(d) Electric charge
- Syllabus
- 2024
- Topic
- —
- Level
- —
Learning objectives
222P Conductors and insulatorsIdentify common materials that are electrical conductors or insulators, including metals and plastics223P Charging by friction practicalPractical: investigate how insulating materials can be charged by friction224P Producing electrostatic chargeExplain how positive and negative electrostatic charges are produced on materials by the loss and gain of electrons225P Attraction and repulsionKnow that there are forces of attraction between unlike charges and forces of repulsion between like charges226P Electrostatic phenomenaExplain electrostatic phenomena in terms of the movement of electrons227P Electrostatic dangersExplain the potential dangers of electrostatic charges, e.g. when fuelling aircraft and tankers228P Electrostatic usesExplain some uses of electrostatic charges, e.g. in photocopiers and inkjet printers
Identify conductors and insulators
An electrical conductor allows charge to move through it easily. An electrical insulator does not allow charge to move through it easily.
| Material or group | Classification | Useful reason or consequence |
|---|---|---|
| metals such as copper, aluminium and iron | conductor | mobile electrons carry charge through the metal |
| graphite | conductor | charge can move through its structure |
| plastics and rubber | insulator | charge does not move through them easily |
| glass and dry wood | insulator | charge is not free to flow through the material |
Conductors are used when charge must flow, such as copper connecting wires. Insulators are used to prevent charge flow, such as plastic wire coverings. An insulating object can also keep electrostatic charge localised on its surface.
A metal object is not automatically discharged: if it is isolated from Earth, charge can remain on it but spreads across the conducting surface. A plastic object is not automatically charged; it is only able to retain transferred charge.
Investigate charging by friction
An insulating material can be charged by rubbing it with a different insulating material. A fair investigation changes one material and measures the resulting electrostatic effect.
Use rods of equal dimensions made from different insulating materials. Discharge each rod before testing, then rub it with the same cloth using the same force, number of strokes and contact length. Bring the rod to the same distance from a suspended charged ball, an electroscope, or a charge meter and record the force, deflection, or charge reading. Repeat and compare mean results.
| Variable role | Example |
|---|---|
| independent | material of the rod |
| dependent | force, deflection, or charge-meter reading |
| controls | cloth material, rubbing force and number, rod dimensions, separation distance, initial charge |
Simply attracting neutral paper shows that the rod is charged, but it does not identify the sign of the charge. Use a charge meter or compare attraction and repulsion with objects of known charge when the sign is required.
How objects become positively or negatively charged
Electrostatic charging transfers electrons between materials. Protons remain bound inside atomic nuclei and do not move from one object to the other.
| Electron change | Resulting charge | Why |
|---|---|---|
| object gains electrons | negative | it now has more electrons than protons |
| object loses electrons | positive | it now has fewer electrons than protons |
| no imbalance | neutral | numbers of electrons and protons are equal |
When two initially neutral insulators are rubbed, electrons can move from one surface to the other. The object that gains those electrons becomes negative; the object that loses the same electrons becomes positive. Charge is transferred, not created from nothing.
Never explain positive charging as gaining protons or positive electrons. A positively charged object has lost negatively charged electrons; its protons have not moved between the materials.
Predict attraction and repulsion
Electrostatic forces act between charged objects: like charges repel and unlike charges attract.
| Charges on the two objects | Force |
|---|---|
| positive and positive | repel |
| negative and negative | repel |
| positive and negative | attract |
Two negatively charged droplets spread apart because each repels the other. A positive ball moves towards a negative rod because the unlike charges attract. The force acts along the line joining the charged objects and becomes weaker as their separation increases.
Repulsion is clear evidence that two objects have the same type of charge. Attraction alone does not prove opposite net charges, because a charged object can also attract a neutral object by rearranging charge within it.
Explain electrostatic attraction by electron movement
A charged object can attract a neutral object by causing electrons in the neutral object to redistribute. The neutral object remains neutral overall, but its nearer and farther sides no longer have the same local charge balance.
Example: negatively charged plastic wrapping is brought near a neutral plate. Its excess electrons repel electrons in the plate away from the nearby surface. That surface is left relatively positive, so the opposite charges close together attract and the wrapping sticks.
A positively charged object produces the reverse electron movement: electrons in the neutral object are attracted towards the near side. In either case, the attractive force from the closer unlike charges is stronger than the repulsion from the farther like charges.
The protons in the neutral object do not travel across it. The phenomenon is explained by movement or slight redistribution of electrons; no net charge has to be transferred to the neutral object for attraction to occur.
Prevent electrostatic sparks
Moving fuel, powder, or dust can transfer electrons by friction. If charge builds up on an isolated aircraft, tanker, pipe, or channel, the resulting large voltage can drive a spark through the air.
During refuelling, fuel flowing through pipes can create separated charge. A spark near flammable fuel vapour can ignite it, causing fire or explosion. In a flour mill, a spark can ignite suspended combustible dust and cause an explosion.
| Control | How it reduces danger |
|---|---|
| earthing | provides a conducting path for electrons to flow to or from Earth, preventing charge build-up |
| bonding two conductors | keeps them at nearly the same potential, reducing the chance of a spark between them |
| conductive equipment and controlled flow | helps charge drain away and reduces rapid charge separation |
Earthing does not remove flammability. It reduces the ignition risk by preventing a dangerous electrostatic voltage and spark, so the causal chain must reach spark prevention rather than stop at 'removing charge'.
Use electrostatic forces in printers
Photocopiers and inkjet printers use controlled electrostatic attraction or repulsion to place toner or ink exactly where it is needed.
| Device | Controlled charge process | Result |
|---|---|---|
| inkjet printer | ink droplets are charged and pass between charged deflection plates; attraction and repulsion change each droplet's path | droplets land at selected positions on the paper |
| photocopier | a charged drum is exposed to the light pattern from the original; illuminated regions lose charge, leaving a charged image pattern that attracts oppositely charged toner | toner is transferred from the drum to charged paper and fixed to form the copy |
In both devices, electrical control changes where charge remains or how a charged particle moves. The force is non-contact, so tiny droplets or toner particles can be positioned without a mechanical tool touching each one.
The toner or ink does not move merely because it is charged. Its direction is set by attraction to opposite charge and repulsion from like charge; changing the surrounding charged pattern changes where it lands.