# Physics Lessons

Topics: Electric charge, Electromagnetism, Capacitor Pages: 11 (3024 words) Published: January 17, 2012
University of the Philippines
College of Science

PHYSICS 72
SET A

Second Semester 2010-2011 First Long Examination

National Institute of Physics

2nd Sem AY 2010-2011

Physics 72

Consider three identical metal spheres, A, B, and C. Sphere A carries a charge of 2.0 µC; sphere B carries a charge of -6.0 µC; and sphere C is neutral. Spheres A and B are touched together and then separated. Spheres B and C are then touched and separated. What will be the new net charge of sphere C? A. -6.0 µC B. -2.0 µC C. 0 D. -1.0 µC E. +4.0 µC Charges Q1 = q and Q2 = +9q are placed as shown below. Of the five positions indicated by the numbered dots, at which point is the electric field E zero? Points 2, 3 and 4 have equal spacing in between Q1 and Q2.

2.

A. B. C. D. E. 3.

1 2 3 4 5

An electric dipole is released from rest in a uniform electric field with the orientation shown. Which of the following correctly describes the rotation and the net force on the dipole? A. No rotation; net force is zero B. Rotation is clockwise; net force is zero C. Rotation is counterclockwise; net force is zero D. Rotation is clockwise; net force is non-zero E. Rotation is counterclockwise; net force is non-zero

1|A

National Institute of Physics

2nd Sem AY 2010-2011

Physics 72

4.

Five particles of the same mass are released into a region of uniform electric field. The numbered lines show the paths taken by the five particles. A particle with a charge +6Q follows path 5 while it moves through this field. Which path would be taken by a charge of -3Q? A. 1 B. 2 C. 3 D. 4 E. 5 An electric dipole of moment p is placed in a uniform external electric field as shown in the diagram below. If the dipole is to have minimum potential energy, in what direction should p be? A. B. C. D. E. positive x. negative x. positive y. negative y. positive z.

5.

6.

7.

Outcast. If a positive point charge is placed near one corner of the cube, which of the following statements will most likely to happen? A. It will move closer to the cube. B. It will move away from the cube. C. It will stay on its position. D. It will move sideward tracing the side of the cube. E. It will move back and forth about its position. Ecstatic. If the three marbles are in contact, determine the relative net charges of each marble (at electrostatic equilibrium) in terms of the electronic charge, |e|. A. A: 0, B: 0, C: 0 B. A: e, B: e, C: e C. A: -e, B: -e, C: -e D. A: 3e, B: e, C: -e E. A: -3e, B: -e, C: e

For the next two numbers, consider three point charges placed at the corners of a square with side-length 2S. An electron is placed at the center of the square. 8. Feel the force. Determine the directions of the net electric field experienced by the electron and the net electric force acting on it. A. E: towards –Q, F: towards –Q

2|A

National Institute of Physics B. C. D. E. 9.

2nd Sem AY 2010-2011

Physics 72

E: towards –Q, F: away from –Q E: away from –Q, F: towards –Q E: away from –Q, F: away from –Q None, since E=0, F=0

Codename E zero. Which of the following steps will make the electron stay at the center? A. Putting +Q on the fourth corner. B. Putting –Q on the fourth corner. C. Putting +2Q on the fourth corner. D. Putting –2Q on the fourth corner. E. The electron is already stationary at the center.

10. Smiley inside. Consider three charged smileys with each smiley surrounded by a Gaussian surface shown to the right. Rank them in terms of the...