🔑 Why metals conduct
Remember, a metal contains free electrons that can move through the whole piece of metal. Because these electrons are free to move, charge can flow through a metal easily.
Electric Charge » Conductors, Insulators and Static Charge
What you'll learn this session
Study time: 30 minutes
Edexcel spec: 2.22P, 2.24P, 2.25P
- Which common materials are electrical conductors and which are insulators
- How rubbing two materials together gives them positive and negative charges
- Why it is always electrons that move, never the positive charges
- The rule for the forces between charges: like charges repel, unlike charges attract
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Unlock This CourseConductors and insulators
Everything in this lesson is examined in Paper 2 only.
Some materials let charge flow through them easily. Others hardly let it flow at all. You need to be able to sort common materials into these two groups.
Key terms:
- Electrical conductor: a material that charge can flow through easily.
- Electrical insulator: a material that charge cannot flow through easily.
| Electrical conductors | Electrical insulators |
|---|---|
| All metals, such as copper, aluminium, iron, silver and gold | All plastics, such as polythene, acetate, PVC and nylon |
| Graphite (the form of carbon in a pencil 'lead') | Rubber |
| Tap water and salty water | Glass |
| The human body | Dry wood and dry air |
The two examples the specification names are metals (conductors) and plastics (insulators). If you remember nothing else from the table, remember those.
🎈 Why plastics insulate
In an insulator the electrons are held tightly in their atoms. There are no free electrons to carry charge through the material, so charge cannot flow through it.
Where does charge come from?
All materials are made of atoms, and atoms contain two kinds of charged particle. Protons have a positive charge and are fixed in the centre of the atom. Electrons have a negative charge and are found around the outside, so they can sometimes be removed.
Normally an object has equal numbers of protons and electrons. The positive and negative charges cancel out, so the object has no overall charge. We say it is neutral.
An object becomes charged when this balance is upset:
- If an object gains electrons, it has more negative charge than positive. It is negatively charged.
- If an object loses electrons, it has more positive charge than negative. It is positively charged.
Key terms:
- Neutral: having equal amounts of positive and negative charge, so no overall charge.
- Electrostatic charge (static charge): charge that builds up on a material and stays there instead of flowing away.
Charging by rubbing
When two different insulators are rubbed together, friction transfers electrons from one material to the other. The material that gains electrons becomes negative. The material that loses electrons becomes positive. Which way the electrons go depends on the two materials.
🔵 Polythene rod and a cloth
Electrons move from the cloth onto the polythene rod. The rod has gained electrons, so it is negatively charged. The cloth has lost electrons, so it is positively charged.
🔴 Acetate rod and a cloth
Electrons move from the acetate rod onto the cloth. The rod has lost electrons, so it is positively charged. The cloth has gained electrons, so it is negatively charged.
Three points are true every time:
- Only electrons move. The positive charges are fixed in the atoms and do not move. A rod never becomes positive by "gaining positive charge". It becomes positive by losing electrons.
- The two charges are equal and opposite. Every electron that leaves one material arrives on the other. No charge is made or destroyed. It is only moved.
- Both materials started neutral. Rubbing does not create charge. It separates charge that was already there.
| Rod | Electrons move | Charge on the rod | Charge on the cloth |
|---|---|---|---|
| Polythene | from the cloth to the rod | negative | positive |
| Acetate | from the rod to the cloth | positive | negative |
Worked example
A girl rubs a plastic ruler with a dry woollen scarf. The ruler becomes negatively charged. Explain how, and state the charge on the scarf.
Friction transfers electrons from the scarf to the ruler. The ruler has gained electrons, so it has more negative charge than positive and is negatively charged. The scarf has lost the same number of electrons, so it has an equal positive charge.
Why the charge stays on an insulator
The word "static" means "not moving". A plastic rod is an insulator, so the extra electrons cannot flow through it. They stay where the rubbing put them, and the charge builds up.
A metal rod held in your hand is different. Metal is a conductor and so is your body. Any charge transferred to the metal flows straight through the rod and through you, so no charge builds up on the rod. This is why static charge is usually found on insulators such as plastics.
Forces between charges
Charged objects push or pull on each other without touching. There is one simple rule to learn:
Like charges repel. Unlike charges attract.
"Like" means the same type of charge. "Unlike" means opposite types.
| Charges | Type | Force |
|---|---|---|
| Positive and positive | like | repulsion (they push apart) |
| Negative and negative | like | repulsion (they push apart) |
| Positive and negative | unlike | attraction (they pull together) |
You may have seen this after pulling off a woolly hat on a dry day. Rubbing gives every hair the same type of charge. Like charges repel, so the hairs push away from each other and stand up.
The specification suggests an optional investigation into the forces between charges. A charged rod is hung from a thread so it can turn freely, and a second charged rod is brought close to one end.
- Two charged polythene rods: both negative, so the hanging rod swings away.
- Two charged acetate rods: both positive, so the hanging rod swings away.
- One polythene and one acetate rod: negative and positive, so the hanging rod swings towards the other rod.
You will meet the full method in the lesson called Practical: Charging Insulators by Friction.
Common mistakes
Do not write that "positive charges move onto the rod". Positive charges never move in these questions. Always explain charging by the loss or gain of electrons. Do not write that friction "creates" charge. It only transfers electrons from one material to the other. And do not forget the cloth: if the rod is negative, the cloth must be positive.
Exam-style question
A student rubs a polythene rod with a dry cloth. The rod becomes negatively charged.
(a) Explain, in terms of electrons, how the rod becomes negatively charged. [2 marks]
(b) State the type of charge on the cloth after rubbing and give a reason for your answer. [2 marks]
(c) The student hangs the charged polythene rod from a thread so that it can turn freely. She brings a second negatively charged polythene rod close to one end. Describe and explain what happens to the hanging rod. [2 marks]
(d) The student rubs a copper rod, held in her hand, with the cloth. No charge builds up on the copper rod. Explain why. [2 marks]
Model answer
(a) Friction transfers electrons from the cloth to the rod (1). The rod gains electrons, which are negatively charged, so it has an overall negative charge (1).
(b) The cloth is positively charged (1), because it has lost electrons, so it has more positive charge than negative charge (1).
(c) The hanging rod moves away from the second rod (1), because both rods have the same type of charge and like charges repel (1).
(d) Copper is a metal, so it is an electrical conductor (1). The charge flows through the rod and through the student's hand, so it does not stay on the rod (1).
Exam tip
When a question says "in terms of electrons", your answer needs three things: which way the electrons move, which object gains them and which loses them, and the charge that each object ends up with. Never mention positive charges moving.