Electrostatic Force

The first movie shows two charged balls (ping-pong balls covered with conductive paint) exerting electrostatic forces on each other. One ball is mounted on a glass rod. The other hangs 2.00 m below the ceiling on a bifilar polyester thread. A plumb bob in the background shows where the hanging ping-pong ball is when there is no horizontal force on it.

You can use this movie to verify Coulomb's force law. With a video analysis application, mark the positions of the two balls in each frame of the movie. The distance the hanging ball moves horizontally can be used to find the horizontal force on it by considering the vector nature of the tension, gravitational and electrostatic forces. The distance between the centers of the balls is the variable in Coulomb's law.

Download Video: MP4: Coulomb.mp4

The second movie is similar, except it shows a vertical charged Teflon rod and a charged hanging ball. Use it to test the formula, often derived in introductory physics textbooks, for the electric field due to a line of charge.

Download Video: MP4: Vertical-Rod_Ball.mp4