What is electricity? The flow of electrons
10 minutes · Beginner
Simulations and quizzes aren't read aloud; pause when you reach them and try them out.
When you charge your phone, switch on a light or light up an LED, the same thing happens: electric charges move through a conductor. In this lesson you'll learn what that movement is, why it passes through some materials and not others, and why a circuit has to be "closed".
It all starts with the atom
All matter is made of atoms. At the center of an atom are positively (+) charged protons and uncharged neutrons. Around them move negatively (−) charged electrons.
In some materials the outermost electrons are only loosely bound to the atom. These are called free electrons. Free electrons can easily hop from atom to atom. An electric current is simply these electrons moving together in one direction.
Conductors, insulators and semiconductors
Conductor
Has lots of free electrons and passes current easily.
Copper, aluminum, gold, silver
Insulator
Has almost no free electrons and doesn't pass current.
Plastic, glass, rubber, dry wood
Semiconductor
Somewhere in between. Its conductivity can be controlled.
Silicon, germanium
That's why cables are copper on the inside and plastic on the outside: the copper carries the current, and the plastic stops it escaping outward (into you). Diodes, transistors and integrated circuits are made from semiconductors. All of modern electronics is built on semiconductors.
Why does a circuit have to be closed?
Electrons only move if there's a path that lets them go all the way round and come back to where they started. This path, leaving one terminal of the battery, going round the circuit and returning to the other terminal, is called a closed circuit. If the path is broken anywhere (an open circuit), the current stops everywhere at once. A light switch does exactly this: it opens or closes the path.
Try it yourself
Open and close the switch. Put different materials into the circuit and watch what the lamp does. Then tick the box that shows the real direction the electrons move.
Copper wire: Conductor. Current flows easily; the lamp is fully bright.
Orange: conventional current, from + to −. Schematics always use this direction.
The direction of current: a small historical accident
When people started studying electricity in the 1700s, the electron hadn't been discovered yet. Scientists assumed current flowed from the positive (+) terminal to the negative (−) one. About 150 years later, when the electron was discovered, it turned out that electrons actually move the opposite way, from − to +.
But by then every book, formula and symbol had been written using the old direction. So even today we use the conventional current direction (+ → −). The arrows in the diode and transistor symbols also point in this direction.
Which one should I use?
When drawing circuits and doing calculations, always use conventional current (+ → −). The electron direction only matters when explaining what goes on inside semiconductors.DC and AC
DC (direct current)
The current always flows in the same direction. Cells, batteries, USB and adapter outputs are DC.
AC (alternating current)
The direction of the current keeps changing. Mains electricity in Turkey and most of Europe is 220–230 V and changes direction 50 times a second (50 Hz); in North America it's 120 V at 60 Hz.
Safety
Every experiment in this course uses DC of 12 V at most. Mains AC, on the other hand, can kill. We'll only deal with mains voltage at the advanced level, after learning the necessary precautions.Common mistakes
- Thinking "the battery produces electricity and the circuit uses it up". The battery provides the push (the voltage) on the electrons. The electrons are already in the wire; they don't disappear, they circulate around the circuit.
- Connecting a single wire and expecting the lamp to light. Without a second path for the current to return, the circuit isn't complete.
- Thinking a pencil contains lead. Pencil "lead" is graphite, and it does conduct electricity, if only poorly.
Test yourself
1. An electric current is actually the movement of what?
2. Which of these is an insulator?
3. Which conventional current direction is used in schematics?
4. What kind of current is at a household socket in Turkey?
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