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

These are condensed notes for OCR GCSE Physics A (Gateway Science, J249), specification Version 5.0 (July 2026), learning outcomes P3.1a to P3.1d. P3.1d is Higher Tier only; P3.1a to P3.1c are assessed in both tiers. For full explanations, read the study guide. Then use the practice questions, the OCR GCSE Physics hub, the printable checklist and the free 10-minute diagnostics.

Key ideas at a glance

Outcome	One-line summary	Tier
P3.1a	All matter has charge, positive or negative; most bodies have equal amounts, so net charge is zero	Both
P3.1b	Rubbing insulators makes static; charge can spark across a gap; charged objects push or pull without touching	Both
P3.1c	Static charge comes from electrons moving from one object to another	Both
P3.1d	An electric field is where a charge feels a force; field lines show direction and strength	Higher Tier only

P3.1a Charge

• Two types: positive and negative.
• Protons are positive, electrons are negative. An atom has equal numbers, so it is neutral.
• A body with equal positive and negative charge has zero net charge.
• Like charges repel. Unlike charges attract.
• Only electrons move. Protons stay in nuclei.

Net charge in a line

Net charge = (positive charge) - (negative charge), counted in the same units.

Positive units	Negative units	Net charge
9	9	0
9	5	+4
9	13	-4

Worked reminder: losing electrons

A rod has 9 positive and 9 negative units. It loses 4 electrons. Negative units are now 9 - 4 = 5. Net charge = 9 - 5 = +4 units. The cloth that gains them has 9 positive and 13 negative units, so its net charge is -4 units. The two net charges add to zero.

Worked reminder: reaching a target charge

An object has 6 positive and 10 negative units, so its net charge is -4. To become neutral it must lose 4 electrons. To end at +2 units it must lose 4 + 2 = 6 electrons. Work from the net charge, then ask whether electrons must be added or removed.

P3.1b Making static and sparking

Method in steps: describing charging by rubbing

1. Rub two different insulators together (for example a rod and a dry cloth).
2. Electrons move from the surface of one to the surface of the other.
3. Both objects now show a charge, and it stays because insulators do not let it flow.
4. Test by bringing the rod near a hanging charged strip or small pieces of paper.

Facts to know

• Static charge only builds up on insulators.
• A conductor held in the hand loses its charge through the body to the ground.
• Typical demonstration: acetate rod positive, polythene rod negative after rubbing with a cloth. Read the question, because the sign can depend on the materials used.
• Spark: charge on a charged object crosses an air gap to a nearby object and flows away in a sudden discharge. The Van de Graaff generator shows this.
• Evidence of non-contact forces: a suspended charged strip is pushed away from, or pulled towards, a second charged object before they touch.
• Closer charges, stronger force. Farther apart, weaker force.

P3.1c Electron transfer

Method in steps: explaining why two rubbed objects are charged

1. Say both objects are insulators and are rubbed together.
2. Say electrons are transferred from one object to the other.
3. The object that gains electrons is negative.
4. The object that loses electrons is positive.
5. Say the two charges are equal in size and opposite in sign.

Must-know distinctions

Idea	Correct statement	Wrong statement
Positive object	Lost electrons	Gained protons
What moves	Electrons	Positive charge
Charge on rubbing	Moved from one object to the other	Made from nothing
Neutral object near a charged one	Charges shift slightly; net charge stays zero	It gains charge

Why a charged object attracts a neutral one

1. A negative balloon is held near a neutral wall.
2. Electrons in the wall’s surface atoms shift slightly away.
3. The surface nearest the balloon becomes slightly positive.
4. The attraction to these nearer charges beats the repulsion from the farther charges.
5. The balloon is pulled to the wall.

Reverse the signs for a positive object: electrons shift towards it.

Gold leaf electroscope

Action	What happens to the leaf	Reason
Negative rod near cap, not touching	Rises	Electrons are pushed down; stem and leaf both negative and repel
Rod removed again	Falls	Electrons return; total charge was zero all along
Negative rod touches the cap	Rises and stays up	Electrons pass onto the electroscope
Same sign rod brought near afterwards	Rises more	More electrons are pushed down
Opposite sign rod brought near afterwards	Falls	Electrons are pulled up towards the cap

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 explains forces without contact: the field acts on the second charge.
• Field lines show the direction of the force on a small positive charge.
• Lines go away from positive and towards negative.
• Closer lines mean a stronger field and a larger force.
• Lines never cross.

Field patterns

Arrangement	Pattern
Positive sphere	Evenly spaced lines pointing out
Negative sphere	Evenly spaced lines pointing in
Positive and negative	Curved lines from positive to negative
Two positive	Lines curve away from each other; very weak field midway

Linking the field to attraction and repulsion

• Unlike charges: lines join one to the other, and each is pulled towards the other.
• Like charges: lines bend away, and each is pushed away from the other.
• Force on a charge is bigger where the field is stronger, which is closer to the source.
• An electron in a field feels a force in the opposite direction to the arrow.

Worked reminder: reading a field diagram

A diagram shows lines leaving a sphere A and ending on a sphere B, packed tightly in the gap. A is positive and B is negative. A small positive charge in the gap is pushed away from A and pulled towards B, so it moves along the line from A to B. The force on it is large here, because the lines are close together.

Practical to remember

Semolina on castor oil, with electrodes connected to a high-voltage supply: the grains line up along the field lines and show the pattern.

Method in steps: explaining a spark with fields (Higher Tier only)

1. Rubbing or a moving belt builds up charge on the object.
2. The field between the object and a nearby earthed conductor gets stronger.
3. When the field is strong enough, the air can no longer insulate.
4. Charge crosses the gap in a spark, and the object discharges.

Quick self-test

1. What is the net charge of a body with equal positive and negative charges?
2. An object has 8 positive units and 11 negative units. What is its net charge?
3. A rod has 9 positive and 9 negative units and loses 4 electrons. What is its net charge?
4. When an acetate rod becomes positive after rubbing, which particles moved, and in which direction?
5. Why can a metal rod held in the hand not keep a static charge?
6. Two suspended strips carry the same type of charge and hang close together. What do you see?
7. A negative rod is brought near the cap of an uncharged electroscope without touching it. Why does the leaf rise?
8. (Higher Tier only) Which way do the arrows point on field lines around an isolated positive sphere?
9. (Higher Tier only) Where is the field strongest on a field line diagram?
10. (Higher Tier only) An electron sits in a field. Compared with the field line arrow, which way is the force on it?
11. (Higher Tier only) What does semolina on castor oil show?

Answers

1. Zero.
2. 8 - 11 = -3 units.
3. Negative units fall to 5, so 9 - 5 = +4 units.
4. Electrons moved, from the rod to the cloth.
5. Charge flows away through the body to the ground, because the metal is a conductor.
6. They repel and swing apart without touching.
7. Electrons in the electroscope are pushed down to the stem and leaf, which both become negative and repel each other.
8. Straight outwards, away from the sphere.
9. Where the lines are closest together.
10. The opposite direction, because an electron is negative.
11. The shape of the electric field: the grains line up along the field lines.

Where marks are usually lost

• Writing that positive charges, or protons, moved from one object to another.
• Describing a charged object as having “gained charge” without saying which charge, or saying electrons were “made”.
• Forgetting to say the other object gains or loses the same amount, so the total stays the same.
• Not saying the materials are insulators, or that the metal rod flows charge away through the hand.
• Writing the net charge with the wrong sign after a transfer. Gaining electrons gives a negative net charge.
• Drawing field lines that cross, that have no arrows or that point the wrong way at a positive source.
• Spreading field lines evenly when asked to show where the field is strongest. Lines must bunch where the field is strongest.
• Explaining a spark by only saying “charge jumps”. For the Higher Tier explanation, link it to a strong field in the gap.

Official syllabus

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