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Pull a jumper over your head on a dry day and you hear a faint crackle; rub a balloon on your hair and the hair stands on end; walk across a nylon carpet and touch a metal door handle, and a tiny spark stings your fingertip. Every one of these small surprises is caused by static electricity — electric charge that has built up on an object and, for the moment, stays put rather than flowing round a circuit. This lesson opens Topic P3 of OCR Gateway Combined Science A by explaining where this charge comes from, why some objects end up positive and others negative, how charged objects push and pull on one another through an electric field, and why static is sometimes a nuisance, sometimes a danger and sometimes genuinely useful.
By the end of this lesson you should be able to explain how an insulator becomes charged by friction in terms of the transfer of electrons, state which way the electrons move and predict the sign of the charge on each object, describe the forces between charged objects, describe an electric field in a basic way, recall that charge is measured in coulombs, and explain everyday examples of static, including its dangers and uses.
This lesson mostly builds AO1 recall and understanding of charge, electron transfer and electric fields, with AO2 application when you predict the sign of the charge on each object and explain everyday examples of static in terms of the underlying physics.
Everything around you is made of atoms, and every atom contains smaller charged particles. At the centre of each atom is a nucleus containing protons, which carry a positive charge, and moving around the nucleus are electrons, which carry an equal-sized negative charge. In a neutral atom the number of protons exactly equals the number of electrons, so the positive and negative charges cancel out and the atom has no overall charge.
Static electricity is all about upsetting this balance. The protons are locked inside the nucleus and cannot move from atom to atom, but the electrons on the outside of an atom can be transferred from one object to another. If an object gains electrons it ends up with more negative charge than positive, so it becomes negatively charged; if it loses electrons it has more positive charge than negative, so it becomes positively charged. The single most important idea to carry through this whole topic is that only electrons move — an object is "positive" because it has lost electrons, never because protons have somehow flowed into it.
Exam Tip: Charge is transferred by moving electrons, never protons. An object is negative because it has gained electrons and positive because it has lost electrons. Always describe charging as electrons moving; answers that talk about protons "flowing" are penalised.
When two insulating materials are rubbed together, the friction transfers electrons from one material to the other. Insulators are used because their electrons are not free to move through the material, so once charge has built up it stays where it is rather than leaking away. (In a conductor such as a metal, any charge you tried to build up would simply flow away to earth through your hand, so no static would collect.)
A classic example is rubbing a polythene rod with a cloth:
With a different material the electrons move the opposite way. Rubbing an acetate (perspex) rod with a cloth:
Notice that the two objects always end up with opposite charges, and the amount of charge on each is equal — the electrons one object loses are exactly the electrons the other gains. Charge is never created or destroyed in this process; it is simply moved from one object to the other.
| Materials rubbed together | Electrons move | Resulting charges |
|---|---|---|
| Polythene rubbed with a cloth | Cloth → polythene | Polythene negative, cloth positive |
| Acetate rubbed with a cloth | Acetate → cloth | Acetate positive, cloth negative |
| Balloon rubbed on hair | Hair → balloon | Balloon negative, hair positive |
Exam Tip: When you describe friction charging, always say which way the electrons move and then give both charges. "Polythene gains electrons so it is negative, and the cloth loses electrons so it is positive" scores both marks; naming only one object's charge usually loses a mark.
Electric charge is a measurable quantity, and its unit is the coulomb, symbol C. One coulomb is actually a very large amount of charge, so the charges built up by rubbing objects together are usually only tiny fractions of a coulomb. You will meet the coulomb again in the next lesson, where it links charge to current through Q=It. For now, simply remember that charge is measured in coulombs and is given the symbol Q.
Charged objects exert forces on one another without touching. The rule is short and must be learnt exactly:
These are non-contact forces: the objects do not need to touch for the force to act, because each charged object is surrounded by an electric field that pushes or pulls on any other charge nearby. The closer the charges are, the stronger the force between them.
graph LR
A["+ charge"] -->|repel| B["+ charge"]
C["− charge"] -->|repel| D["− charge"]
E["+ charge"] -->|attract| F["− charge"]
This simple rule explains the charged-rod demonstrations done in class. If a negatively charged polythene rod is brought near another negatively charged polythene rod hanging on a thread, the hanging rod swings away (repulsion). If instead a positively charged acetate rod is brought near the negative polythene rod, the polythene rod swings towards it (attraction). Testing the force is, in fact, how you can find the sign of an unknown charge: bring up a rod of known charge and see whether the object is attracted or repelled.
Exam Tip: Repulsion is the only sure test of charge. An uncharged object is attracted to a charged one too (by induction), so attraction on its own does not prove an object is charged — but if two objects repel, they must both be charged, with the same sign.
The reason two charges can push or pull on each other across a gap is that every charged object is surrounded by an electric field. An electric field is a region around a charged object in which another charge experiences a force. Place a small charge anywhere inside this region and it feels a push or a pull; take it well away and it feels nothing. The field is what carries the influence of one charge across the empty space to another, which is exactly why the forces between charges are non-contact forces.
The field is strongest close to the charged object and gets weaker as you move further away. This fits what you already know — two charges push or pull more strongly when they are near each other than when they are far apart.
We picture an electric field using field lines, which follow two rules you should know:
Around a small charged sphere the field is radial: the lines point straight outwards (for a positive charge) in all directions, like the spokes of a wheel, and are most crowded near the charge.
For a negative charge the picture is identical except that every arrow points inwards, towards the charge, because that is the way a small positive test charge would be pushed.
Exam Tip: Two facts win most electric-field marks: arrows point from + to − (the direction a positive charge would be pushed), and closely spaced lines mean a stronger field. Always draw an arrow on every field line — a line without an arrow loses the direction mark.
You may have seen a charged rod pick up tiny scraps of paper, even though the paper is uncharged. This happens by induction. When a negatively charged rod is held near the neutral paper, its field repels the electrons in the surface of the paper, pushing them slightly to the far side. This leaves the near side of the paper slightly positive and the far side slightly negative. Because the positive near side is closer to the rod, the attraction is stronger than the repulsion, and the paper is pulled towards the rod. This is why a charged object can attract small, light, uncharged objects — and, again, why attraction on its own cannot prove that something is charged.
If enough charge builds up on an object, the electric field around it becomes very strong. A strong enough field can tear electrons off the atoms in the surrounding air — a process called ionising the air. The air, normally an insulator, suddenly contains free charged particles and becomes a conductor, so charge rushes across the gap as a spark: a flash of light and a crackle. This is a discharge — the object loses its excess charge and returns to neutral.
The everyday "shock" from a car door works this way. As you move, friction between your shoes and the floor charges your body. When you touch a metal object, electrons flow suddenly between you and it to equalise the charge, and you feel this rush as a small electric shock. The spark you sometimes see in the dark is the air being made to conduct briefly.
Exam Tip: A spark happens when the electric field becomes strong enough to ionise the air, making it conduct. Charge then jumps across the gap and the object discharges. Link it as "more charge → stronger field → more likely to spark".
Static electricity shows up everywhere once you know what to look for.
Nuisances. Clothes made of synthetic fibres cling together and crackle when taken out of a tumble dryer, because friction has charged them and unlike charges attract. Dust is attracted to charged screens by induction, which is why they seem to get dusty so quickly.
Dangers. The most important hazard is when sparks ignite flammable vapours or dusts:
The general safety idea is earthing: providing a conducting path for charge to flow away gradually, so a dangerous build-up never happens.
Uses. Static is genuinely useful when charge is made to attract things on purpose:
| Situation | Static effect | What is done |
|---|---|---|
| Refuelling an aircraft | Charge build-up could spark and ignite fuel | Earth the aircraft so charge flows away |
| Spray painting a car | Charged droplets repel and spread; attracted to oppositely charged car | Used deliberately for an even coat |
| Clothes from a dryer | Friction charges fibres; unlike charges attract | Antistatic sheets reduce charging |
Exam Tip: For dangers, the standard remedy is earthing (a conducting path lets charge flow away safely). For uses like spray painting, the key points are that like-charged droplets repel and spread out, and the oppositely charged object attracts the paint — quote both.
| Misconception | The correct idea |
|---|---|
| "Protons move to make an object positive" | Only electrons move; an object is positive because it has lost electrons |
| "Charging creates new charge" | Charge is only transferred; the electrons lost by one object are gained by the other |
| "You can charge a metal rod by rubbing it in your hand" | Charge would flow away through your hand to earth; friction charging needs insulators |
| "Attraction proves an object is charged" | An uncharged object is also attracted by induction; only repulsion proves charge |
| "Like charges attract" | Like charges repel; only unlike (opposite) charges attract |
| "Field lines point from − to +" | Field lines point from + to − (the force on a positive charge) |
Question (6 marks): A polythene rod is rubbed with a cloth and becomes negatively charged. Explain, in terms of electrons, why the rod becomes negative and what happens to the cloth. Then describe what you would observe if this rod were brought close to a second, identically charged polythene rod hanging from a thread.
Mid-band response: "When you rub the rod it gets electrons, so it is negative. The cloth loses electrons. If you bring two negative rods together they push apart because they are the same charge."
Examiner-style commentary: The electron transfer and the repulsion are both correct, but the cloth's resulting charge is not named and the explanation is brief. To climb a band, state explicitly that the cloth becomes positive because it has lost electrons, say that the charges are equal and opposite, and describe the observation (the hanging rod swings away) more fully.
Stronger response: "Rubbing transfers electrons from the cloth to the polythene rod. Because the rod gains electrons it has more negative charge than positive, so it becomes negatively charged. The cloth loses those electrons, so it becomes positively charged. The two have equal and opposite charges. If the charged rod is brought near a second rod that is also negative, the hanging rod swings away because like charges repel."
Examiner-style commentary: A clear, correct answer that names both charges and explains the repulsion. To reach the top band, emphasise that no charge is created — only moved — and note that this is a non-contact force that gets stronger as the rods get closer.
Top-band response: "Friction between the cloth and the rod transfers electrons from the cloth to the polythene. Because the rod gains electrons, it now has more electrons (negative) than protons (positive), so it becomes negatively charged. The cloth has lost exactly those electrons, so it is left with more protons than electrons and becomes positively charged — the charges are equal in size and opposite in sign, because charge is only transferred, not created. When the rod is brought near a second polythene rod that is also negatively charged, the hanging rod swings away from it. This is because like charges repel: the two negative charges exert a non-contact force of repulsion on each other through their electric fields, and this force gets stronger the closer the rods are brought together."
Examiner-style commentary: Full marks. It describes the electron transfer with direction, names both resulting charges, stresses conservation of charge, predicts the observation, and explains it with the like-charges-repel rule, including the non-contact nature and the effect of distance.
This content is aligned with OCR Gateway Combined Science A (J250), Topic P3 Electricity and magnetism. Refer to the official OCR specification for exact wording.