Electric Charge

Where do we start? How about with some basics. Electricity, from the Greek word elektron means "amber" - the petrified tree resin that carried the mosquito with the dinosaur DNA in "Jurassic Park". Amber could be rubbed with a cloth and it would attract dust or small pieces of paper, much like a comb can, or our glass rod in class. We call this static electricity and it is a phenomenon we are very familiar with. An object becomes charged after rubbing with another object through a transfer of free electrons. You feel the effect of this when you shuffle across the carpet on a cool, dry day and touch a doorknob. (Why on dry days? Read a little farther!) When an object is charged, it has a net electric charge There are two types of charges - positive and negative. Opposite charges attract, and like charges repel. This is important to remember when we are working with
charges and sign conventions. When an object has equal amounts of positive and negative charge, we say there is no net charge and the object is neutral.
There is another cool thing about charge that is echoed in other areas of physics. If two objects are rubbed together and one gains a positive charge, the other object will gain a negative charge of the same magnitude. In fact, the net amount of electric charge produced in any process is zero. This is called the Law of Conservation of Electric Charge. If one region gains a positive charge, then there is an equal amount of negative charge somewhere nearby. No exception to this has ever been found.

Electricity comes from atoms. Or specifically, from the surplus or absence of the normal amount of electrons an atom should have. We call atoms with a charge ions and they can be positive (some electrons are missing) or negative (there is a surplus of electrons). So where is all this charge going or coming from? Water molecules in the air attract free electrons due to the fact that one side of a water molecule tends to be charged compared to the other side of the molecule. The molecule is said to be polar. Although the net charge of a water molecule is neutral, its make up, shown here, tends to make one side positive or negative. This will cause electrons to be loosely held to the water. The earth itself is a huge reservoir for electrons, and electrons tend to flow pretty freely through objects to the earth. We call this process grounding.

We mentioned that rubbing objects together may also cause a transfer of charge. This is due in part to the way atoms are put together. While the protons and neutrons in the nucleus are fairly stable and fixed, the electrons orbiting the nucleus are more or less free to come and go. Rubbing two objects together can cause a transfer of electrons from one object to the other. The object that gains electrons becomes the negatively charged object, and the object that loses electrons becomes the positively charged object. Robert Millikan performed an experiment in 1909 using oil drops and electric apparatus to determine the charge on an electron. This is
discussed in a little more detail in a lesson on the electron.  He determined that charge is quantized, that is, it exists in discrete units. We say a unit of charge is symbolized by the letter e, which refers to an electron. An electron has 1 unit of charge, so an object may +e, +2e, +3e, -5e, etc units of charge. It cannot have .5e. Or any other fraction of a unit. Charge is measured, in the SI system in Coulombs (C). 1 e has a charge of 1.602 x 10-19 C, or, there are 6.2 x 1018 electrons present in one Coulomb of charge. More on this in the lesson on Coulomb's Law.

Some more definitions: Objects that allow a transfer of electrons freely, such as metals, are called conductors. Objects that don't, such as wood, glass, ceramic, and rubber, are called insulators. Some materials are good conductors but only under certain conditions. Otherwise they are insulators. These materials are called semiconductors and these are really useful in making electronic components such as computer chips. And some superconductors, under certain conditions of temperature, can become perfect conductors.

Static electricity is all around us. Sometimes the build up of excess charge can be really large, and we can see the discharge path to ground, such as with our van de Graf generator, or with a lightning bolt. This is a little bit different than current flow to ground, which we will discuss later.

Can we measure static charge? Sure. A device called an electroscope can be used to detect the presence of charge. Generally, two pieces of foil are placed in a jar and connected at the top. When a charge is placed on the foil, the leaves will move apart. The more charge, the greater the distance the foil moves. Unfortunately, the electroscope won't tell us charge unless we first remove the charge, then apply a known charge to the leaves and test new charges against that. New devices, called electrometers, replace electroscopes and are much more sensitive.

For more on Electric Charge, visit physics.bu.edu/py106/notes/Charge.html

For more on Static Electricity, visit