OCR GCSE Physics A (J249) — Electricity
Ever pulled a woolly jumper over your head in the dark and heard a distinct crackling sound? This everyday phenomenon is static electricity in action. It is produced when two insulators are rubbed together, creating friction.
This friction provides the energy to cause electrons to move from one surface to another. Because electrons carry a negative charge, this electron transfer creates a charge imbalance between the two materials. The material that gains electrons becomes negatively charged, while the material that loses electrons is left with an equal positive charge.
Importantly, only electrons can move. Protons are fixed inside the atomic nucleus and do not move during charging by friction.
Why does static charge only build up on certain materials? An insulator (like plastic, glass, or rubber) has electrons that are tightly bound to its atoms. Because these electrons are held in fixed positions and cannot flow, the electric charge remains stationary (static) on the surface.
A conductor (like copper or aluminium) contains delocalised electrons that can flow freely. If a conductor is rubbed, any charge produced is immediately conducted away to the ground (a process called earthing), preventing any charge from building up.
When identifying charge in the laboratory, examiners expect you to know two standard examples of insulators:
You do not need to physically touch an object to push or pull it. Electrostatic force is a non-contact force created by charged objects. Every charged object is surrounded by an , and any other charged object entering this region will experience a force.
We can prove this using a laboratory observation with suspended rods:
What causes the massive flash of lightning during a thunderstorm? Lightning is essentially a giant electrostatic spark. As static charge builds up on an object (or a cloud), the potential difference between that object and the Earth rapidly increases.
When this potential difference becomes large enough, it creates a powerful electric field in the air gap. This field causes ionisation of the air, aggressively stripping electrons from the air molecules.
Normally, air is an insulator, but ionised air suddenly becomes an electrical conductor. Current then flows rapidly through the air, resulting in a visible spark or electrostatic discharge.
Refuelling an aircraft is a highly dangerous process that requires careful physics engineering. Friction between the fast-moving liquid fuel and the refuelling pipe causes a massive build-up of static charge. If this resulted in a spark, the highly flammable fuel vapor could easily explode.
To prevent this, the aircraft and the fuel tanker are physically connected using a copper earthing wire. This allows any built-up charge to safely flow away to the ground, neutralising the objects and completely removing the risk of a spark.
Students often say that 'positive charges move' or 'protons are transferred' when a material becomes positively charged. Actually, ONLY electrons move; an object becomes positive entirely by the loss of electrons.
EXAM TECHNIQUE: In 6-mark questions asking you to describe sparking, examiners are specifically looking for the terms 'potential difference' and 'ionisation' — you will lose marks if you just say 'the electricity jumps across the gap'.
When explaining how static electricity is produced, always use the exact phrase 'transfer of electrons' rather than 'transfer of charge', as the latter is considered too vague by OCR examiners.
Static electricity
An imbalance of electric charges on the surface of a material, typically an insulator, caused by the gain or loss of electrons.
Insulator
A material that does not allow the easy flow of electric charge because its electrons are tightly bound to atoms in fixed positions.
Conductor
A material, usually a metal, that allows electric charge to flow freely due to the presence of delocalised electrons.
Electrostatic force
A non-contact force of attraction or repulsion exerted by charged objects.
Ionisation
The process by which an atom or molecule acquires a charge by gaining or losing electrons, caused in this context by a strong electric field.
Earthing
The process of connecting a charged object to the Earth with a conductor so that electrons can safely flow to or from the Earth, neutralising the object.
Potential difference
The work done or energy transferred per unit of charge moving between two points.
Friction
The force created when two surfaces rub against each other, providing the energy for electron transfer.
Electrons
Negatively charged subatomic particles that orbit the nucleus of an atom and are the only charges that move during static charging.
Electron transfer
The movement of electrons from one surface to another, resulting in one becoming positively charged and the other negatively charged.
Delocalised electrons
Electrons in a conductor that are not bound to a specific atom and are free to move through the material.
Non-contact force
A force that acts on an object without any physical contact between the objects.
Electric field
A region around a charged object where another charged object will experience an electrostatic force.
Repulsion
The electrostatic force that pushes two objects with the same charge away from each other.
Attraction
The electrostatic force that pulls two objects with opposite charges towards each other.
Electrostatic discharge
The sudden flow of electric current between two charged objects, often through ionised air, resulting in a spark.
Put your knowledge into practice — try past paper questions for Physics A
Static electricity
An imbalance of electric charges on the surface of a material, typically an insulator, caused by the gain or loss of electrons.
Insulator
A material that does not allow the easy flow of electric charge because its electrons are tightly bound to atoms in fixed positions.
Conductor
A material, usually a metal, that allows electric charge to flow freely due to the presence of delocalised electrons.
Electrostatic force
A non-contact force of attraction or repulsion exerted by charged objects.
Ionisation
The process by which an atom or molecule acquires a charge by gaining or losing electrons, caused in this context by a strong electric field.
Earthing
The process of connecting a charged object to the Earth with a conductor so that electrons can safely flow to or from the Earth, neutralising the object.
Potential difference
The work done or energy transferred per unit of charge moving between two points.
Friction
The force created when two surfaces rub against each other, providing the energy for electron transfer.
Electrons
Negatively charged subatomic particles that orbit the nucleus of an atom and are the only charges that move during static charging.
Electron transfer
The movement of electrons from one surface to another, resulting in one becoming positively charged and the other negatively charged.
Delocalised electrons
Electrons in a conductor that are not bound to a specific atom and are free to move through the material.
Non-contact force
A force that acts on an object without any physical contact between the objects.
Electric field
A region around a charged object where another charged object will experience an electrostatic force.
Repulsion
The electrostatic force that pushes two objects with the same charge away from each other.
Attraction
The electrostatic force that pulls two objects with opposite charges towards each other.
Electrostatic discharge
The sudden flow of electric current between two charged objects, often through ionised air, resulting in a spark.
