4.2.4 Electrical Conductors and Insulators · 4. Electricity and Magnetism | Cambridge IGCSE Co-Ordinated Sciences 0654: Physics

Section 1 of 9Two kinds of material

Materials are grouped by how easily electric charge can move through them.

Two kinds of material

Conductor

A conductor allows charge to flow freely; an insulator does not.

Metals such as copper, aluminium and iron are good conductors.

Graphite and salt solutions also conduct.

Plastics, rubber, glass, dry wood, air and pure water are insulators.

Between the two are semiconductors such as silicon, which are used in electronic devices.

Section 2 of 9Why metals conduct

A metal contains a regular lattice of positive ions surrounded by a "sea" of free electrons that are not bound to any one atom.

Why metals conduct

When a potential difference is applied across the metal, these electrons drift through the lattice and carry charge from one end to the other.

In an insulator, the electrons are all held tightly within atoms or bonds, so there are almost no charges able to move.

Even a very large voltage produces a negligible current, until the voltage is so large that the material breaks down,

as happens in lightning when air conducts.

Section 3 of 9Conductors and insulators in use

1

Copper wires carry current in cables because copper has a very low resistance and is easy to draw into wire.

2

The plastic around the wires insulates them, so that charge cannot leak to the user or to another wire.

3

Plugs are made of plastic with metal pins; the pins must conduct, while the casing must insulate.

4

Overhead power lines hang from glass or ceramic insulators attached to the metal pylons.

5

Rubber gloves and mats protect people working near high voltages.

Section 4 of 9Charge on conductors and insulators

When an insulator is charged by rubbing, the charge stays where it was produced because it cannot move through the material.

1

A rubbed area of a plastic rod remains charged only in that region.

2

A conductor behaves differently: any charge added to a metal spreads over its surface because electrons repel each other and are free to move.

3

If a charged metal object is touched by a hand, the charge leaks through the body to the ground and the object is discharged.

4

This is why metal objects held in the hand cannot easily be charged by rubbing: the charge passes through the body to earth as fast as it is produced,

unless the metal is mounted on an insulating handle.

The same idea explains why you must hold a metal rod with an insulating handle in a charging demonstration, and why a gold-leaf electroscope has a metal cap, stem and leaf mounted on an insulating base.

Section 5 of 9Earthing

Earthing means connecting a conductor to the ground using a wire.

1

The Earth is so large that it can supply or accept enormous amounts of charge without any change in potential.

2

If a positively charged conductor is earthed, electrons flow from the ground to neutralise it.

3

If a negatively charged conductor is earthed, the excess electrons flow into the ground.

4

In both cases the object ends with no net charge.

This is related to the safety features of mains wiring (Electrical Safety), and to the sparks described in Uses and Dangers of Static Electricity. Charge transfer by friction is covered in Investigating Charging by Friction.

Section 6 of 9Resistivity and choosing materials

Even among conductors there is a wide range.

1

Copper and silver have very low resistance; nichrome has a much higher resistance and is used in heaters.

2

The resistance of a wire depends on its material, its length and its cross-sectional area: a longer wire has more resistance,

and a thicker wire has less.

3

Engineers choose materials to suit the job.

4

Power cables need low resistance to avoid wasting energy,

so thick copper or aluminium is used,
while a toaster element needs a higher resistance so that it gets hot.

5

Insulators are chosen for their ability to withstand high voltages without breaking down, and for being flexible, waterproof and durable.

6

Good conductors of electricity are often good conductors of heat as well, because free electrons carry both energy forms in metals.

Section 7 of 9Worked examples

A student holds a copper rod in the bare hand and rubs it with a cloth. The rod shows no sign of charge. Explain why, and say how to change the experiment.
Classify each as conductor or insulator: steel spoon, rubber band, graphite pencil lead, glass beaker, salt water.

Section 8 of 9Key terms

Term	Meaning
Conductor	Material in which charge can flow easily
Insulator	Material in which charge cannot flow easily
Free electron	Electron not bound to an atom, able to move through a metal
Earthing	Connecting an object to the ground so charge can flow away
Semiconductor	Material with conductivity between that of conductors and insulators

Section 9 of 9Exam technique

1

Saying that insulators have no electrons – they have electrons, but these are not free to move.

2

Thinking charge cannot move at all in a conductor when it is placed on the surface – it spreads out over the surface.

3

Believing that salt water conducts because of electrons – it is mobile ions.

4

Saying earthing "adds" charge to an object – it allows charge to flow away or in, to neutralise it.

5

Believing that a metal can never be charged – it can, if it is insulated from the ground.