Two ping pong balls filled with air are tied to a very light 0.5 meters-long string. The loose ends of the strings are taped to the same position on a horizontal ceiling. When both are equally rubbed with the same material, they develop a static charge and repel each other, making an angle of 30° with respect to the vertical. (m = 3 grams) 

1. With this experiment alone, can you tell what type of charge is on the balloon? Explain. 

2. Is this a stable or unstable equilibrium? 

3. Estimate the number of excess charges on each balloon. Clearly state any assumptions made in your estimation. 

• Short review of Coulomb's law 

• Flux and electric flux

• General conceptual understanding of Gauss’s law 

• Mathematical representation of Gauss’s law 

• How to drive the electric field of a point charge using Gauss’s law