What you'll learn
Static electricity covers the behaviour of stationary electric charges on insulating materials. CIE IGCSE Physics examinations test your understanding of how objects become charged, the interactions between charged materials, and practical applications including hazards and safety measures. Questions typically involve explaining observations, predicting outcomes of charging experiments, and applying knowledge to real-world contexts.
Key terms and definitions
Static electricity — the study of stationary electric charges, typically on insulating materials that cannot conduct electricity easily.
Charging by friction — the process where electrons are transferred from one material to another when two insulating materials are rubbed together, leaving one positively charged and the other negatively charged.
Conductor — a material that allows electric charge to flow freely through it because it contains mobile charged particles (typically electrons in metals).
Insulator — a material that does not allow electric charge to flow easily because the electrons are held tightly to atoms and cannot move freely.
Electric field — the region around a charged object where another charged object experiences a force.
Earthing — the process of connecting a charged conductor to the ground with a conducting path, allowing excess charge to flow away safely.
Electrostatic induction — the redistribution of charges within an object caused by the presence of a nearby charged object, without direct contact.
Coulomb's law principle — like charges repel each other, unlike charges attract each other; the fundamental rule governing electrostatic forces.
Core concepts
The nature of electric charge
All matter contains charged particles. Atoms consist of a positively charged nucleus surrounded by negatively charged electrons. Normally, atoms are electrically neutral because they contain equal numbers of protons (positive) and electrons (negative).
Objects become charged when they gain or lose electrons:
- Negative charge: object has gained extra electrons
- Positive charge: object has lost electrons
- Neutral: object has equal numbers of protons and electrons
Protons remain fixed in the nucleus and cannot be transferred during normal charging processes. Only electrons can move between materials.
Charging by friction
When two insulating materials are rubbed together, electrons transfer from one material to the other. This occurs because different materials hold electrons with different strengths.
The process follows these steps:
- Two uncharged insulators are brought into contact and rubbed together
- Friction between the surfaces causes electrons to transfer from one material to the other
- One material loses electrons and becomes positively charged
- The other material gains electrons and becomes negatively charged
- Both objects carry equal but opposite charges
Common examples tested in CIE IGCSE examinations:
- Polythene rod rubbed with cloth: polythene becomes negative (gains electrons), cloth becomes positive (loses electrons)
- Perspex (acetate) rod rubbed with cloth: perspex becomes positive (loses electrons), cloth becomes negative (gains electrons)
- Balloon rubbed on hair: balloon becomes negative, hair becomes positive; the balloon then attracts small pieces of paper or can stick to walls
The charge remains on insulators because electrons cannot flow freely through the material. On conductors, any excess charge spreads rapidly across the surface and can leak away to earth.
Forces between charged objects
Charged objects exert forces on each other without touching. The direction of the force depends on the types of charge:
- Like charges repel: two positive charges or two negative charges push apart
- Unlike charges attract: a positive and negative charge pull together
- Uncharged objects and charged objects: uncharged objects are attracted to charged objects through induction
The size of the electrostatic force depends on:
- The magnitude of the charges (larger charges produce stronger forces)
- The distance between the charges (closer charges experience stronger forces)
These forces become weaker as the distance increases, following an inverse square relationship at A-level, though the specific mathematical relationship is not required for CIE IGCSE.
Demonstrating charge with a gold-leaf electroscope
The gold-leaf electroscope detects the presence and type of electric charge. It consists of a metal cap connected to a metal stem, with a thin gold leaf attached to the stem, all housed in an earthed metal case with glass windows.
Detecting charge:
- When a charged rod touches the metal cap, charge spreads through the metal stem and onto the gold leaf
- The stem and leaf now carry the same type of charge and repel each other
- The gold leaf rises away from the stem
- The greater the charge, the larger the deflection of the leaf
Testing the sign of charge using induction:
- First, charge the electroscope with a known charge (e.g., negative) by touching with a charged rod
- Bring an unknown charged object near (but not touching) the metal cap
- If the leaf rises further, the unknown object has the same charge as the electroscope
- If the leaf falls, the unknown object has opposite charge to the electroscope
This occurs because the nearby charged object induces a redistribution of charge in the metal components through electrostatic induction.
Electric fields
An electric field exists in the region around any charged object. Another charged object placed in this field experiences a force.
Electric field lines represent the field visually:
- Field lines point from positive charges toward negative charges
- The direction of the field line shows the direction of force on a positive charge
- Closer field lines indicate a stronger field
- Field lines never cross each other
Common field patterns:
- Single positive charge: field lines radiate outward in all directions
- Single negative charge: field lines point inward from all directions
- Two opposite charges: field lines run from positive to negative, curving through space
- Two like charges: field lines repel away from the region between the charges
- Parallel plates: uniform field with parallel, evenly-spaced field lines perpendicular to the plates
Hazards and applications of static electricity
Hazards requiring prevention:
Lightning: clouds become charged through friction between ice particles and water droplets in turbulent air. When the charge becomes large enough, a massive discharge occurs between cloud and ground or between clouds. Lightning conductors protect buildings by providing a safe conducting path to earth, with a thick copper conductor running from a metal spike above the building down to a metal plate buried underground.
Refuelling aircraft and vehicles: fuel flowing through pipes causes friction, generating static charge. If the charge builds up and sparks, it can ignite fuel vapour, causing an explosion. Prevention: connect the fuel tanker and aircraft/vehicle to earth with a conducting cable before refuelling begins. The charge flows harmlessly to the ground.
Working with flammable gases or liquids: similar risk of sparks igniting vapours. Workers may wear conducting shoes and work on conducting floors connected to earth, allowing any charge to leak away continuously.
Sensitive electronic components: static discharge can damage delicate circuits. Workers wear earthing straps (anti-static wrist straps) connected to earth, preventing charge build-up.
Useful applications:
Electrostatic paint spraying: paint droplets are all given the same charge as they leave the spray nozzle. The object being painted is given the opposite charge. Droplets repel each other (producing a fine spray) and are attracted to the object, giving even coverage with less waste. Paint reaches shadowed areas and the reverse side of objects.
Photocopiers and laser printers: a drum is given a pattern of charge matching the image to be copied. Charged toner particles stick to the charged areas of the drum. Paper is pressed against the drum, transferring the toner pattern. Heat then fuses the toner permanently to the paper.
Electrostatic precipitators: remove smoke particles from industrial chimneys. Smoke passes through a charged metal grid, and particles become charged. These charged particles are attracted to oppositely charged metal plates and stick to them. The plates are periodically cleaned, removing the pollution before it enters the atmosphere.
Defibrillators: deliver a controlled electric shock through the chest to restart a stopped heart or correct an abnormal heart rhythm, though this is more about discharge than static phenomena specifically.
Conductors, insulators and earthing
The behaviour of charge depends critically on whether materials are conductors or insulators.
Conductors (metals, graphite, human body):
- Contain mobile electrons that can move freely
- Charge spreads rapidly across the entire surface
- Charge can flow to earth if a conducting path exists
- Cannot be charged by friction (charge immediately leaks away)
Insulators (plastics, rubber, glass, dry air):
- Electrons are held tightly to atoms
- Charge remains where it is placed
- Can be charged by friction
- Charge persists for extended periods
Earthing removes charge from conductors:
- A conducting wire connects the charged object to the ground
- Excess electrons flow to earth (if object was negatively charged)
- Electrons flow from earth to the object (if object was positively charged)
- The object becomes electrically neutral
Earthing only works effectively on conductors. Insulators remain charged because electrons cannot flow through them to reach the earth connection.
Worked examples
Example 1: Explaining charging by friction
A polythene rod is rubbed with a woollen cloth. The polythene rod becomes negatively charged.
(a) Explain how the polythene rod becomes negatively charged. [3 marks]
Answer:
- Electrons are transferred from the cloth to the polythene rod [1]
- The polythene rod gains electrons [1]
- An object with excess electrons is negatively charged [1]
(b) State the charge on the cloth after rubbing. [1 mark]
Answer:
- Positive [1]
(c) A second, uncharged polythene rod is brought close to the charged rod. Predict what happens and explain your answer. [2 marks]
Answer:
- The uncharged rod is attracted to the charged rod [1]
- Because the charged rod causes a redistribution of charge in the nearby rod / electrostatic induction occurs [1]
Example 2: Applications and hazards
An aircraft is being refuelled from a tanker.
(a) Explain how dangerous static charge can build up during refuelling. [2 marks]
Answer:
- Friction between the fuel and the pipe [1]
- Causes electrons to transfer, leaving the fuel/aircraft charged [1]
(b) Describe how the danger is reduced. [2 marks]
Answer:
- The aircraft and tanker are connected together by a conducting cable/earthed [1]
- Charge flows away to earth / charge equalises between aircraft and tanker [1]
Example 3: Electric fields
Two metal plates are connected to a high voltage supply. One plate is positive and the other is negative.
(a) Draw at least four electric field lines between the plates. [2 marks]
Answer:
- Four or more lines from positive to negative plate [1]
- Lines approximately parallel and evenly spaced [1]
(b) A small positively charged particle is placed between the plates. State the direction it will move. [1 mark]
Answer:
- Toward the negative plate [1]
Common mistakes and how to avoid them
Mistake: Stating that protons move during charging by friction. Correction: Only electrons move between materials. Protons are fixed in the nucleus and cannot be transferred during normal charging processes.
Mistake: Confusing which material becomes positive and which becomes negative when two specific materials are rubbed together. Correction: Learn the common examples (polythene becomes negative when rubbed with cloth; perspex becomes positive when rubbed with cloth). The material that holds electrons more weakly loses them and becomes positive.
Mistake: Claiming that conductors can be charged by friction. Correction: Conductors cannot hold static charge because excess charge immediately spreads across the surface and leaks away, often to earth through the person holding the conductor. Only insulators retain charge when rubbed.
Mistake: Stating that opposite charges repel or like charges attract. Correction: Like charges (both positive or both negative) repel; unlike charges (one positive and one negative) attract. This is the fundamental rule of electrostatic force.
Mistake: Explaining that earthing works by "removing charge" without mentioning electron flow. Correction: Be specific that earthing provides a conducting path for electrons to flow either to earth (if object is negative) or from earth (if object is positive), neutralising the object.
Mistake: Drawing electric field lines that cross each other or point in inconsistent directions. Correction: Field lines never cross. They always point from positive toward negative charges. For a uniform field between parallel plates, draw parallel, evenly-spaced lines perpendicular to the plates.
Exam technique for Static Electricity
"Explain" questions: Examiners expect clear cause-and-effect reasoning. For charging by friction, state what particles move (electrons), in which direction, and why this creates the observed charge. Answers like "it becomes charged by rubbing" score zero marks without mentioning electron transfer.
Hazard questions: CIE examiners want both the mechanism (how charge builds up through friction) and the danger (sparks igniting flammable vapours). For prevention methods, specify earthing or bonding and explain that charge flows away. "Removing static" is too vague for full marks.
Field line diagrams: Use a ruler where appropriate (for parallel plate fields). Show direction with arrows. Common allocation is 1 mark for correct direction, 1 mark for correct pattern (parallel/radial/curved as appropriate).
Application questions: When asked about photocopiers, paint sprayers, or precipitators, structure answers as: (1) what is charged and how, (2) what force acts (attraction or repulsion), (3) the practical outcome. Each step typically earns one mark.
Quick revision summary
Static electricity involves stationary charges on insulators. Charging by friction transfers electrons between materials, making one positive and one negative. Like charges repel; unlike charges attract. Electric fields show force direction on positive charges. Conductors allow charge to spread and flow to earth; insulators retain charge. Hazards include lightning and fuel ignition, prevented by earthing. Applications use controlled charging and electrostatic forces for paint spraying, photocopying, and pollution control. Only electrons move during normal charging; protons remain in nuclei.