Physics 212Spring 2009

Problem Set #1

NOTE: Show ALL work and ALL answers on a piece of separate loose leaf paper. Not on this sheet.

Read: 15.1 – 15.3 and then do the following problems.

1) A plastic rod has a charge of –2.0 C. How many electrons must be removed so that the charge on the rod becomes +3.0C?

2) Three identical metal spheres, A, B, and C initially have net charges as shown. Spheres A and B are now touched together and then separated. Sphere C is then touched to sphere A and separated from it. Lastly, sphere C is touched to sphere B and then separated from it. (a) How much charge ends up on sphere C? What is the total charge on the three spheres (b) before they are allowed to touch each other and (c) after they have touched?

3) Two small spheres are initially neutral and separated by a distance of 0.50 m. Suppose that 3.0 x 1013 electrons are removed from one sphere and placed on the other. (a) What is the magnitude of the electrostatic force that acts on each sphere? (b) Is the force attractive or repulsive? Explain how you can tell.

4) Three charges are fixed to an xy coordinate system. A charge of +18 C is on the y axis at y = +3.0 m. A charge of –12 C is at the origin. Lastly, a charge of + 45 C is on the x axis x =+3.0 m. Calculate the magnitude and direction of the net electrostatic force on the charge x = +3.0 m.

5) Skid of 40 kg and Mitch of 60 kg are standing on ice. They are each carrying massless spheres of charge +500 C while standing 2 m apart. (a) Calculate the electrostatic force on each sphere. Are the forces the same or different? Explain. (b) Calculate the accelerations for Skid and Mitch at the moment they are 2 m apart. Are they the same or different? Explain. (c) As Skid and Mitch move farther apart do their accelerations increase, decrease, or remain the same? Explain.

6) Two small metallic spheres, each of mass 0.20 g, are suspended by light strings from a common point as shown. The spheres are given the same electric charge, and it is found that the two come to equilibrium when each string is at an angle of 5 with the vertical. If each string is 30.0 cm long, what is the magnitude of the charge on each sphere?

7) Three charges are arranged as shown. Calculate the magnitude and direction of the net electrostatic force on each of the three charges.

8) Two charges are sitting 1.5 m apart with a force of 3 N between them. They are now moved farther apart to 2.25 m and one of the charges is increased by a factor of 4. What is the magnitude of the new force between the two charges?

9) The distance between two charges is increased by a factor of 3 while the values of the charges are both decreased by a factor of ½. By what fraction does the magnitude of the force change?

10) If the magnitude of the force between two identical charges is to remain constant while the distance between the two charges increases by a factor of 2, by what factor will each charge change?

ANSWERS:1) 3.1 x 1013 electrons 7) 46.7 N (left), 157.3 N (right), 2) a) 1.5q 110.6 N (left)

b) 4q 8) 5.33 N c) 4q 9) 1/36

3) a) 0.83 N 10) 24) 0.38 N, 496) 7.2 x 10-9 C

None-1q+5q

CBA

Skid Mitch

Ice

5

6 C 1.5 C -2 C

3 cm 2 cm