Chapter 3: Resistance and Static Electricity
As we have learned, some kinds of atoms contain loosely attached electrons. Electrons can be made to move easily from one atom to another. When those electrons move among the atoms of matter, a current of electricity is created.
Take a piece of wire. The electrons are passed from atom to atom, creating an electrical current from one end to the other. Electrons are very, very small. A single copper penny contains more than 10,000,000,000,000,000,000,000 (1×1022) electrons.
Electricity “flows” or moves through some things better than others do. The measurement of how well something conducts electricity is called its resistance.
Resistance in wire depends on how thick and how long it is, and what it’s made of. The thickness of wire is called its gauge. The smaller the gauge, the bigger the wire. Some of the largest thicknesses of regular wire is gauge 1.
Different types of metal are used in making wire. You can have copper wire, aluminum wire, even steel wire. Each of these metals has a different resistance; how well the metal conducts electricity. The lower the resistance of a wire, the better it conducts electricity.
Copper is used in many wires because it has a lower resistance than many other metals. The wires in your walls, inside your lamps and elsewhere are usually copper.
A piece of metal can be made to act like a heater. When an electrical current occurs, the resistance causes friction and the friction causes heat. The higher the resistance, the hotter it can get. So, a coiled wire high in resistance, like the wire in a hair dryer, can be very hot.
Some things conduct electricity very poorly. These are called insulators. Rubber is a good insulator, and that’s why rubber is used to cover wires in an electric cord. Glass is another good insulator. If you look at the end of a power line, you’ll see that it is attached to some bumpy looking things. These are glass insulators. They keep the metal of the wires from touching the metal of the towers.
Another type of electrical energy is static electricity. Unlike current electricity that moves, static electricity stays in one place.
Try this experiment.
Rub a balloon filled with air on a wool sweater or on your hair. Then hold it up to a wall. The balloon will stay there by itself.
Tie strings to the ends of two balloons. Now rub the two balloons together, hold them by strings at the end and put them next to each other. They’ll move apart.
Rubbing the balloons gives them static electricity. When you rub the balloon it picks up extra electrons from the sweater or your hair and becomes slightly negatively charged.
The negative charges in the single balloon are attracted to the positive charges in the wall.
The two balloons hanging by strings both have negative charges. Negative charges always repel negative charges and positive always repels positive charges. So, the two balloons’ negative charges “push” each other apart.
Static electricity can also give you a shock. If you walk across a carpet, shuffling your feet and touching something made of metal, a spark can jump between you and the metal object. Shuffling your feet picks up additional electrons spread over your body. When you touch a metal doorknob or something with a positive charge the electricity jumps across the small gap from your fingers just before you touch the metal knob. If you walk across a carpet and touch a computer case, you can damage the computer.
One other type of static electricity is very spectacular. It’s the lightning in a thunder and lightning storm. Clouds become negatively charged as ice crystals inside the clouds rub up against each other. Meanwhile, on the ground, the positive charge increases. The clouds get so highly charged that the electrons jump from the ground to the cloud, or from one cloud to another cloud. This causes a huge spark of static electricity in the sky that we call lightning.
You can find out more about lightning at Web Weather for Kids – www.ucar.edu/40th/webweather/
You’ll remember from Chapter 2 that the word “electricity” came from the Greek words “elektor,” for “beaming sun” and “elektron,” both words describing amber. Amber is fossilized tree sap millions of years old and has hardened as hard as a stone.
Around 600 BCE (Before the Common Era) Greeks noticed a strange effect: When rubbing “elektron” against a piece of fur, the amber would start attracting particles of dust, feathers and straw. No one paid much attention to this “strange effect” until about 1600 when Dr. William Gilbert investigated the reactions of magnets and amber and discovered other objects can be made “electric.”
Gilbert said that amber acquired what he called “resinous electricity” when rubbed with fur. Glass, however, when rubbed with silk, acquired what he termed “vitreous electricity.”
He thought that electricity repelled the same kind and attracts the opposite kind of electricity. Gilbert and other scientists of that time thought that the friction actually created the electricity (their word for the electrical charge).
In 1747, Benjamin Franklin in America and William Watson in England both reached the same conclusion. They said all materials possess a single kind of electrical “fluid.” They didn’t really know anything about atoms and electrons, so they called how it behaved a “fluid.”
They thought that this fluid can penetrate matter freely and couldn’t be created or destroyed. The two men thought that the action of rubbing (like rubbing amber with fur) moves this unseen fluid from one thing to another, electrifying both.
Franklin defined the fluid as positive and the lack of fluid as negative. Therefore, according to Franklin, the direction of flow was from positive to negative. Today, we know that the opposite is true. Electricity flows from negative to positive. Others took the idea even further saying this that two fluids are involved. They said items with the same fluid attract each other. And opposite types of fluid in objects will make them repel each other.
All of this was only partially right. This is how scientific theories develop. Someone thinks of why something occurs and then proposes an explanation. It can take centuries sometime to find the real truth. Instead of electricity being a fluid, it is the movement of the charged particles between the objects. the two objects are really exchanging electrons.
Learn about Electrical Circuits and electrons in Chapter 4.