Electric Charge
Edexcel IGCSE PhysicsΒ· 2.22Pβ2.28PΒ· 12 min read
1. Conductors, Insulators and Charging by Friction Practicalβ β βββHL onlyβ± 3 min
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Materials are classified as conductors or insulators based on their ability to allow electron flow. Most metals (copper, aluminium, iron) and graphite are conductors, while plastics, glass, rubber and dry wood are common insulators. Only insulators can build up a static electrostatic charge, as electrons cannot flow freely through them.
Charging by friction
The transfer of electrons between two insulating materials when they are rubbed together, producing equal and opposite static charges on each material.
Classify the following materials as conductors or insulators: copper wire, plastic ruler, aluminium foil, glass window pane, rubber eraser.
- 1
Recall that most metals are conductors, as they allow free electron flow.
- 2
Copper and aluminium are metals, so they are classified as conductors.
- 3
Plastic, glass and rubber do not allow free electron flow, so they are classified as insulators.
For the required practical, you will rub pairs of insulating rods (e.g. polythene, acetate) with cloths, and test charge interaction using a hanging charged rod. The independent variable is the pair of materials rubbed, the dependent variable is the type of charge produced, and control variables include the dryness of materials and the length of time rubbing takes.
Exam tip:
When answering practical questions, always explicitly state that materials must be dry, as moisture creates a conductive path that prevents charge build-up.
2. Electron Transfer and Charge Interaction Rulesβ β β ββHL onlyβ± 4 min
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All atoms contain positively charged protons (fixed in the nucleus) and negatively charged electrons (in outer shells, free to transfer between materials). When two insulators are rubbed together, the material that gains electrons becomes negatively charged, and the material that loses electrons becomes positively charged. Protons never move during this process.
Explain why a polythene rod becomes negatively charged when rubbed with a dry cotton cloth.
- 1
Friction from rubbing transfers electrons between the two insulating materials.
- 2
Polythene attracts electrons more strongly than cotton, so it gains electrons from the cloth.
- 3
Electrons have a negative charge, so the excess of electrons gives the polythene rod a negative charge. The cotton cloth loses electrons, so it becomes positively charged.
What charge does an object develop if it loses electrons during friction?
Positive
Negative
Neutral
Reveal answer
Positive βLosing negative electrons leaves the object with a surplus of positive protons, so it is positively charged.
What happens when two negatively charged balloons are held close together?
They attract each other
They repel each other
No force acts between them
Reveal answer
They repel each other βLike charges always repel, so two negative charges will push each other apart.
Exam tip:
Never state that protons move in any electrostatic explanation: this is a common exam error that will lose you marks.
3. Dangers of Electrostatic Chargeβ β βββHL onlyβ± 2 min
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Electrostatic charge build-up on insulated objects can create a large potential difference, leading to a spark jumping between the charged object and a nearby earthed conductor. This is dangerous in areas with flammable materials, as the spark can ignite fuel vapour or dust, causing explosions.
Explain why aircraft are bonded to fuel tanks via a metal wire during refuelling.
- 1
Friction between the fuel and the fuel pipe causes electrostatic charge to build up on the pipe and aircraft body.
- 2
Without bonding, a large potential difference could cause a spark to jump between the aircraft and the fuel tank.
- 3
The spark would ignite flammable fuel vapour around the refuelling point, causing an explosion.
- 4
The metal bonding wire provides an earthed conductive path for excess charge to flow to the ground, preventing spark formation.
Exam tip:
For full marks on danger explanation questions, always follow the full causal chain: friction β charge build-up β spark β ignition β risk of explosion.
4. Uses of Electrostatic Chargeβ β β ββHL onlyβ± 3 min
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Electrostatic charge is used in a range of everyday technologies, including photocopiers, laser printers and inkjet printers. All these applications rely on the attraction between opposite charges to control the position of small particles (toner or ink droplets).
Describe how electrostatic charge is used in an inkjet printer to produce a printed image.
- 1
Tiny ink droplets are given a controlled electrostatic charge as they leave the printer nozzle.
- 2
The charged droplets pass between two oppositely charged metal plates, which create an electric field.
- 3
The charge on the plates is adjusted to steer the charged droplets to the exact correct position on the paper, using the rule that opposite charges attract.
- 4
Unwanted droplets are steered into a waste collector, while the correct droplets form the printed image on the page.
In photocopiers and laser printers, a charged drum is exposed to light to create a pattern of positive charge corresponding to the image to be printed. Negatively charged toner powder is attracted to the charged areas of the drum, then transferred to positively charged paper and fused in place with heat.
Exam tip:
You only need to learn two uses for the exam: photocopiers/laser printers and inkjet printers. No other applications are required.
5. Common Pitfalls
Wrong move:
Saying protons move when an object is charged by friction
Why:
Protons are tightly bound in the atomic nucleus and cannot transfer between materials
Correct move:
Always state only electrons move; objects gaining electrons become negative, those losing electrons become positive
Wrong move:
Stating charged objects only attract other charged objects
Why:
Charged objects induce an opposite surface charge on neutral objects, leading to attraction
Correct move:
Mention charged objects attract both oppositely charged objects and neutral objects via induction
Wrong move:
Explaining earthing 'blocks' charge rather than transferring it
Why:
Earthing provides a conductive path for electron flow to or from the ground to neutralise charge
Correct move:
State earthing allows excess electrons to flow to the ground, or electrons to flow from the ground to neutralise positive charge
Wrong move:
Linking static charge to current electricity formulas (Q=It)
Why:
This subtopic covers static charge only; current electricity is out of scope for these questions
Correct move:
Only reference one-off electron transfer between materials, not continuous charge flow in circuits
Wrong move:
Skipping the spark-ignition link when explaining fuelling dangers
Why:
Examiners require a full causal chain to award full marks for explanation questions
Correct move:
Follow the chain: friction β charge build-up β potential difference β spark β ignition of flammable vapour β explosion risk
6. Quick Reference Cheatsheet
Concept | Key Exam Point | Common Question Prompt |
|---|---|---|
Conductors/Insulators | Conductors (metals, graphite) allow electron flow; insulators (plastic, glass, rubber) do not | Name one material that cannot build up static charge: |
Charging by friction | Only electrons transfer; gain eβ» = negative, lose eβ» = positive | Explain why a rubbed acetate rod becomes positively charged: |
Charge rules | Like charges repel, unlike charges attract; charged objects attract neutral objects via induction | State what happens when two negative charges are held close together: |
Dangers | Charge build-up causes sparks that ignite flammables; mitigated by earthing | Explain why fuel tankers have earthing straps: |
Uses | Photocopiers use charged toner; inkjet printers steer charged ink droplets | Give one use of electrostatic charge in technology: |
7. Frequently Asked
Do protons move when an object is charged by friction?
No, protons are tightly bound in the nucleus of atoms and cannot move between materials. Only outer electrons are transferred during charging by friction.
Why does a charged balloon stick to a neutral wall?
The charged balloon induces an opposite charge on the surface of the neutral wall. The attractive force between the opposite charges holds the balloon to the wall.
Do I need to learn any formulas for this topic?
No, all questions for this subtopic are qualitative, requiring explanation rather than calculation. No formulas are provided or needed for electrostatics questions in Edexcel IGCSE Physics Paper 2.
Going deeper
- official_specEdexcel IGCSE Physics 4PH1 SpecificationSee section 2.22P to 2.28P
- practical_guideCharging by Friction Practical HandbookCovers variable identification and safe technique
What's Next
Now you have mastered static electric charge for Edexcel IGCSE Physics Higher Paper 2, you are ready to move on to current electricity concepts, including current, potential difference and circuit components, which make up the majority of the Electricity unit. You should also practise past paper explanation questions for this topic to refine your answer structure, as 2β3 mark explanation questions are common for electrostatics. Ensure you can correctly identify independent, dependent and control variables for the charging by friction practical, as AO3 practical skills questions are often paired with this subtopic. Remember this content is only examined in Paper 2, so Foundation tier and Double Award students do not need to revise it.