Atom structure: where electrons live
An atom has a tiny centre called the nucleus (it carries positive charge). Around it are very light, negative particles called electrons. The electrons close to the nucleus are held strongly. The outer electrons are held weakly, and they decide how a material behaves with electricity.
In your 3D scene, each atom is a big ball and its electron is a small blue dot circling it.
Charge carriers: who carries the current?
Electric current is a flow of charge. The thing that moves is called a charge carrier. In metals the carriers are free electrons. A free electron is an outer electron that is not tied to one atom. It wanders through the whole metal, but with no push it goes in all directions, so there is no current. A battery adds a push, and the electrons drift the same way: that drift is the current.
(In salty water the carriers are charged atoms called ions, and in a semiconductor there is a second kind of carrier called a hole. Here we stay with electrons.)
Conductors
A conductor lets current pass easily because it has a huge number of free electrons. Examples: copper, aluminium, silver, iron and graphite. Metals are good conductors. Silver is the best, but copper is cheaper, so wires are made of copper.
Insulators
An insulator holds all its electrons tightly. There are almost no free charge carriers, so almost no current flows. Examples: rubber, plastic, glass, dry wood, dry cloth, air. Insulators protect us: the plastic cover on a wire, the rubber handle of a screwdriver, rubber gloves for an electrician. A very strong push (like lightning) can force even air to conduct for a moment.
Semiconductors
A semiconductor sits between the two. Examples: silicon and germanium. When very cold, it behaves almost like an insulator. When it is warmed, or when light falls on it, more electrons break free and it conducts better. This is opposite to a metal: a metal conducts slightly worse when hot, a semiconductor conducts better.
Makers also add a tiny amount of another element (called doping) to decide how many carriers there are. That is how diodes, transistors and computer chips are made. Read more in the p-n junction diode lesson.
Try it: test your own materials
- Make a circuit: one cell, one small bulb, and two loose wire ends (a gap in the circuit).
- Touch the two ends to a coin, a key, a pencil lead, a rubber, a plastic spoon and a dry stick.
- Before each touch, predict: bulb on or off? Then check.
- Sort the objects into two groups. Keep it safe: use only a small cell, never the wall socket.
In the 3D scene, also try silicon at different heat settings to see the bulb change brightness.
Key formulas and definitions
- Conductor: many free electrons, low resistance (copper, silver, aluminium)
- Insulator: almost no free electrons, very high resistance (rubber, glass, plastic)
- Semiconductor: few free electrons, resistance falls when heated or doped (silicon, germanium)
- Current = flow of charge carriers (electrons in metals)
- Metal: resistance rises with temperature; semiconductor: resistance falls with temperature
Worked examples
1. A sheet of material does not let a bulb light when it joins a battery and a bulb. Is it a conductor, an insulator or can you not tell?
It could be an insulator, but it could also be a poor semiconductor at low temperature. If the bulb stays completely off even with a strong battery, we call it an insulator.
2. Sort into conductor, insulator, semiconductor: copper, glass, silicon, dry wood, aluminium, germanium.
Conductors: copper, aluminium. Insulators: glass, dry wood. Semiconductors: silicon, germanium.
3. Why is a copper wire covered with plastic?
Copper is a conductor so current flows along it. Plastic is an insulator, so the current cannot pass to our hand or to other wires. It keeps us safe and prevents short circuits.
Common mistakes
- Thinking insulators have no electrons. They have electrons, but all are held tightly.
- Thinking the electrons in a wire race from battery to bulb in a flash. They drift slowly; the push travels fast.
- Thinking heating always helps conduction. It helps semiconductors but makes metals conduct slightly worse.
- Calling water an insulator. Pure water conducts very badly, but tap water has dissolved salts and conducts, so it is dangerous with electricity.