HW 1 Due Tuesday, July 10In Class

PHY 114, Summer 2007

Langenbrunner

HW 1 – due Tuesday, July 10in class

(Please do the problems on a different sheet of paper. Write your name, the date, and the homework number on top of the sheet. Show all steps, mark your answer clearly, and don’t forget to staple.)

1. The figure below shows five pairs of plates: A, B, and D are charged plastic plates and C is an electrically neutral copper plate. The electrostatic forces between the plates are shown for three of the pairs. For the remaining two pairs, do the plates attract or repel each other?

2. Two identical, electrically isolated conducting spheres A and B are separated by a (center-to-center) distance a that is large compared to the spheres. Sphere A has a positive charge of +Q, and sphere B is electrically neutral. Initially, there is no electrostatic force between the spheres. Suppose the spheres are connected for a while by a neutral conducting wire. What is the electrostatic force between the spheres after the wire is removed?

3. The figure below shows four situations in which charged particles are fixed in place on an axis. In which situations is there a point to the left of the particles where an electron would be in equilibrium?

4. A central particle of charge –q is surrounded by charged particles, as shown (call the radii of the two “rings” r and R, where rR). What is the magnitude and direction of the net electrostatic force on the central particle due to the other particles? (do not spend much time on this one….)

5. A positively charged ball is brought close to a neutral isolated conductor. The conductor is then grounded while the ball is kept close. Is the conductor charged positively or negatively, or it is neutral, if

a) the ball is first taken away and then the ground connection is removed?

b) the ground connection is first removed and then the ball is taken away?

6. A point charge of +3.00 x 10-6 C is 12.0 cm distant from a second point charge of -1.5 x 10-6 C. Calculate the magnitude of the force on each charge. Is it repulsive or attractive?

7. The charges and coordinates of two charged particles held fixed in the xy plane are q1=+3.0μC, x1=3.5 cm, y1=0.5 cm, and q2=-4.0μC, x2=6.5 cm, y2=0. Find the magnitude and direction of the electrostatic force on q2.

8. What is the total charge in coulombs of 75.0 kg of electrons?

9. An electron is in a vacuum near the surface of the Earth. Where, relative to the first electron, should a second electron be placed so that the electrostatic force it exerts on the first electron balances the gravitational force on the first electron due to the Earth?

10. In the figure below, two tiny conducting balls of identical mass m and identical charge q hang from nonconducting threads of length L.

a) If x is the distance between the charges, find an expression for x in terms of q, L, and m (NOT θ) when the system is in equilibrium.

b) How can you approximate this expression for cases where xL?

11. The figure below shows a plastic rod of uniform charge distribution. If a point charge +q is placed at point P, what will be the force (magnitude and direction) on that charge from the rod? Your answer should be in terms of r, q, and Q.