Week 1 of Fall 2012
Kevin Le, kevinle@usc.edu
Electricity and Magnetism
www-scf.usc.edu/˜kevinle
Electric Charges, Coulomb’s Law, and the Electric Field
1. I give you a sphere charged with 1 µC attached to an insulated stick and one more identical sphere
similarly insulated. You may discharge any sphere by touching it to the ground. How will you
1
produce a sphere with µC? If you cannot ground anything but are given more identical spheres,
8
1
5
how many more will you need to get me µC? How will you get me
µC?
8
16
2. At the point x = 3a is a point charge Q. What is the force exerted on a charge 2Q located at the
origin?
3. At the point x = −a is a point charge Q. At the point x = 2a is a point charge −Q. What is the
total force exerted on a charge 2Q?
4. At six corners of a hexagon inscribed in a circle of radius r = a are placed electrons and at the center
is placed a proton. What is the force on the proton? Now remove the electron in the northeast
corner and recompute the total force on the proton.
5. At the origin is a point charge 2Q. What is the electric field at the point (3a, 4a)? Sketch the electric
field lines.
6. At the point x = −2a is a point charge Q. At the point x = a is a point charge −Q. What is the
electric field at the point (0, 2a)? Sketch the electric field lines.