P8: Static Electricity
Static charge, electric fields and sparks — dangers and uses of static electricity including earthing and electrostatic applications.
📋 Key Definitions
Static electricity: A build-up of electric charge on the surface of an insulating material. It is caused by friction between two insulators — electrons are transferred from one material to the other.
Electric field: A region around a charged object where another charged object experiences a force. Field lines point from positive to negative charges.
Sparking: When the potential difference between a charged object and the earth (or another object) is large enough, electrons can jump across the gap, creating a spark.
⚡ How Static Charge Builds Up
By friction: When two insulating materials are rubbed together, electrons are transferred from one to the other. The material that gains electrons becomes negatively charged. The material that loses electrons becomes positively charged.
- Only electrons move — positive charges (protons) stay in the nucleus
- Conductors cannot hold static charge because electrons flow away easily
- Insulators hold static charge because electrons cannot flow through the material
Example — Polythene and Acetate Rods
When a polythene rod is rubbed with a duster, electrons transfer FROM the duster TO the rod. The rod becomes negatively charged; the duster becomes positively charged.
When an acetate rod is rubbed with a duster, electrons transfer FROM the rod TO the duster. The rod becomes positively charged; the duster becomes negatively charged.
🧲 Attraction and Repulsion
Like charges repel. Unlike charges attract. Two positive charges repel; two negative charges repel; a positive and negative charge attract.
- A charged object can attract an uncharged object — the charge induces an opposite charge in the nearby object (induced charge)
- For example: a negatively charged balloon attracts small pieces of uncharged paper
📐 Electric Fields
Electric field: The region around a charged object where a force is exerted on other charged objects. The field is strongest close to the charge.
- Field lines show the direction of the force on a positive charge
- Field lines point away from positive charges and towards negative charges
- The closer the field lines, the stronger the field
- A uniform field between parallel plates has equally spaced, parallel field lines pointing from positive to negative plate
| Shape | Description | Field Lines |
|---|---|---|
| Point charge (positive) | Radial field around a single positive charge | Point outwards from the charge in all directions |
| Point charge (negative) | Radial field around a single negative charge | Point inwards towards the charge from all directions |
| Parallel plates | Uniform field between two charged plates | Straight parallel lines from positive plate to negative plate |
💥 Sparking
Sparking occurs when the potential difference between a charged object and the earth (or another conductor) is large enough to make electrons jump across the gap. The air is ionised, allowing current to flow briefly.
- A spark is a sudden flow of charge through the air
- The higher the potential difference and the smaller the gap, the more likely a spark
- Sparks can ignite flammable gases or fuels — this is a major danger of static electricity
⚠️ Dangers of Static Electricity
| Danger | Description | Prevention |
|---|---|---|
| Lightning | Charge builds up in clouds. When PD is large enough, a spark (lightning) discharges to the ground | Lightning conductors provide a safe path to earth |
| Refuelling aircraft | Friction during refuelling can build up static charge. A spark could ignite fuel vapour | The aircraft is earthed before refuelling so charge cannot build up |
| Petrol stations | Static charge on a person or the fuel pipe could spark and ignite fuel vapour | Earthing the fuel pipe, keeping the nozzle in contact with the car |
| Flammable factories | Static build-up on machinery or workers could cause sparks | Anti-static clothing, earthing of equipment, humidifying the air |
Earthing prevents dangerous static build-up by providing a conducting path to the ground, allowing charge to flow away safely.
✅ Uses of Static Electricity
| Use | How it works | Process |
|---|---|---|
| Photocopier | Static charge attracts toner to the image areas on a drum | 1. Charged drum exposed to light from document — charged areas where light hits are discharged. 2. Dark areas (image) remain charged and attract oppositely charged toner. 3. Toner transferred to paper and fixed with heat. |
| Electrostatic sprayer | Droplets are given a charge so they spread evenly and are attracted to the target | 1. Paint/pesticide given a static charge at the nozzle. 2. Charged droplets repel each other for even spray. 3. Attracted to earthed target, covering even the back. |
| Inkjet printer | Charged ink droplets are deflected by electric plates | 1. Ink droplets are given a charge. 2. Electric field between charged plates deflects them. 3. Varying charge controls where each droplet lands on the paper. |
| Dust extraction | Charged plates attract dust and smoke particles from the air | 1. Waste gases pass over charged wire, giving particles a charge. 2. Charged particles attracted to oppositely charged metal plates. 3. Clean gas exits; particles removed periodically. |
📝 Worked Examples
Worked Example 1 — Explaining Static Charge
Question: A balloon is rubbed on a woolly jumper and then sticks to a wall. Explain why the balloon becomes charged and why it sticks to the wall.
Solution:
- When the balloon is rubbed on the jumper, friction causes electrons to transfer from the jumper to the balloon.
- The balloon gains electrons and becomes negatively charged. The jumper loses electrons and becomes positively charged.
- When the charged balloon is brought near the wall, it induces an opposite charge on the surface of the wall (the negative balloon repels electrons in the wall, leaving a positive area on the surface).
- The attraction between the negatively charged balloon and the positively charged area of the wall causes the balloon to stick.
Worked Example 2 — Dangers of Static in Refuelling
Question: Explain why an aircraft must be earthed before refuelling, and what could happen if it is not.
Solution:
- During refuelling, friction between the fuel and the pipe causes static charge to build up on the fuel and the aircraft.
- If the aircraft is not earthed, the charge cannot flow away. The potential difference between the aircraft and the ground increases.
- If the PD becomes large enough, a spark could jump between the aircraft and the ground (or the fuel nozzle).
- This spark could ignite the fuel vapour, causing an explosion.
- Earthing provides a conducting path to the ground so charge flows away safely, preventing any dangerous build-up.
Worked Example 3 — Electric Field Explanation
Question: Describe the electric field between two parallel metal plates, one positively charged and one negatively charged. Explain how this field would affect a small positive charge placed between them.
Solution:
- The electric field between parallel plates is uniform — the field lines are straight, parallel and equally spaced.
- Field lines point from the positive plate to the negative plate.
- A small positive charge placed between the plates would experience a force in the direction of the field lines (from positive to negative plate).
- The force would accelerate the positive charge towards the negative plate.
- The force is the same strength everywhere in the uniform field because the field lines are equally spaced.
Worked Example 4 — Photocopier Process
Question: Describe how a photocopier uses static electricity to produce a copy of a document.
Solution:
- A selenium-coated drum is given a uniform positive charge.
- An image of the document is projected onto the drum. Where light falls on the drum, the charge leaks away (those areas become neutral).
- Dark areas of the image (where no light falls) remain positively charged on the drum.
- Negatively charged toner powder is spread over the drum. It is attracted to the positively charged areas (the dark image areas).
- The toner is transferred onto a sheet of paper and fixed in place by heated rollers.
- The drum is cleaned and recharged for the next copy.
❓ Practice Questions
Q1: Foundation Explain how static electricity builds up when two insulators are rubbed together. State which particles move.
Q2: Foundation State the rule for attraction and repulsion of charges. Give one example of each.
Q3: Foundation Explain what earthing means and why it is used when refuelling aircraft.
Q4: Higher Describe the electric field around a single positive charge. State how the field lines show the direction of force and the strength of the field.
Q5: Higher Explain how an electrostatic paint sprayer gives a more even coat than a conventional sprayer. Refer to charge, repulsion and attraction in your answer.
Q6: Higher A factory produces flammable solvents. Suggest two ways to reduce the risk of a static spark causing a fire, and explain how each works.
✅ Answers
- When two insulators are rubbed together, friction transfers electrons from one material to the other. The material that gains electrons becomes negatively charged; the material that loses electrons becomes positively charged. Only electrons move — protons do not move because they are in the nucleus.
- Like charges repel, unlike charges attract. Repulsion example: two negatively charged balloons pushed close together will repel each other. Attraction example: a negatively charged balloon and a positively charged duster will attract each other.
- Earthing means connecting the object to the ground with a conducting wire. When an aircraft is earthed before refuelling, any static charge that builds up flows safely to the ground through the earth wire. This prevents charge building up to a dangerous level where a spark could jump and ignite fuel vapour.
- The electric field around a positive point charge is a radial field. The field lines point outwards from the charge in all directions (away from positive). The direction of the field lines shows the direction of the force on a positive charge placed in the field. The closer the field lines are to each other (near the charge), the stronger the field. Further away, the field lines spread out and the field gets weaker.
- In an electrostatic sprayer, the paint droplets are given a static charge as they leave the nozzle. Because like charges repel, the charged droplets repel each other and spread out into a fine, even mist. The target (e.g. a car body) is earthed, so the oppositely charged droplets are attracted to it. This means paint wraps around the target, reaching the back and edges, giving a more even coat with less waste.
- 1) Earth all machinery and containers — this provides a conducting path to the ground so static charge cannot build up; any charge flows safely away. 2) Humidify the air — water vapour in the air helps charge to leak away from charged surfaces, preventing large build-ups of static charge. Both methods reduce the risk of a spark that could ignite the flammable solvent vapour.
🎯 Exam Tips
- When explaining static charge, always mention electrons transferring — never say "positive charges move" because protons stay in the nucleus.
- "Gains electrons = negative, loses electrons = positive" — memorise this.
- For dangers of static, always explain: friction causes charge build-up → large PD → spark → ignites flammable substance.
- For earthing: it provides a conducting path to the ground so charge can flow away safely.
- In electric field questions: field lines go from + to -, and they show the force on a positive test charge.
- For photocopier/sprayer questions, use the terms: charge, attract, repel, earth — and explain the process step by step.
- If asked to draw field lines between parallel plates: straight, parallel, equally spaced, from + to -.
🔢 Maths Skills
Mathematical Skills
While there is little numerical maths in static electricity, you must be able to interpret electric field diagrams quantitatively. The spacing of field lines indicates field strength — closer lines mean a stronger field. Charge is quantised: all charges are whole-number multiples of the electron charge (e = 1.6 × 10⁻¹⁹ C). When electrons transfer, the charge on each object is equal and opposite (conservation of charge).
Maths Example
An acetate rod loses 3 × 10¹⁰ electrons when rubbed with a duster. Calculate the charge on the rod and the duster.
Charge on one electron = 1.6 × 10⁻¹⁹ C
Total charge transferred = 3 × 10¹⁰ × 1.6 × 10⁻¹⁹ = 4.8 × 10⁻⁹ C
Rod charge = +4.8 × 10⁻⁹ C (lost electrons). Duster charge = −4.8 × 10⁻⁹ C (gained electrons). The charges are equal and opposite.
⚠️ Common Misconceptions
Watch Out!
1. Wrong: Positive charges move during static charging Correct: Only electrons move — protons are fixed in the nucleus. A positive charge is caused by losing electrons, not by positive charges moving
2. Wrong: Static charge is "created" when objects are rubbed Correct: Charge is conserved — electrons are transferred from one object to another. The total charge remains the same; no charge is created or destroyed
3. Wrong: Conductors can hold static charge Correct: Conductors allow charge to flow away, so they cannot hold static charge. Only insulators can hold static charge because electrons cannot move through the material
✍️ 6-Mark Question
Extended Answer
6 marks: Explain the dangers of static electricity during the refuelling of an aircraft and how earthing prevents these dangers.
During refuelling, fuel flows through the pipe at high speed. Friction between the fuel and the pipe causes electrons to transfer, building up static charge on the fuel and the aircraft body. If the aircraft is not earthed, this charge cannot flow away and the potential difference between the aircraft and the ground increases. If the potential difference becomes large enough, electrons can jump across the gap between the aircraft and the fuel nozzle or the ground. This creates a spark, which ionises the air and allows a sudden flow of charge. Fuel vapour is highly flammable, so the spark could ignite it, causing a fire or explosion. Earthing prevents this by connecting the aircraft to the ground with a conducting wire before refuelling begins. This provides a path for any static charge to flow safely to the ground, preventing charge from building up to a dangerous level. The fuel nozzle is also bonded to the aircraft so there is no potential difference between them, eliminating the risk of a spark.
Mark scheme: 1 mark — friction causes charge build-up during refuelling, 1 mark — charge builds up if not earthed / PD increases, 1 mark — large PD causes a spark, 1 mark — spark ignites flammable fuel vapour, 1 mark — earthing provides conducting path to ground, 1 mark — charge flows away safely so no dangerous build-up
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
A factory produces flammable solvents. Workers wear overalls, and machinery mixes the solvents in metal vats. The factory has the following data:
| Condition | Relative humidity | Static incident rate (per month) |
|---|---|---|
| Winter (heating on) | 25% | 8 |
| Summer (windows open) | 60% | 2 |
| After humidifiers installed | 55% | 1 |
(a) Explain the relationship between humidity and the rate of static incidents shown in the data.
(b) The factory manager proposes two safety measures: (1) earth all metal vats and (2) require anti-static overalls. Evaluate which measure is more effective at reducing spark risk and why.
(c) A worker suggests wearing insulating rubber-soled shoes to prevent electric shock. Explain why this actually increases the risk of a static spark.
Answers: (a) Lower humidity means drier air, so charge cannot leak away through moisture in the air — static builds up more easily. The data shows more incidents in dry winter conditions (8/month) and fewer in humid summer (2/month). Installing humidifiers reduced incidents to 1/month, confirming the relationship. (b) Earthing the metal vats is more effective because it directly eliminates charge build-up on the vats (the most likely source of a spark near flammable solvent). Anti-static overalls prevent charge on workers but do not address charge on the vats and pipes where the solvent is located. Both are important, but earthing addresses the greater risk. (c) Rubber-soled shoes insulate the worker from the ground, preventing charge from flowing away through the body. This means static charge builds up on the worker, increasing the potential difference between the worker and any earthed object. When the worker touches something earthed (e.g. a metal vat), a spark can jump — exactly the danger we are trying to avoid. Anti-static shoes that conduct are needed instead.
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