OCR GCSE Physics A: Static electricity and electric fields (J249)

This guide teaches static electricity for OCR GCSE Physics A (Gateway Science, J249), using the specification Version 5.0 (July 2026). It covers learning outcomes P3.1a to P3.1d of topic P3.1 Static and charge. Statement P3.1d is in bold in the specification, so it is tested only in the Higher Tier papers. P3.1a to P3.1c are assessed in both tiers. The later outcomes in P3.1 (current as a flow of charge, and charge = current x time) are covered separately and are not repeated here.

Related pages: revision notes and practice questions. Course links: OCR GCSE Physics hub, printable checklist and free 10-minute diagnostics.

What this unit covers

Outcome	What you must be able to do	Tier
P3.1a	Describe charge as a property of all matter; positive and negative charge; most bodies have equal amounts, so zero net charge	Both
P3.1b	Describe how rubbing produces static electricity and sparking; give evidence for forces between charged objects not in contact; know that static charge only builds up on insulators	Both
P3.1c	Explain static electricity in terms of the transfer of electrons between objects	Both
P3.1d	Explain the concept of an electric field and how it helps explain static electricity; relate fields to forces of attraction and repulsion	Higher Tier only

P3.1a Charge is a property of all matter

Charge is a property that all matter has. It comes in two types, named positive and negative. Atoms contain a nucleus with positive protons, surrounded by negative electrons. An atom has equal numbers of each, so it has no overall charge.

The same applies to everyday objects. In most bodies there are equal numbers of positive and negative charges, so the body has zero net charge. We call it neutral. “Net” means the total once positive and negative are added together.

The rules for the force between two charged objects:

• Like charges repel (positive and positive, or negative and negative).
• Unlike charges attract (positive and negative).

You can see this with charged rods. Hang one rod from a thread, then bring a second rod near it. If both were rubbed in the same way, the hanging rod swings away. If they were charged oppositely, it swings towards the other rod.

Which charge moves?

Only electrons move when an object is charged. Protons are fixed inside nuclei. A material becomes positive because it loses electrons, not because it gains positive charge. Writing “positive charge moved across” is a common way to lose a mark.

Worked example 1: counting charge

A model uses charge units where one electron is -1 unit and one proton is +1 unit. A plastic strip has 12 positive units and 12 negative units. A cloth has 20 positive and 20 negative. After rubbing, 5 electrons have moved from the cloth to the strip.

1. Strip: negative units are now 12 + 5 = 17, positive units still 12. Net charge = 12 - 17 = -5 units.
2. Cloth: negative units are now 20 - 5 = 15, positive units still 20. Net charge = 20 - 15 = +5 units.
3. Check: -5 + 5 = 0. The total charge is the same as before rubbing (both objects were neutral), so no charge has been created or destroyed. It has only moved.

The strip is negative and the cloth is positive, with equal and opposite charges.

Worked example 2: what does it take to neutralise?

An object has 15 positive units and 12 negative units. Net charge = 15 - 12 = +3 units. It is short of electrons by 3. To become neutral it must gain 3 electrons. To reach a net charge of -2 units it must gain 3 + 2 = 5 electrons.

P3.1b Making static electricity

Charging by rubbing

Rub two different insulating materials together and both become charged. Typical school demonstrations use a polythene rod and an acetate rod, each rubbed with a dry cloth. Which material ends up positive depends on the pair of materials. In the usual demonstration, the acetate rod becomes positive and the polythene rod becomes negative. You will be told the sign in a question, or you can work it out from what the question says happens to the electrons.

Static charge only builds up on insulators. The charge stays where it was made because it cannot move through the material. A conductor, such as a metal rod held in your hand, lets electrons flow away through your body to the ground, so it does not stay charged. A metal object held by an insulating handle can keep its charge, because the charge has nowhere to go.

Sparking

Rubbing can build up a large amount of charge on an insulator. If a charged object comes close to another object that can take charge, the charge can cross the gap in a sudden spark. A Van de Graaff generator is a standard demonstration: a moving belt builds up charge on a metal dome, and a spark jumps to a nearby earthed sphere.

Evidence for forces without contact

Charged objects exert forces on each other without touching. The evidence:

• A charged rod hung on a thread swings towards or away from a second rod before they touch.
• A suspended charged strip is pushed away by a second strip of the same charge, with air between them.

A force like this is called a non-contact force. The two objects always pull or push on each other with equal-sized forces, even if one is much more charged than the other.

Distance and force

The closer two charged objects are, the stronger the force between them. Move the hanging rod closer and it swings further.

P3.1c Explaining static with electron transfer

When two insulators are rubbed together, electrons are transferred from the surface of one to the surface of the other. The material that gains electrons has more negative than positive charge, so it is negative. The material that loses electrons is left with more positive than negative charge, so it is positive. The two charges are equal in size.

A full explanation has four parts:

1. Name the objects and say they are insulators.
2. Say electrons move from one to the other when rubbed.
3. State which gains electrons (negative) and which loses them (positive).
4. Say the amounts are equal, because electrons are only moved, not made.

Why a charged object attracts a neutral one

A charged balloon sticks to a wall that has no net charge. After a balloon is rubbed on a jumper, it holds extra electrons and is negative. Near the wall, the electrons in the wall’s surface atoms shift slightly away from the balloon. The surface nearest the balloon becomes slightly positive and the side further away slightly negative. The attraction to the nearer positive charges is a little greater than the repulsion from the farther negative charges, because the nearer charges are closer. The balloon is pulled to the wall. The wall is still neutral overall.

The gold leaf electroscope

A gold leaf electroscope has a metal cap, a metal rod and a thin gold leaf hanging from the rod, in an insulated case. The leaf hangs down when the electroscope is uncharged.

Rod near the cap, not touching. Take a negatively charged rod. Electrons in the metal are repelled down towards the leaf and rod. Both gain extra negative charge and repel each other, so the leaf lifts. The cap is short of electrons. The electroscope’s total charge is still zero. Take the rod away and the electrons return, so the leaf falls.

Rod touching the cap. Some electrons pass from the rod to the electroscope. It now has a net negative charge, so the leaf stays raised after the rod is removed. If you then bring a second negative rod close, the leaf rises further. A positive rod makes it fall, as electrons are drawn towards the cap and the leaf has less negative charge.

P3.1d Electric fields (Higher Tier only)

Higher Tier only. An electric field is the space surrounding a charged object in which any other charge experiences a force. It is a way to explain how a force can act without contact. The charged object does not reach out and touch the other one. It creates a field, and the field exerts a force on any charge placed in it.

Field lines

Electric fields are drawn using field lines with arrows. The conventions:

• The arrow shows the direction of the force on a small positive charge placed there.
• Lines start on positive charge and end on negative charge.
• Lines never cross.
• The closer together the lines, the stronger the field and the bigger the force on a charge placed there.

Because a positive charge is pushed away from another positive and pulled towards a negative, the arrows point away from positive objects and towards negative ones.

Arrangement	What the lines look like
Single positive sphere	Straight lines, evenly spaced, pointing outwards
Single negative sphere	Straight lines, evenly spaced, pointing inwards
One positive, one negative sphere	Curved lines from the positive to the negative; closest together in the gap between them
Two positive spheres	Lines curve away from each other; the field is very weak midway between equal charges

On a single sphere the lines spread out as you move away, so they are closer near the surface. The field is strongest there, and weaker far away.

Fields and attraction or repulsion

Unlike charges attract because the field of one pulls on the other; the lines run from one to the other. Like charges repel because each charge feels a push from the field of the other. The force on a charge is larger where the lines are close together and smaller where they are far apart. This explains why the force gets weaker as the distance between charged objects increases.

The semolina demonstration shows the shape of a field. Two electrodes are placed in a dish of castor oil with semolina sprinkled on the surface. When the electrodes are connected to a high-voltage supply, the grains line up along the field lines.

Worked example 3: field lines around two opposite charges

Describe the field between a positive and a negative sphere, and say where a small positive charge feels the biggest force.

1. The lines run from the positive sphere to the negative sphere, curving between them.
2. A small positive test charge is repelled by the positive sphere and attracted by the negative one, so it moves along a line towards the negative sphere.
3. The lines are closest together in the gap between the spheres and near their surfaces, so the field is strongest there. The force on the test charge is biggest in these regions.

Worked example 4: explaining a spark

Explain, using the idea of an electric field, why a spark jumps from a charged dome to an earthed sphere.

1. Charge builds up on the dome, so the field between the dome and the sphere gets stronger.
2. When the field is strong enough, the air in the gap can no longer insulate. Charge is pulled across the gap.
3. The flow of charge across the gap is the spark. The dome loses charge.

Common errors

• Saying positive charges move. Only electrons transfer, so a positive object has lost electrons.
• Calling a metal object charged by rubbing while it is held in the hand. The charge flows away.
• Drawing field lines that cross, or with arrows pointing towards a positive charge.
• Writing “electrons are attracted to the positive ones” when asked about a neutral object: say the charges in the object shift, but its net charge is still zero.
• Using “the field pushes electrons” for every case: the arrow shows the force on a positive charge, so the force on an electron points the opposite way.

Next: condense this with the revision notes, then test yourself with the practice questions and the diagnostics.

Official syllabus

OCR GCSE (9-1) Gateway Science Physics A (J249), Version 5.0 (July 2026), for first assessment in 2018, published by OCR. This guide covers learning outcomes P3.1a to P3.1d.