Learning Introductory Physics with Activities

You have probably experienced the phenomenon of static electricity. For example, when you first take clothes out of a dryer, some may stick together, and make a spark as they are pulled apart. Another example occurs if you take a woolen sweater off quickly—you can hear and feel the crackling of the static electricity pulling on your clothes, and perhaps even your hair. If you rub a balloon on a wall for a few seconds, it will stick to the wall. Probably the most annoying effect of static electricity is getting shocked by a doorknob (or a friend) after shuffling your feet as you walk across a carpet.

This phenomena has been known for centuries. The ancient Greek philosopher, Thales of Miletus (624–546 BCE), recorded that when amber - a hard, translucent, fossilized resin from extinct trees - was vigorously rubbed with a piece of fur, a force was created that caused the fur and the amber to be attracted to each other. Additionally, he found that the rubbed amber would not only attract the fur, and the fur attract the amber, but they both could affect other objects, even if not in contact with those objects. All of these situations involve objects being charged.

Charging an object, by adding excess charge, can occur in several ways. One way to charge an object is by rubbing it against another object. This only works for certain materials because all charge properties are related to the atomic structure of atoms and molecules. Rubbing a piece of plastic PVC pipe with wool will cause charges to be transferred from the wool to the plastic. This process is called charging through friction.

When two materials slide against one another, there is friction between the two contact surfaces. Friction can break the bonds holding a molecule together. Charging through friction occurs when an electrically neutral molecule is broken apart into a positive and negative ion by rubbing it against another object. Often electrons are shared in large molecules. When the molecular bonds are broken, part of the molecule has net positive charge and part of the molecule has net negative charge since. This is how a plastic rod can be charged by rubbing it with wool.