 C at x = 0 m and Q2 = -2x10-9 C at x = 1.6 m. Where on the x axis is the electric field zero?

1. -4.0 m
2. -0.6 m
3. 0.6 m
4. 1.0 m
5. 2.2 m

10. A conducting sphere is enclosed by two different conducting shells as shown in Fig. 4. The charge on the sphere is 1C. The charge on the inside surface of the outer most shell is -1C. What is the net charge on the middle shell?

1. -2C
2. -1C
3. 0 C
4. 1C
5. 2C

11. Two stationary point charges are arranged on the x-axis as follows: Q1 = 2.4x10-10 C at x = 0 m and Q2 = -1.2x10-10 C at x = 1 m. An electron is placed at x = 2 m and let go. What is the velocity of the electron when it reaches x = 3 m.

1. 2.5x105 m/s
2. 4.3x105 m/s
3. 5.6x105 m/s
4. 6.6x105 m/s
5. 7.5x105 m/s

12. In Figure 5 a uniform electric field of magnitude 0.5 N/C points uniformly upward. What is the work done by the electric field on a positive 1C charge, which moves from point A to point B?

1. -2 J
2. -0.5 J
3. 0.5 J
4. 2 J
5. Not enough information is given.

13. Three point charges are brought from infinity to the following positions on the x-axis: Q1 = 5.2 micro Coulomb at x = -1 m, Q2 = 2.6 micro Coulomb at x = 0 m, and Q3 = 5.2 micro Coulomb at x = 1 m. What is the potential energy of this charge configuration?

1. 0 J
2. 0.12 J
3. 0.24 J
4. 0.36 J
5. 0.48 J

14. Six point charges are arranged at the center of a hexagon as shown in Fig. 6. Which of the following is true of the electric field, E, and electrical potential, V, at the center of the hexagon?

1. V is zero and E is zero.
2. V is zero and E is not zero.
3. V is not zero and E is zero.
4. V is not zero and E is not zero.
5. Not enough information is given.

15. A cathode-ray tube (TV tube) contains two sets of deflectors, which are oppositely charged parallel plates (see Figure 7). In order to focus the beam in the upper right corner of the screen, how do you charge the deflectors?

1. Positively charge the right horizontal deflection plate and positively charge the upper vertical deflection plate.
2. Positively charge the right horizontal deflection plate and negatively charge the upper vertical deflection plate.
3. Negatively charge the right horizontal deflection plate and negatively charge the upper vertical deflection plate.
4. Negatively charge the right horizontal deflection plate and positively charge the upper vertical deflection plate.
5. Leave all plates uncharged.

16. In the capacitor network of Fig. 8 the charge on the 2 micro Farad capacitor is 2 micro Coulombs. What is the charge on the 1 micro Farad capacitor?

1. 0.25 micro Coulomb
2. 0.75 micro Coulomb
3. 1 micro Coulomb
4. 1.5 micro Coulomb
5. 2 micro Coulomb

17. In the capacitor network of Fig. 8 the voltage across the 4 micro Farad capacitor is 1 Volt. What is the voltage across the 2 micro Farad capacitor?

1. 0.25 V
2. 0.42 V
3. 0.5 V
4. 1 V
5. 2V

18. An air capacitor consisting of two closely spaced parallel plates has a capacitance of 1000 pF. The magnitude of the charge on each plate is 4 micro Coulombs. If a dielectric material with dielectric constant K = 2 is inserted between the plates of the capacitor, what will happen to the energy stored in the capacitor? (The charge remains unchanged.)

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

19. What is the effective capacitance of the capacitor network in Figure 9?

20. Which statement is true about the capacitance of the three networks shown in Figure 10?

1. Figs. 10.1 and 10.2 are the same and Fig. 10.3 is different.
2. Figs. 10.2 and 10.3 are the same and Fig. 10.1 is different.
3. Figs. 10.1 and 10.3 are the same and Fig. 10.2 is different.
4. All three are the same.
5. All three are the different.

1. Answer = (2) 1.8x10-4 N
2. Answer = (1) 30 degrees
3. Answer = (4) 8.6x10-9 N
4. Answer = (2) 2.2x106 m/s
5. Answer = (3) The overall charge on the electroscope is zero, and the foil is negatively charged.
6. Answer = (3) -0.5 micro Coulomb
7. Answer = (1) Fig. 3.1
8. Answer = (5) 14 cm
9. Answer = (1) -4.0 m
10. Answer = (3) 0 C
11. Answer = (1) 2.5x105 m/s
12. Answer = (3) 0.5 J
13. Answer = (4) 0.36 J
14. Answer = (2) V is zero and E is not zero.
15. Answer = (1) Positively charge the right horizontal deflection plate and positively charge the upper vertical deflection plate.
16. Answer = (2) 0.75 micro Coulomb
17. Answer = (4) 1 V
18. Answer = (2) The energy will go down by a factor of 2.