On a dry winter day in Ontario you shuffle across a carpet, reach for a metal doorknob and zap. You rub a balloon on your hair and it sticks to the wall. You pull a sweater over your head and your hair stands straight up. All of these come from the same tiny particle: the electron. In this lesson you will find out where electric charge comes from, why some objects attract and others repel, and how you can charge an object on purpose. This is the starting point for everything else in the unit, from circuits to power plants.
All matter is made of atoms, and atoms contain three kinds of particles. Protons have a positive (+) charge and sit in the tiny centre of the atom, the nucleus. Neutrons have no charge and are also in the nucleus. Electrons have a negative (−) charge and move around outside the nucleus. A proton's charge and an electron's charge are the same size but opposite signs. An object is neutral when it has equal numbers of protons and electrons, so the charges cancel. An object becomes charged when this balance changes. In solid objects, that happens because electrons move: protons are locked inside the nucleus and do not leave. An object that gains electrons becomes negative. An object that loses electrons becomes positive, because it now has more protons than electrons.
Static electricity is charge that builds up on the surface of an object and stays there ("static" means not moving) until it has a path to escape. When it suddenly escapes, as in the spark from a doorknob, we call it an electric discharge. Static charge can be useful, too: laser printers use a charged drum to attract toner powder into the shape of letters, and factories charge paint droplets so they are pulled onto a car body and less paint is wasted. Charge is measured in coulombs (C), and its symbol in formulas is . One electron carries a very tiny charge: it takes about electrons to make 1 C of charge.
The laws of electric charges
- Opposite charges attract. A positive object and a negative object pull toward each other.
- Like charges repel. Two negative objects push each other away, and so do two positive objects.
- Charged objects attract neutral objects. A charged object pushes or pulls on the electrons in a nearby neutral object, shifting them slightly to one side. The near side ends up with the opposite charge, so it is attracted. This shifting is called charge separation (or polarization). It is why a charged balloon sticks to a neutral wall.
Three ways to charge an object
| Method | What happens | Charge left on the object |
|---|---|---|
| Friction | Two different materials are rubbed together. Electrons move from the material that holds them weakly to the one that holds them strongly. | The two objects get equal and opposite charges. |
| Contact (conduction) | A charged object touches a neutral object. Some electrons move between them, so the charge is shared. | The same sign as the charged object. |
| Induction | A charged object is brought near (not touching) a neutral object that is connected to the ground. Electrons flow to or from the ground. The ground connection is removed, then the charged object is taken away. | The opposite sign to the charged object. |
The electrostatic series
Which material wins the electrons when two things are rubbed together? Scientists have tested many pairs and ranked materials in an electrostatic series (also called the triboelectric series). Materials near the top hold their electrons weakly and tend to lose them. Materials near the bottom hold electrons strongly and tend to gain them. Here is a short version, from tends to lose electrons (becomes +) to tends to gain electrons (becomes −): glass, human hair, nylon, wool, silk, paper, cotton, ebonite (hard rubber), polyester, plastic wrap, vinyl (PVC). Fur is also near the top. Different sources order a few materials slightly differently, and humidity changes the results, but the overall pattern holds.
Detecting charge with an electroscope
An electroscope detects electric charge. Its knob, rod and leaves are metal, so charge spreads through all of them. The insulating stopper keeps the charge from leaking into the jar. If you touch the knob with a negatively charged rod, extra electrons spread over the knob, rod and both leaves. Both leaves are now negative, so they repel and spread apart. To find the sign of an unknown charge, first charge the electroscope with a known sign. Then bring the unknown object near the knob. If the leaves spread farther apart, the object has the same sign. If the leaves move closer together, the object has the opposite sign (or it may be neutral, so check with a repulsion test).