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Ways To Charge Conductors

Key idea: O Level static electricity: how to charge conductors by conduction and induction (with earthing).

  • 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. Charging by conduction (contact)

Charging by conduction happens when a charged object touches a conductor and electrons transfer.

B. Charging by induction

Charging by induction charges a conductor without contact. A charged object is brought near, and with earthing, electrons enter/leave the conductor so it gains a net charge.

What you need for this course

You should be able to describe an experiment that charges a conductor by induction.

2. Key Ideas

  • In a conductor, electrons can move freely, so charge spreads out (mainly on the surface).
  • Earthing (grounding) provides a path for electrons to flow to/from the Earth.
  • In induction, the order matters:
    • remove the earth connection first
    • then remove the charged rod
  • After induction, the conductor ends with a net charge opposite to the charged rod.

3. Detailed Explanations

A. Charging by conduction (contact)

When a charged object touches a neutral conductor, electrons move until charge redistributes.

Result: the conductor ends up with the same sign as the object that touched it.

Core versus supporting detail

The current syllabus outcome specifically assesses an induction experiment. Contact charging is useful supporting comparison, but induction is the core method to describe precisely.

B. Charging by induction (with earthing)

Example: negatively charged rod

  1. Bring a negatively charged rod near (not touching) a neutral metal sphere.
  2. Electrons in the sphere move away from the rod (charge separation).
  3. While the rod is still near, connect the sphere to earth:
    • electrons flow from the sphere into Earth
  4. Disconnect the earth first (rod still near).
  5. Remove the rod. The sphere is left positively charged.
Charging a conductor positively by inductionFour numbered panels show a negative rod separating charge in a metal sphere, earthing allowing electrons to leave, Earth being disconnected while the rod remains, and the rod being removed to leave a positive sphere.1 Rod near2 Earth3 Earth off4 Rod away−−+ +− −Electrons moveaway from rod−−+ +− −Electrons to Earth−−+ ++Disconnect Earthwhile rod is near+ ++ +Positive chargespreads evenlyover sphere
Scroll diagram horizontally to read all labels.
A negative rod repels electrons to Earth. Disconnect Earth before removing the rod so the sphere remains positively charged.

If the rod is positively charged

The steps are the same, but the electron flow reverses:

  • electrons flow from Earth into the sphere
  • the sphere is left negatively charged
Safety

Static electricity can cause sparks. For real-world hazards and prevention, see Electrostatic Hazards.

4. Common Mistakes

  • Saying protons move (in solids, only electrons move).
  • Charging by induction but removing the rod before disconnecting earth (the charge will leak away).
  • Saying induced charge is the same sign as the rod (it is the opposite sign after earthing).

5. Exam Tips

  • Write the induction sequence clearly:
    1. rod near → 2) earth on → 3) earth off → 4) rod away
  • Use the “electron story”:
    • positive rod attracts electrons toward it
    • earthing supplies electrons
    • remove earth first so electrons can’t return
  • If asked “why no contact?”, write: “induction charges without contact”.

6. Worked Examples

Modelled example 1

Contact vs induction

Core

Problem

Contrast charging a neutral conductor by conduction with charging it by induction, including contact and final charge sign.
Study the worked solution

  1. Describe conduction

    Method

    Touch the charged object to the conductor so electrons transfer directly.

    Reason

    Contact provides a path for charge redistribution.

    Working

    Conduction: contact; conductor usually ends with the same sign as the charging object.

  2. Describe induction

    Method

    Bring the charged object near without touching, earth the conductor and remove earth first.

    Reason

    The nearby charge redistributes electrons and Earth supplies or removes electrons.

    Working

    Induction: no contact; conductor ends with the opposite sign after correct earthing.

Guided practice 2

Induction sign

About 5 min

Problem

A positively charged rod is used to charge a metal sphere by induction with earthing. What is the sphere’s final charge, and which way do electrons flow while it is earthed?

Follow attraction of mobile electrons

Final sphere charge

Hints

Hint 1: use electron attraction
The positive rod attracts mobile electrons toward the near side.
Hint 2: include Earth
While connected, additional electrons can flow from Earth into the sphere.
View solution step by step

  1. Track electron flow

    Method

    Move electrons from Earth into the sphere while the positive rod remains nearby.

    Reason

    The rod’s positive charge attracts electrons.

    Working

    Electrons: Earth → sphere.

  2. Preserve the net charge

    Method

    Disconnect Earth first, then remove the rod, leaving the sphere negative.

    Reason

    The extra electrons cannot flow back after the earth path is broken.

    Working

    Excess electrons → negative sphere.

Common misconception 3

Why remove earth first?

Find and correct the mistake

Learner response

A student completes induction by removing the charged rod while the conductor is still earthed, then disconnecting Earth. Diagnose why this fails.

Choose the step that must come first

First removal

View solution step by step

  1. Explain the failed order

    Method

    State that removing the rod first removes the field causing charge separation.

    Reason

    With Earth still connected, electrons can then flow until the conductor becomes neutral.

    Working

    Rod away while earthed → charge leaks through Earth.

  2. Give the correct order

    Method

    Disconnect Earth first while the rod is near, then remove the rod.

    Reason

    Breaking the electron path traps the acquired net charge.

    Working

    Rod near → Earth off → rod away.

Examiner practice 4

Charging by contact (sign)

3 marks

Examination question

A negatively charged rod touches a neutral metal sphere and is removed. State the sphere’s final sign, explain the electron transfer and contrast the sign expected from charging by induction with the same rod. [3 marks]

Track contact transfer, then contrast the no-contact method

View solution step by step

  1. Explain conduction

    2 marks

    Method

    State that electrons transfer from the negative rod onto the sphere, leaving it negative.

    Reason

    Direct contact lets excess electrons redistribute onto the conductor.

    Working

    Contact with negative rod → negative sphere.

  2. Contrast induction

    1 mark

    Method

    State that correct earthed induction with the same negative rod would leave the sphere positive.

    Reason

    Induction drives electrons from the sphere to Earth without contact.

    Working

    Induction → opposite sign.

Challenge 5

Induction with a negative rod (electron story)

Minimal support

Opposite-rod transfer

A negatively charged rod is brought near a neutral metal sphere. The sphere is earthed, Earth is removed first, and then the rod is removed. Determine electron flow and final sphere charge.

Reverse the electron story while preserving the sequence

Hints

Hint 1: use electron repulsion
A negative rod pushes mobile electrons away from it.
Hint 2: follow the Earth path
While earthed, those electrons can leave the sphere for Earth.
View solution step by step

  1. Track earthing

    Method

    Move electrons from the sphere into Earth.

    Reason

    The nearby negative rod repels the sphere’s mobile electrons.

    Working

    Electrons: sphere → Earth.

  2. Preserve and identify the charge

    Method

    Remove Earth first, then the rod, leaving the sphere positive.

    Reason

    The sphere retains an electron deficit after the path to Earth is broken.

    Working

    Electron deficit → positive sphere.

7. Mind Stretchers

Mind stretcher 1: Induction without earthingExtension

A charged rod is brought near a neutral isolated metal sphere, but there is no earthing and no contact. After the rod is removed, is the sphere charged? Explain.

Show Answer

No. The charges separate while the rod is nearby, but without earthing there is no net gain/loss of electrons. When the rod is removed, charges redistribute and the sphere becomes neutral again.

Mind stretcher 2: Two different rodsExtension

Using induction, which rod would leave the sphere positive: a positive rod or a negative rod?

Show Answer

A negative rod. Electrons are repelled from the sphere to Earth, so the sphere is left with a deficit of electrons (positive).

8. Practice and next step

Practise writing the four-stage sequence—rod near, Earth on, Earth off, rod away—for both rod signs in Static Electricity structured practice. Then continue to Electric Fields.

Continue with the next resource in this course.

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