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Electrical Insulators & Conductors

Key idea: O Level static electricity: how conductors and insulators differ, and why earthing works for conductors.

  • SEC G3 Physics 2027
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Learning objectives

  • state that there are positive and negative charges and that charge is measured in coulombs
  • state that unlike charges attract and like charges repel
  • Describe an electric field as a region where charge experiences force
  • Recall that the field lines of an isolated point charge give the direction of the force on a positive test charge
  • Draw the directed electric field of an isolated point charge
  • Interpret the electric-field pattern between two isolated point charges
  • Draw the electric-field pattern between two isolated point charges
  • Explain charging by rubbing as electron transfer
  • Describe experiments that charge a conductor by induction
  • Describe potential electrostatic-charging hazards
  • Describe an electrostatic precipitator and transfer the principle

1. Definition

A. Conductor

A conductor is a material in which electrons can move freely through the material (e.g. metals).

B. Insulator

An insulator is a material in which electrons are tightly bound and cannot move freely (e.g. plastic, rubber).

2. Key Ideas

  • In a conductor, charge can move and spread out easily (mainly on the surface).
  • In an insulator, charge is usually stuck where it was created (e.g. where you rubbed it).
  • Earthing works well for conductors because electrons can flow to/from Earth.
FeatureConductorInsulator
Electron mobilityHighLow
Charge after rubbingSpreads out quicklyLocalised (“stuck”)
Earthing works?Yes (charge flows away)Usually not effective
ExamplesCopper, aluminium, graphitePlastic, rubber, glass, dry air

3. Detailed Explanations

A. Why conductors conduct

In conductors (especially metals), some electrons are not strongly attached to any one atom. These electrons can move through the material, so charge can flow and redistribute.

B. Why insulators insulate

In insulators, electrons are held tightly in atoms/molecules. Since they cannot move freely, charge does not flow easily through the material.

C. Why does excess charge sit on the surface of a conductor?

Excess charge spreads out as far as possible because like charges repel. In electrostatic equilibrium, it ends up mainly on the outer surface of the conductor.

D. Why humidity reduces static effects (everyday)

Moist air provides a slightly conducting path so charge can leak away more easily. In dry air, charge stays on insulating surfaces longer, so static effects are stronger.

Link

Earthing and charging by induction rely on conductors: Ways to Charge Conductors.

4. Common Mistakes

  • Thinking earthing works the same way for insulators (charge cannot flow away easily).
  • Thinking conductors “create charge” (they mainly allow charge to move).
  • Mixing up “insulator” with “uncharged” (an insulator can be charged; it just holds charge in place).

5. Exam Tips

  • Use the words “electrons move freely” (conductor) and “electrons tightly bound” (insulator).
  • For “why does earthing work?”, write: “electrons can flow to/from Earth through a conductor”.
  • For “where is the charge?”, write: “mainly on the outer surface (conductor)”.

6. Worked Examples

Modelled example 1

Classify materials

Core

Problem

Classify copper, rubber, glass and aluminium as conductors or insulators, then state the particle-level criterion.
Study the worked solution

  1. Apply the material model

    Method

    Place copper and aluminium in the conductor group.

    Reason

    Metals contain mobile electrons that can redistribute through the material.

    Working

    Conductors: copper, aluminium.

  2. Complete the classification

    Method

    Place rubber and glass in the insulator group.

    Reason

    Their electrons are tightly bound and do not move freely through the material.

    Working

    Insulators: rubber, glass.

Guided practice 2

Earthing

About 4 min

Problem

Explain why an earth connection neutralises a charged metal rod effectively but not a charged plastic rod.

Try this before viewing the solution

Hints

Hint 1: trace electrons, not positive charge
Ask whether electrons can move through each rod to or from Earth.
View solution step by step

  1. Trace the metal path

    Method

    Electrons can flow through the metal between the rod and Earth.

    Reason

    Metal is a conductor with mobile electrons.

    Working

    The flow continues until the rod is neutral.

  2. Contrast the plastic rod

    Method

    Charge remains localised on the plastic.

    Reason

    Plastic is an insulator, so electrons cannot move freely along it to the earth connection.

    Working

    Earthing is ineffective for the charged region.

Common misconception 3

Where is the charge?

Find and correct the mistake

Learner claim

A charged metal sphere is left in electrostatic equilibrium. A learner says excess charge remains evenly throughout its volume. Locate and correct the error.

Try this before viewing the solution

Main charge location

View solution step by step

  1. Use mobility and repulsion

    Method

    Allow excess electrons or electron deficits to redistribute through the conductor.

    Reason

    Like charges repel and mobile charge moves until electrostatic equilibrium.

    Working

    Charge spreads as far apart as possible.

  2. State the equilibrium location

    Method

    Place most excess charge on the outer surface.

    Reason

    This distribution maximises separation for the isolated sphere.

    Working

    Excess charge: mainly outer surface.

Examiner practice 4

Why wires are covered

3 marks

Examination question

Explain why an electrical wire uses a metal core covered by plastic. [3 marks]

Try this before viewing the solution

View solution step by step

  1. Core function

    1 mark

    Method

    Use metal because charge can flow through it.

    Reason

    Its mobile electrons make it a conductor.

    Working

    Metal carries the current.

  2. Covering property

    1 mark

    Method

    Use plastic because its electrons are not free to move.

    Reason

    Plastic is an electrical insulator.

    Working

    It prevents easy charge flow through the covering.

  3. Safety consequence

    1 mark

    Method

    Link the insulation to reduced current through a person touching the intact wire.

    Reason

    The conducting core is kept physically isolated.

    Working

    Reduced electric-shock risk.

Challenge 5

Charge “leaking away”

Minimal support

Contact-discharge transfer

A charged metal object loses charge quickly when held by a grounded person, while a similarly charged plastic object often retains it. Explain the complete contrast.

Try this before viewing the solution

Hints

Hint 1: connect material to the Earth path
The hand provides a conducting route only if charge can first move through the object to the contact point.
View solution step by step

  1. Trace the metal discharge

    Method

    Electrons move through the metal and the person to or from Earth.

    Reason

    Both provide a conducting path, so the metal neutralises.

    Working

    Mobile charge reaches the earth connection.

  2. Contrast the plastic

    Method

    Charge remains localised on the plastic away from the contact path.

    Reason

    Its electrons cannot move freely through the object.

    Working

    The plastic retains charge much longer.

7. Mind Stretchers

Mind stretcher 1: Why a charged metal doesn’t stay charged when heldExtension

If you hold a charged metal rod in your hand, it often becomes neutral quickly. Explain.

Show Answer

Your body and the ground act as an earth connection. Since the rod is a conductor, electrons can flow through you to Earth (or from Earth) until the rod is neutral.

Mind stretcher 2: Same charge, different behaviourExtension

Both a plastic rod and a metal rod are charged. Why does the plastic rod often keep its charge longer?

Show Answer

Plastic is an insulator, so charge cannot move away easily and does not leak to Earth quickly. Metal is a conductor, so charge can flow and is easily discharged through contact/earthing.

8. Practice and next step

Use Static Electricity structured practice to explain electron mobility rather than merely naming a material. Then continue to Charging by Induction, where conductor mobility and earthing become essential.

Continue with the next resource in this course.

Course and syllabus information
Course
SEC G3 Physics
Edition
SEC G3 Physics 2027