Look at the cable of a lamp or a phone charger. On the outside, it is soft plastic, but hidden inside are thin, shiny metal wires, usually made of copper. Why two different materials? Because electricity flows easily through some materials and hardly at all through others.
What makes the difference?
Everything is made of atoms, and every atom has tiny electrons around it. Electricity in a wire is a flow of electrons, all drifting the same way.
In metals, some electrons are not tied to any one atom. They can wander freely between the atoms. Give them a push, and they flow — so electricity passes through easily. Materials like this are called conductors.
In materials like plastic or rubber, every electron is held tightly by its own atom. Even with a push, the electrons stay where they are, so electricity can hardly pass. Materials like this are called insulators.
Which materials are which?
| Conductors | Insulators |
|---|---|
| copper | plastic |
| aluminium | rubber |
| iron and steel | glass |
| silver | porcelain |
| gold | dry wood |
| pencil lead (graphite) | air |
Of all the metals, silver conducts best. Copper comes a close second and costs much less, so most wires are made of copper. Big power lines are often made of aluminium, because it is light and cheap.
A good conductor has a low resistance: it hardly holds the electricity back. An insulator has a huge resistance.
Go deeper: How big is the difference? for grown-ups and the extra curious
How well a material conducts is measured by its resistivity: the resistance of a block that is 1 metre long and 1 square metre across. Copper has about . Hard rubber and plastics have or more. So a copper rod lets electricity through more than a billion billion times more easily than a rubber or plastic rod of the same size.
Why is a cable copper inside and plastic outside?
A cable has two jobs. The copper inside carries the electricity to the lamp. The plastic outside keeps the electricity in, so it cannot flow into your hand or into anything else.
Insulators keep us safe in lots of places. Plugs have plastic cases, and light switches are made of plastic. Power lines hang from tall towers on glass or porcelain holders, called insulators. These stop the electricity from running down the towers.
Can water carry electricity?
Pure water is actually a poor conductor. But the water around us is never pure. Tap water, sea water and sweat all have salts and other substances dissolved in them. These let electricity through.
Your body is full of salty water, so it conducts electricity too. That is what makes the electricity from a socket so dangerous: it can flow right through you.
Can air carry electricity?
Air is an insulator. That is why big power lines high above the ground can be bare metal: the air around them keeps the electricity in. You may wonder how birds can sit on those bare wires. Find out in why birds on wires are safe.
But a huge push can break through the air. The air then glows and lets electricity jump across. That is a spark — or, much bigger, a bolt of lightning. The push is called voltage, and for lightning it reaches hundreds of millions of volts.
Go deeper: When ions carry the current for grown-ups and the extra curious
In salty water there are no free electrons. Instead, the salt breaks up into charged atoms called ions, and these drift through the water.
In air, a spark starts when a strong push — about 3 million volts per metre in dry air — tears electrons off the air’s molecules. The air turns into a glowing gas that conducts, called a plasma.
Try it: which things conduct?
With a grown-up, you can build a simple tester. You need a 4.5-volt battery, a small torch bulb in a holder and three wires with clips. Three AA batteries in a holder work too. The tester is a simple electric circuit.
- Connect the battery, the bulb and the wires into a loop, but leave a gap between two loose wire ends.
- Touch both loose ends to the object you are testing, a little apart from each other.
- If the bulb lights up, the object is a conductor. If it stays dark, it is an insulator.
Test a spoon, a coin, a key, a pencil lead, a plastic ruler, a rubber band and a piece of paper. The pencil lead may make the bulb glow only faintly.
Go deeper: In between: semiconductors for grown-ups and the extra curious
Some materials, such as silicon, are semiconductors. They conduct much worse than metals, but much better than insulators. How well they conduct can be changed: by adding tiny amounts of other substances, by light, by heat or by a small voltage. That makes them perfect for computer chips, LEDs and solar panels.
Go deeper: Superconductors: no resistance at all for grown-ups and the extra curious
Cool certain materials far enough, and their resistance vanishes completely. This was discovered in 1911, when mercury cooled to −269 °C suddenly lost all its resistance. In a superconducting ring, a current can keep flowing for years with no battery at all. Superconducting coils make the powerful magnets inside hospital MRI scanners.
Check yourself
Why are electric cables covered in plastic?
Show the answer
Plastic is an insulator that keeps the electricity inside — Plastic holds on to its electrons, so electricity cannot pass through it. It keeps the electricity inside the copper wire.
Plastic holds on to its electrons, so electricity cannot pass through it. It keeps the electricity inside the copper wire.