Letting energy through, or not
Why are electrical wires made of copper but covered in plastic? Why do pots have metal bottoms but plastic or wooden handles? The answer is that some materials let electricity and heat flow easily, while others block them.
Electrical conductors and insulators
| Conductor | Insulator |
|---|---|
| Lets electric current flow easily | Resists the flow of electric current |
| Has many free electrons that can move | Electrons are held tightly in place |
| Copper, silver, gold, aluminum, iron | Rubber, plastic, glass, dry wood, air, ceramic |
Why metals conduct
In metals, the outer electrons aren’t attached to any one atom. They drift freely through the metal like a sea. When a battery is connected, these free electrons flow, creating an electric current.
In insulators, electrons are held tightly by their atoms and can’t move easily.
The best conductors
| Material | Notes |
|---|---|
| Silver | Best conductor of all, but expensive |
| Copper | Nearly as good and much cheaper; used in most wiring |
| Gold | Doesn’t corrode; used on electronic connectors |
| Aluminum | Light; used in overhead power lines |
Water
Pure water is actually a poor conductor. But tap water, rainwater and especially salt water contain dissolved ions that let current flow. That’s why electricity and water are a dangerous mix, and why you should never use electrical devices with wet hands.
Your body
Your body contains salty water, so it can conduct electricity. That’s why electric shocks are dangerous.
Semiconductors
Semiconductors, like silicon and germanium, are in between. Scientists can control how well they conduct by adding tiny amounts of other elements. This makes them perfect for building:
- computer chips
- transistors (billions in every smartphone)
- solar cells
- LEDs
Silicon is made from sand, and it’s the reason the San Francisco area’s tech region is nicknamed Silicon Valley.
Superconductors
Some materials, when cooled to extremely low temperatures, become superconductors with zero electrical resistance. Current can flow through them forever without losing energy. They’re used in MRI machines and particle accelerators. Scientists are searching for materials that superconduct at room temperature.
Thermal conductors and insulators
Heat moves in a similar way.
| Thermal conductor | Thermal insulator |
|---|---|
| Transfers heat quickly | Slows heat transfer |
| Metals: copper, aluminum, iron | Wood, plastic, wool, foam, fibreglass, air |
| Pot bottoms, radiators, heat sinks | Pot handles, oven mitts, winter coats, home insulation |
Metals are good thermal conductors for the same reason they conduct electricity: free electrons carry energy quickly.
Why does metal feel colder?
On a cold day, a metal railing and a wooden bench are the same temperature. But metal feels much colder. That’s because metal conducts heat away from your hand quickly, while wood doesn’t.
Air is a great insulator
Trapped air is one of the best insulators. That’s why:
- down jackets and wool sweaters keep you warm (they trap air)
- double-paned windows have air or gas between the panes
- foam coolers keep drinks cold
- snow insulates the ground and animals in burrows
- igloos stay much warmer inside than the air outside
Safety and design
| Object | Conductor part | Insulator part |
|---|---|---|
| Electrical cord | Copper wire inside | Plastic coating |
| Frying pan | Metal pan | Plastic or wooden handle |
| Electrician’s tools | Metal tip | Rubber-coated handles |
| Power line | Aluminum cable | Glass or ceramic insulators on the poles |
| House wall | Fibreglass or foam insulation |
Testing materials
You can test materials with a simple circuit: a battery, a bulb and two wires with a gap. Put an object in the gap. If the bulb lights, the object is a conductor.
Common mistakes
- Thinking all metals conduct equally. Silver and copper are much better than iron or lead.
- Thinking pure water conducts well. Dissolved substances make it conduct.
- Thinking insulators make heat or cold. They only slow the flow of heat.
Try it yourself
- Build a conductivity tester and test a coin, a paper clip, an eraser, foil, a pencil lead, a plastic spoon and a key.
- Put metal, plastic and wooden spoons in a cup of hot water. After two minutes, carefully feel the handles. Which got hottest?
- Wrap three cups of hot water in different materials (foil, wool, newspaper). Measure temperatures every five minutes.
Quick recap
- Conductors let electricity or heat flow; insulators slow it.
- Metals conduct because they have free electrons.
- Semiconductors are in between and power modern electronics.
- Trapped air is an excellent thermal insulator.
- Good design uses conductors and insulators together for safety.