A characteristic negative electrostatic effect is electrostatic attraction. A negatively charged object attracts positively charged objects and even electrically neutral objects. For example, a hard rubber rod receives a negative charge when rubbed with fur. A charged rubber rod attracts bits of paper and even lifts them off the ground in defiance of gravity.
If a negatively charged rubber rod approaches an ungrounded needle, the rod induces a positive charge in the near end of the needle, leaving the far end negatively charged. If the needle is temporarily grounded, electrons escape from the far end of the needle into the ground. After removing the ground, the needle remains positively charged, even after the rubber rod is removed from its proximity. This negative electrostatic effect is called electrostatic induction.
Another characteristic negative electrostatic effect is electrostatic repulsion. Two negatively charged objects repel one another. For example, assume that a ping-pong ball coated with graphite has received a negative charge by electrostatic induction. The ping-pong ball hangs from the ceiling by a string. If a negatively charged rubber rod is held close to the ping-pong ball, the ping-pong ball swings away from the rubber rod because the two objects have the same electrical charge.
When the difference in potential between a negatively charged object and another nearby object becomes great enough, electrons jump from the negatively charged object to its neighbor. A visible spark reveals the path that the electron travels and, if human beings are involved, a shock results. Such electrostatic discharges occur on a grand scale when the electric potential of a rain cloud becomes so great that electrons violently jump to the earth in the form of lightning.
Under certain conditions, negative static electricity dissipates harmlessly into the atmosphere. If the charged object has a point, the electric charge gradually leaks out of the point. Moisture also promotes leakage.
Most colloids suspended in a river have a negative charge. The mutual repulsion of the negatively charged particles helps keep colloids from settling out of the water. When a river reaches the sea, the negatively charged colloidal particles are attracted to positively charged ions in seawater. These ions act as coagulants, and the colloidal particles settle to the bottom.