In order to understand even the simplest concepts of electronics, you must first understand what electricity is. After all, the whole purpose of electronics is to get electricity to do useful and interesting things.
The concept of electricity is both familiar and mysterious. We all know what electricity is, or at least have a rough idea, based on practical experience. In particular, consider these points:
We are very familiar with the electricity that flows through wires. That electricity comes from power plants that burn coal, catch the wind, or harness nuclear reactions.
It travels from the power plants to our houses in big cables hung high in the air or buried in the ground. Once it gets to our houses, it travels through wires through the walls until it gets to electrical outlets. From there, we plug in power cords to get the electricity into the electrical devices we depend on every day.
We know that electricity isn't free.
We know that electricity can be stored in batteries. When the batteries die, all their electricity is gone.
We know that some kinds of batteries are rechargeable, which means that when they've been drained of all their electricity, more electricity can be put back into them by plugging them into a charger, which transfers electricity from an electrical outlet into the battery.
We know that electricity can be measured in volts. Household electricity is 120 volts (abbreviated 120 V). Flashlight batteries are 1.5 volts. Car batteries are 12 volts.
We also know that electricity can be measured in watts. Incandescent light bulbs are typically 60, 75, or 100 watts. Compact fluorescent lights (CFLs) have somewhat smaller wattage ratings. Microwave ovens and hair dryers are 1,000 or 1,200 watts.
We also may know that there's a third way to measure electricity, called amps. A typical household electrical outlet is 15 amps (abbreviated 15 A).
And finally, we know that electricity can be very dangerous.
But, what is electricity really? Let’s start with three very basic concepts of electricity: namely, electric charge, electric current, and electric circuit.
Electric charge refers to a fundamental property of matter that even physicists don't totally understand. Suffice it to say that two of the tiny particles that make up atoms — protons and electrons — are the bearers of electric charge. There are two types of charge: positive and negative. Protons have positive charge, electrons have negative charge.
Electric current refers to the flow of the electric charge carried by electrons as they jump from atom to atom. Electric current is a very familiar concept: When you turn on a light switch, electric current flows from the switch through the wire to the light, and the room is instantly illuminated.
Electric current flows more easily in some types of atoms than in others. Atoms that let current flow easily are called conductors, whereas atoms that don't let current flow easily are called insulators.
An electric circuit is a closed loop made of conductors and other electrical elements through which electric current can flow. For example, a very simple electrical circuit consists of three elements: a battery, a lamp, and an electrical wire that connects the two.
Circuits can get much more complex, consisting of dozens, hundreds, or even thousands or millions of separate components, all connected with conductors in precisely orchestrated ways so that each component can do its bit to contribute to the overall purpose of the circuit. But all circuits must obey the basic principle of a closed loop.
All circuits must create a closed loop that provides a complete path from the source of voltage (in this case, the battery) through the various components that make up the circuit (in this case, the lamp) and back to the source (again, the battery).