Q.

A neutral Ping Pong ball dangles from the ceiling by a thread. A person chafes a rubber rod with animal fur and then touches the Ping Pong ball with the rubber rod. Afterwards what is the charge of the Ping Pong ball?

Amswer

Chafing the rubber rod with animal fur gives the rubber rod a negative charge. The statement that the rod turns into charged is based on the fact that the experiment has been done millions of times with that result. The reality that the charge is negative is simply a definition Ben Franklin's definition. A neutral entity that comes in contact with a charged object acquires the same kind of charge as the charged object. This is called as charging by contact. The Ping Pong ball obtains a negative charge because it comes in contact with the negatively charged rubber rod.