Physics 106: Introduction to Classical Physics
Spring Semester, 2005

Week 1

1) Purcell 1.1

2) Purcell 1.3

3) Purcell 1.4

4) Purcell 1.6. Also show the direction of the forces.

5) Consider a cube of side a with a charge of size +Q on each corner.

a) Find the potential energy for this configuration of charges.

b) Now imagine a charge of size -Q is placed at the center of the cube. Without writing down any equations, determine the electrostatic force that this charge feels. Be sure to give an argument how reached your result. (Hint: consider the symmetry of the arrangement.)

c) Now imagine that one of the eight positive charges is removed from the cube. What is the force that the negative charge at the center now feels? (Hint: Don't try to compute the vector sum of the forces from the seven remaining charges. Instead, consider your answer to part b) and think about how else might you change the charge distribution so that you have zero net charge at one corner.)