Think about all the things you use daily that require electricity: lights, radios, audio equipment, telephones, microwave ovens, hair dryers, furnaces, air conditioners. And what about the computer you are using right now.
Everywhere you turn, we humans are using electricity. Have you ever wondered exactly what electricity is and how it works?
Let's enter the world of Circuits and Switches.
Lesson 1: What Is Electricity?
Turn on a light, pop some toast in the toaster, electricity is there and ready to go to work for you. But what exactly is it? To find out, you have to get really small. You have to go down to the atomic level.
An electron is a negatively charged particle in an atom. Everything (you, your desk, a cheese sandwich) is made of atoms. These atoms contain electrons, particles with a tiny negative electric charge. In some materials, electrons can jump from atom to atom. But to start the electrons jumping, there needs to be an imbalance. There needs to be more electrons at one end of the conductor than at the other.
In a battery, for instance, the negative pole has lots of electrons and the positive pole has few. When you connect the two poles through the circuitry of a device like a flashlight, the electrons have a path they can use to flow from the negative to the positive pole to correct the imbalance. This is what we call "electric current." In this example, we get the electricity to do some work—lighting a bulb.
You've probably experienced electricity yourself. Ever touch a door knob and get a shock? In this case, there was an imbalance of electrons between you and the doorknob. In an instant, this imbalance was corrected.
Lesson 2: Plumbed for Electricity
Is your house plumbed for electricity?
Ever wonder why electricity is always there waiting in the wire for you to use? A good way to understand it is to compare household electricity to household plumbing. Turn a faucet handle and out pours the water. Why? Because there is:
A supply of water somewhere
A pipe to carry the water
A pressure difference between the water supply and your faucet
A faucet to turn the water on and off
Compare this to how you get electricity to a lamp. In this case, there is:
A power plant to supply moving electrons
Wire to conduct the moving electrons
More moving electrons at the power plant than in your lamp
We generally think of electric circuits as small things, like a lamp or a radio. What about the path electricity takes to get to your home? It's a circuit, too—a super humongous circuit! View the video to follow the path from the power generation plant to your toaster.
Resistance is a physical property of materials. If a material has a high resistance, it opposes the passage of a steady electric current. The lower the resistance, the easier it is to force electrons to leave atoms and move through the material. Metals, by nature, have a low resistance. That’s why metals like copper are used as electrical conductors. Rubber, wood, and plastics, on the other hand, have a high resistance. That means it's nearly impossible to force their electrons to leave their atoms. A plastic coating around wire acts as an insulator, preventing the flow of electricity. Use the slider control to increase or decrease current.
As convenient as it is to be able to turn on electricity, think how bad it would be if you couldn't turn it off. An electric furnace that never shut down could turn a home into a toaster. An electric burner you couldn't turn off could be a real fire hazard. Switches that allow you to turn on and off lights or a toaster oven are called mechanical switches. They're called "mechanical" because they have moving parts.
Computers use switches to represent information such as numbers and letters. These are more complicated than the switch you use to turn on a light. In fact, just representing a single number requires eight switches.
Mechanical switches are too slow and bulky to handle all the processing required to even do something as simple as write an email. To make computers work, inventors had to make a nonmechanical switch—a super-fast switch with no moving parts.
What was needed was a switch that could be turned on and off by electricity. This required finding a semiconductor, a naturally poor conductor that could be easily modified to conduct electricity under certain conditions.
The best material for the job turned out to be silicon, an abundant nonmetallic element. Using silicon as a poor conductor, the transistor was invented and the digital age became possible.
How Nonmechanical Switches (Transistors) Work
You can see in the diagram that two terminals—the source (where current goes in) and the drain (where current goes out)—are negatively charged. They are made of n-type silicon ("n" for negative). Both terminals sit on a positively charged well of silicon connected to the gate terminal. The well silicon is called p-type silicon ("p" for positive). When a charge is applied to the gate terminal, electrons in the p—silicon are drawn to the space between the source and drain terminals and form an electron channel. Electrons now flow from the source to the drain. In this position, the transistor is on. Remove the charge from the gate terminal and the transistor returns to its off state.