Physics 72: Chapter 23 Problem Set Name: Lecturer: Mr Carlos Baldo III
Summer 2010 Score: Section: X7-7
Encircle the letter of the correct answer. Solve each problem clearly and completely for full credit. 1.
Conducting spheres A, B, and C are connected to each other by conducting wires as shown in the figure. If the radius of A and C are the same (rA = rC) and the radius of B is four times the radius of A, which of the following statements is correct, at electrostatic equilibrium? A.
Q A = Q B = QC V A = V B = V C ;
B.
Q A < Q B < QC ; V A = V B = V C
C.
Q A = QC < Q B V A < V B = V C ;
D.
Q A = QC < Q B ; V A = V B = V C
E.
Q A = QC > Q B V A = V B = V C ;
2.
A test charge is placed at a distance r away from the origin and immersed in an electric field E(r) = kQ/r2 (with k = constant, Q > 0). If r is quadrupled, the electron's potential energy is multiplied by a factor of: A. 2 B. 4 C. 1/2 D. 1/4 E. 1/16
3.
The potential at the center of a conducting sphere with radius R is -6V. What is the potential at a distance r=0.5R at any given time? Assume zero potential at infinity. A. -12V B. -6V C. Zero D. +6V E. +12V
4.
How much work is needed by the electric field in assembling the system?
2kQ a A. 2kQ 2 â&#x2C6;&#x2019; a B. 4kQ 2 a C. 5.
4kQ 2 a 2 kQ a
â&#x2C6;&#x2019;
2
D. E.
A negative charge Q is released near the 0V plate (shown in the figure). Electric forces then accelerate it towards the other plate. At which initial point will the charge gain the most speed?
Physics 72: Chapter 23 Problem Set 6.
Summer 2010
What is the corresponding electric field plot for the given potential plot (enclosed in broken line)?
A
D
B
C
E
7.
Which of the following statements is/are true for a conductor in electrostatic equilibrium? A. The conductor is an equipotential region. B. Electric field is continuous at the surface of a conductor. C. Electric potential is continuous at the surface of a conductor. D. A and B E. A and C
8.
A positively charged ball moves freely from points O to points P. Which of the following statements is/are true? A. The potential at point O is higher than at P. B. The potential energy increases as the ball moves. C. The electric field does positive work on the ball. D. A and B E. A and C
9.
Which of the following is / are TRUE? A. The electric field can be parallel to any equipotential surface. B. Equipotential surfaces can intersect. C. The electric field does no work moving a charge along an equipotential surface. D. A and B E. B and C
10. The electric potential energy of a dipole ( q = 1.0 x 10-9 C, d = 2.0 x 10-6 m, k = 9.0 x 109 N.m2/C2 ) is: A. -18.0 x10-3 J B. -4.5 x10-3 J C. 0.0 J D. 4.5 x10-3 J E. 18.0 x10-3 J 11. A positive charge q moves along a length L parallel to a uniform electric field. The work done by the electric field is W. If the length L is increased four times, what will then be the work done by the electric field? A. 4 W B. 2 W C. 1/2 W D. Âź W E. Zero
Physics 72: Chapter 23 Problem Set
Summer 2010
For the next two numbers, consider an electron moves from x to y in a region with uniform electric field as shown in the figure. 12. Which of the following statement/s is/are TRUE? A. The electric field is doing positive work in moving the electron from x to y. B. The potential energy decreases as the electron moves from x to y. C. External force is needed to move the electron from x to y. D. A and B E. B and C 13. If the final potential energy is UO, what is the initial potential energy? A. Uo + eEod B. Uo – eEod C. eEod – Uo D. eEod E. zero 14. A voltmeter tells you that the magnitude of the potential difference between two points, A and B is 10.0 V. You are told that point A is at 5.0 V. An electron will freely move from point B to point A. This means that the electric potential at point B is _________. A. +15.0 V B. + 5.0 V C. Zero D. -5.0 V E. -15.0 V 15. Consider 3-point charges, which are initially infinitely far apart, placed at the corners of an equilateral triangle with side-length d. Two of the point charges are identical and have a charge q. If zero work is required to place the 3 charges at the corners of the triangle, what must be the value of the third charge? A. + q B. + q/2 C. Zero D. – q/2 E. – q 16. Shown in the figure below are point charges that are equidistant from the origin. Which of the following charge combinations will yield a minimum potential at the origin?
17. A negatively charged gummy bear moves from Punta Verde to Kota Kinabalu. Which of the following statements is/are TRUE? A. The potential at Kota Kinabalu is lower than that at Punta Verde B. The potential energy decreases as the gummy bear moves from Punta Verde to Kota Kinabalu C. The electric field does positive work as the gummy bear moves to Kota Kinabalu D. A and B E. B and C F. A, B and C
Physics 72: Chapter 23 Problem Set
Summer 2010
18. A charge q is placed on each corner of a square with side length 2a. What is the energy needed to bring another charge q to the center of the square? A. Zero
B.
C.
D.
E.
kq 2 2a 2 kq 2 a 2 2kq 2 a 2 4kq 2 a 2
For the next two numbers, consider the figure on the right which shows two parallel conducting plates connected to a 6V power supply. The dotted lines indicate the corresponding equipotential lines. 19. How much energy is used by the electric force in moving an electron from X to Y? Note: e = 1.6 x 10-19 C A. + 3.2 x 10-19 J B. + 1.6 x 10-19 J C. zero D. - 1.6 x 10-19 J E. - 3.2 x 10-19 J 20. What is the work done by the electric force in moving an electron from Y to Z? Note: e = 1.6 x 10-19 C A. + 3.2 x 10-19 J B. + 1.6 x 10-19 J C. zero D. - 1.6 x 10-19 J E. - 3.2 x 10-19 J 21. An electron is placed in point A and released from rest. It then freely moves from point A to B. What is the final speed of the electron at point B? Note me is the mass of the electron. A. zero
B.
2eV me
C.
4eV me
D.
6eV me
E.
12eV me
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