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Physics · Electricity

Static charge

Explain how insulating materials become charged when rubbed, and describe attraction, repulsion and sparking in terms of the transfer of electrons.

  • 6 key terms
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Warm-up

Answer each one, then check.

  1. 1

    What charge do electrons have?

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    Negative

  2. 2

    What is an insulator?

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    A material that does not conduct electricity

  3. 3

    What is a non-contact force?

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    A force between objects that are not touching

  4. 4

    What charge does a proton have?

    Show answerHide answer

    Positive

  5. 5

    Like charges...

    Show answerHide answer

    Repel

Learning Objectives

  1. 1Describe the production of static electricity by rubbing.
  2. 2Explain charging in terms of the transfer of electrons.
  3. 3Describe evidence that charged objects exert non-contact forces.
  4. 4Explain sparking.

STATIC ELECTRICITY

When insulating materials are rubbed together, electrons are transferred: the material that gains electrons becomes negatively charged and the one that loses them is left with an equal positive charge.

Objects with the same type of charge repel; objects with different types of charge attract.

Explaining Charging

Explain why a polythene rod becomes negatively charged when rubbed with a cloth.

Show the solutionHide the solution
  1. 1 Friction Rubbing transfers energy so electrons can move
  2. 2 Electrons move Negatively charged electrons are rubbed off the cloth and on to the rod
  3. 3 The rod Gains electrons, so it becomes negatively charged
  4. 4 The cloth Loses electrons and is left with an equal positive charge

AnswerElectrons transfer from the cloth to the rod; the rod becomes negative and the cloth positive.

Sparking

Explain how a spark can jump from a charged object to an earthed conductor.

Show the solutionHide the solution
  1. 1 Charge builds A large amount of charge collects on the object
  2. 2 Strong field The electric field between the object and the conductor becomes very strong
  3. 3 Air ionises The air becomes ionised and conducts, so charge flows as a spark

AnswerCharge builds up until the field is strong enough to make air conduct, and the charge flows as a spark.

Evidence of Non-Contact Forces

  • Repulsion

    Two charged rods of the same charge push each other apart.

  • Attraction

    A charged rod attracts small pieces of paper or a stream of water.

  • Hanging balls

    Suspended charged balls move apart or together without touching.

  • Balloon on a wall

    A rubbed balloon can stick to a wall.

Common Mistakes

Be precise.

  • Only electrons move

    Positive charges (protons) are fixed in the nucleus and do not move when rubbed.

  • Insulators

    Only insulators keep the charge; a metal rod held in the hand loses its charge.

  • Say gained or lost

    Say 'gained electrons', not 'gained negative charge'.

Rub and Predict

A student rubs a cloth on a balloon. The balloon is then attracted to the wall. Explain what has happened to the electrons and what type of charge the balloon has.

1. Say which way electrons moved.

2. State the charge on each object.

A good answer shows: Electrons are transferred from the cloth to the balloon. The balloon gains electrons and becomes negatively charged; it attracts the wall.

Can I...?

  1. 1Explain how rubbing makes charge.
  2. 2Say which particles move.
  3. 3State the charge on each object.
  4. 4Describe attraction and repulsion.
  5. 5Describe evidence for non-contact forces.
  6. 6Explain sparking.
  7. 7Explain why insulators keep charge.
  8. 8Use the terms gained and lost electrons.

Summary & Exam Focus

  • Rubbing insulators transfers electrons.
  • The material that gains electrons is negative; the one that loses electrons is positive.
  • Like charges repel, unlike attract.
  • Sparks occur when the field is strong enough to ionise air.

Exam focus

Explain how a polythene rod becomes negatively charged when rubbed with a cloth. (3 marks) (3 marks)

Say which way the electrons move.

Key terms

The vocabulary this lesson expects you to use. Each one is linked from the first place it appears above.

Static electricity
Charge that builds up on an insulator.
Electron
A negatively charged particle.
Non-contact force
A force between objects that are not touching.
Earthing
Connecting to the ground so charge can flow away.
Repel
Push apart.
Attract
Pull together.

Practice questions

Have a go at each one before you open its answer.

  1. Question 1 Explain 3 marks

    A polythene rod is rubbed with a cloth and becomes negatively charged. Explain how the rod becomes negatively charged.

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    Model answer

    Electrons are transferred from the cloth to the rod when they are rubbed together. The rod gains electrons so it becomes negative.

    Mark scheme

    • Electrons transferred — 1 mark
    • From the cloth to the rod — 1 mark
    • Rod gains electrons so it is negative — 1 mark
  2. Question 2 Use the diagram 3 marks

    The diagram shows two charged balls hanging from threads. Ball A is negatively charged. (a) State the type of charge on ball B. (b) Name the type of force acting between the balls.

    Two hanging balls, A negative and B unknown, pushed apart.
    Show answerHide answer

    Model answer

    (a) Negative (they repel). (b) A non-contact force (electrostatic repulsion).

    Mark scheme

    • Negative — 1 mark
    • Because they repel (like charges) — 1 mark
    • Non-contact force — 1 mark
  3. Question 3 Explain 3 marks

    A student walks across a nylon carpet and then gets a small electric shock when touching a metal door handle. Explain why.

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    Model answer

    Electrons are rubbed off the carpet onto the student (or the other way round), so the student becomes charged. When the student touches the metal handle, charge flows through the metal to earth, causing a spark or shock.

    Mark scheme

    • Rubbing transfers electrons — 1 mark
    • The student becomes charged — 1 mark
    • Charge flows to earth through the handle — 1 mark
  4. Question 4 Explain 2 marks

    Explain why a charged balloon can stick to a wall even though the balloon and wall do not touch.

    Show answerHide answer

    Model answer

    The charged balloon exerts a non-contact force (electrostatic force of attraction) on the wall.

    Mark scheme

    • Non-contact force — 1 mark
    • Attraction between charged balloon and wall — 1 mark
  5. Question 5 Explain 4 marks

    Lightning is a very large spark. Explain how charge builds up in a cloud and what causes the lightning.

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    Model answer

    Collisions between ice particles in the cloud transfer electrons, so parts of the cloud become charged. A large potential difference forms between the cloud and the ground. The electric field becomes strong enough to ionise the air, which conducts, and charge flows as lightning.

    Mark scheme

    • Electrons transferred by collisions or friction — 1 mark
    • Cloud charged, large pd between cloud and ground — 1 mark
    • Strong electric field ionises the air — 1 mark
    • Charge flows as a spark — 1 mark

Quick check

  1. Which particles move when an insulator is rubbed?

    1. AProtons
    2. BElectrons
    3. CNeutrons
    4. DAtoms
    Show answerHide answer

    B: Electrons

    Only electrons are free to move.

  2. A material that gains electrons becomes...

    1. Apositively charged
    2. Bneutral
    3. Cnegatively charged
    4. Dmagnetic
    Show answerHide answer

    C: negatively charged

    It has more electrons than protons.

  3. Two negatively charged objects...

    1. Aattract
    2. Bhave no force
    3. Cbecome positive
    4. Drepel
    Show answerHide answer

    D: repel

    Like charges repel.

  4. Positive and negative objects...

    1. Aattract
    2. Brepel
    3. Chave no force
    4. Dlose charge
    Show answerHide answer

    A: attract

    Unlike charges attract.

  5. A charged object attracting paper is an example of...

    1. Aa contact force
    2. Ba non-contact force
    3. Cfriction
    4. Dtension
    Show answerHide answer

    B: a non-contact force

    The objects are not touching.

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