What you'll learn
Rub a balloon on your hair and it sticks to the wall; walk across a carpet and you might get a shock from a door handle. These everyday effects are caused by static electricity. For AQA GCSE Physics you need to understand how objects become charged by friction, how charges attract and repel, why sparks happen, and the idea of an electric field. This guide covers charging by friction, the transfer of electrons, the forces between charges, the dangers and uses of static electricity, and electric fields. By the end you should be able to explain how objects become charged, predict attraction or repulsion, and describe an electric field.
Key terms and definitions
Static electricity — Electric charge that builds up on an insulator and does not flow away.
Charge — A property of matter that can be positive or negative; measured in coulombs (C).
Electron — A negatively charged particle that can be transferred between objects.
Insulator — A material that does not allow charge to flow through it easily.
Friction — The rubbing of two surfaces together, which can transfer electrons.
Electric field — A region around a charged object where another charge experiences a force.
Discharge — The sudden movement of charge, often seen as a spark.
Earthing — Connecting a charged object to the ground so charge can flow safely away.
Core concepts
Charging by friction
When two insulating materials are rubbed together, electrons are transferred from one to the other by friction. Electrons are negatively charged, so:
- The material that gains electrons becomes negatively charged.
- The material that loses electrons becomes positively charged.
Only electrons move — the positive charges (protons) stay fixed in the nuclei. So an object becomes positively charged by losing electrons, not by gaining positive charge. For example, when a polythene rod is rubbed with a cloth, electrons move onto the rod, making it negative, and the cloth is left positive.
Why insulators hold charge
Charge builds up only on insulators, because the electrons cannot flow away. On a conductor, any extra charge would simply flow away to earth. This is why static effects are seen with materials like plastic, rubber and hair, not with metals.
Forces between charges
Charged objects exert forces on each other without touching:
- Like charges repel (two positives, or two negatives, push apart).
- Opposite charges attract (a positive and a negative pull together).
The closer the charges, the stronger the force. This is why a charged balloon (negative) is attracted to a wall — it induces an opposite charge on the surface — and why two rods with the same charge push apart.
Sparks and discharge
If enough charge builds up on an object, the voltage between it and a nearby earthed object can become large. If it becomes large enough, electrons can jump across the gap through the air, producing a spark — this is called discharge. This is what happens when you touch a metal door handle after building up charge by walking on a carpet: the charge suddenly flows, and you feel a small shock. Lightning is a very large-scale example of discharge.
Dangers of static electricity
Static electricity can be dangerous where sparks could ignite flammable substances. For example, when refuelling an aircraft or tanker, the flow of fuel can build up charge, and a spark could ignite the fuel vapour. To prevent this, the equipment is earthed — connected to the ground by a conductor — so that charge flows safely away instead of building up. Earthing removes the risk of a dangerous spark.
Uses of static electricity
Static electricity is also useful. In some devices, charged surfaces attract oppositely charged particles. For example, in electrostatic dust precipitators used to clean smoke, particles are charged and then attracted to oppositely charged plates, removing them from the gases. In inkjet printers and paint sprayers, charging the droplets helps direct them accurately onto the surface.
Electric fields
An electric field is a region around a charged object where another charged object experiences a force. The field is strongest close to the charged object and gets weaker further away. Electric fields can be shown using field lines:
- Field lines point away from a positive charge and towards a negative charge.
- The lines are closer together where the field is stronger (near the charge).
When another charge is placed in the field, it experiences a force. The idea of a field explains how charges can affect each other without touching: one charge creates a field, and the other charge feels a force in that field.
Explaining attraction and repulsion with fields
The concept of an electric field explains the non-contact forces between charges. Each charged object is surrounded by a field, and when a second charged object enters that field, it experiences a force. If the charges are opposite, the force pulls them together (attraction); if they are the same, the force pushes them apart (repulsion). The strength of the force depends on how much charge is present and how far apart the objects are — the force is stronger when the charges are larger or closer together, which matches the fact that field lines are closer together near the charge. This is why the field model is so useful: it turns "action at a distance" into something we can picture and reason about.
Induced charge and why a balloon sticks
A charged object can attract an uncharged object through a process called induced charge. When a negatively charged balloon is brought near a wall, it repels some of the electrons in the surface of the wall, leaving the near surface slightly positive. The opposite charges then attract, and the balloon sticks. This is why a charged rod can pick up small pieces of paper even though the paper has no overall charge — the charge on the rod induces an opposite charge on the near side of the paper. Being able to explain induced charge is a common higher-mark question.
Worked examples
Example 1: Explaining charging
A plastic rod is rubbed with a cloth and becomes negatively charged. Explain what happened. Electrons were transferred from the cloth to the rod by friction. The rod gained electrons, so it became negatively charged, and the cloth lost electrons, so it became positively charged.
Example 2: Predicting the force
Two rods both carry a negative charge and are brought close together. What happens, and why? They repel each other, because like charges repel. Both are negative, so they push apart.
Example 3: Explaining a spark
Explain why you sometimes get a shock when touching a metal door handle. Walking on a carpet can transfer electrons, charging your body. When you touch the earthed metal handle, the built-up charge suddenly flows through the small gap and into the handle, discharging as a spark, which you feel as a shock.
Example 4: Why earthing prevents danger
Explain how earthing prevents a dangerous spark when refuelling a tanker. Earthing connects the equipment to the ground with a conductor, so any charge that builds up flows safely away to earth instead of accumulating. This prevents the charge from building up enough to cause a spark that could ignite the fuel.
Common mistakes and how to avoid them
The most common error is saying an object gains "positive charge" when it becomes positive. Only electrons move — a positive charge is caused by losing electrons, never by gaining positive particles. State it in terms of electrons.
Students often forget that static charge builds up on insulators. On a conductor, the charge would flow away, so you would not see static effects. Mention insulators when explaining charging.
Another mistake is getting attraction and repulsion the wrong way round. Like charges repel, opposite charges attract — learn this firmly.
When explaining sparks and shocks, remember that discharge is the sudden movement of charge (electrons jumping the gap), not the charge disappearing. And earthing works by providing a conductor for charge to flow safely to the ground.
Finally, when drawing electric fields, remember the lines point away from positive and towards negative, and are closer together where the field is stronger.
Exam technique for "Static electricity and electric fields"
Charging questions almost always want you to describe the transfer of electrons by friction, and to state which object gains or loses electrons and therefore its charge. Always answer in terms of electrons moving.
For danger and use questions, explain the role of charge building up and either the risk of a spark (danger) or the attraction of charged particles (use), and mention earthing as the safety measure. Give a specific example such as refuelling or a dust precipitator.
For electric fields, be ready to describe a field as a region where a charge feels a force, draw the field lines with correct direction, and state that the field is stronger where the lines are closer together. Use precise terms — electron, insulator, discharge, earthing — throughout.
Quick revision summary
- Static electricity builds up on insulators when electrons are transferred by friction.
- Gaining electrons makes an object negative; losing electrons makes it positive (only electrons move).
- Like charges repel, opposite charges attract; the force is stronger when charges are closer.
- A large build-up of charge can cause a spark (discharge); earthing lets charge flow safely away to prevent dangerous sparks.
- Uses include dust precipitators, inkjet printers and paint sprayers, which rely on attraction between opposite charges.
- An electric field is a region where a charge feels a force; field lines point away from positive and towards negative, and are closer where the field is stronger.