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4.1 Electrical Charges and Fields

GCSE Physics · AQA · Chapter 4: Electric Circuits

A free GCSE Physics lesson for AQA (8463) from OBA School: charging by friction, forces between charges, electric fields and sparking (physics only). Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Explain how rubbing two insulators together charges them by transferring electrons
  • State that like charges repel and unlike charges attract, and that this is a non-contact force
  • Describe the electric field around a charged object and draw the field pattern for an isolated charged sphere
  • Explain how the strength of the field and force changes with distance
  • Explain sparking in terms of the electric field and electrons

Key words

  • Static charge: Charge that builds up on an insulator and stays in place because it cannot flow away.
  • Electron: A tiny negative particle outside the nucleus. Electrons are the only particles that move when objects are charged by rubbing.
  • Ion: An atom (or molecule) that has gained or lost electrons, so it is charged.
  • Electric field: The region around a charged object where another charged object feels a force.
  • Field line: A line showing the direction of the force on a small positive charge placed in the field.
  • Non-contact force: A force that acts between objects that are not touching, such as the electrostatic force.
  • Sparking: A sudden flow of electrons through the air when the electric field is strong enough to pull electrons out of air molecules.

Charges inside atoms

Every atom has a tiny nucleus of protons and neutrons, with electrons moving around it.

  • proton; Where it is nucleus; Charge positive (+)
  • neutron; Where it is nucleus; Charge zero (uncharged)
  • electron; Where it is around the nucleus; Charge negative (−), equal in size to a proton’s charge

An uncharged atom has equal numbers of protons and electrons, so the charges cancel. Protons are locked in the nucleus, so only electrons can be moved on to or off an atom:

  • gaining electrons → more electrons than protons → negative ion
  • losing electrons → fewer electrons than protons → positive ion

Exam tip: An object never becomes positive by “gaining protons”. It becomes positive by losing electrons.

Charging by friction

When two different insulating materials are rubbed together, friction moves electrons from the surface atoms of one material on to the other. Charge cannot flow through an insulator, so it stays where it is – this is static electricity.

  • A polythene rod rubbed with a dry cloth gains electrons from the cloth: the rod becomes negative and the cloth becomes positive.
  • An acetate (or perspex) rod rubbed with a dry cloth loses electrons to the cloth: the rod becomes positive and the cloth becomes negative.
  • The two charges are equal and opposite, because every electron that leaves one material arrives on the other.

Worked example: a rubbed balloon

A balloon is rubbed on a wool jumper and becomes negatively charged.
The balloon gained electrons from the wool, so the wool lost electrons.

Answer: the wool is left with an equal positive charge

Key fact: Rubbing a metal held in your hand does not leave it charged: metals are conductors, so electrons flow away through you to the ground.

Forces between charged objects

Hang a charged rod from a thread so it can turn freely, then bring a second charged rod near one end.

Like charges repel. Unlike charges attract.

The rods do not touch, so the electrostatic force is a non-contact force. It gets stronger as the charged objects get closer.

Worked example: testing an unknown charge

A rod Z repels a rubbed polythene rod (which we know is negative).
Repulsion only happens between like charges.

Answer: rod Z is negatively charged

Exam tip: Attraction is not proof of charge: a charged rod also attracts small uncharged objects such as bits of paper. Only repulsion proves that both objects are charged with the same sign.

Electric fields

A charged object creates an electric field in the space around it. A second charged object placed in the field feels a force. This is how the non-contact force acts.

We draw field lines to show the field. The arrow on each line shows the direction of the force on a small positive charge:

  • Lines start at right angles to the surface of the sphere.
  • The lines are closest together near the sphere, where the field is strongest.
  • Further away the lines spread out: the field, and so the force on another charge, gets weaker as distance increases.

Sparking

If a very large charge builds up, the field near the object becomes very strong. The field can then pull electrons out of air molecules (ionising them). These electrons speed up, hit other air molecules and knock out more electrons. Suddenly a large number of electrons flows through the air between the charged object and a nearby conductor – a spark.

Worked example: a shock from a door handle

Walking on a nylon carpet in rubber-soled shoes rubs electrons on to you (or off you), and the insulating soles stop the charge leaking away.
As your finger approaches a metal door handle, the field in the small gap becomes very strong.

Answer: electrons flow suddenly between you and the handle, which you feel as a small shock (and may see as a spark)

Key fact: Sparks can ignite fuel vapour, so fuel tankers are connected to earth by a conducting wire before fuel is pumped. The wire lets charge flow away safely.

Common mistakes

  • Saying protons (or “positive charges”) move when objects are rubbed. Only electrons are transferred.
  • Forgetting that both materials become charged. The cloth gets an equal and opposite charge to the rod.
  • Drawing field lines with arrows pointing towards a positive sphere. Lines always point away from + and towards −.
  • Saying the force between charges needs contact. It is a non-contact force that acts through the electric field.
  • Using attraction to prove two objects are both charged – only repulsion proves this.

Revision questions

Try these, then check your answers below each one.

Question 1. A polythene rod becomes negatively charged when it is rubbed with a dry cloth. What has happened? (Choose from: Electrons moved from the cloth to the rod, Protons moved from the rod to the cloth, Electrons moved from the rod to the cloth, Positive charge was created on the cloth.)

Answer: Electrons moved from the cloth to the rod

Working: Rubbing transfers electrons. The rod gains electrons from the cloth, so it has more electrons than protons and is negative.

Question 2. True or false? When an acetate rod is rubbed and becomes positive, the cloth is left with an equal negative charge.

Answer: True

Working: True. The electrons lost by the rod are gained by the cloth, so the cloth’s negative charge equals the rod’s positive charge.

Question 3. Which describes the electric field lines around an isolated negatively charged sphere? (Choose from: Straight lines pointing in towards the sphere, Straight lines pointing out away from the sphere, Circles around the sphere, There are no field lines around a negative sphere.)

Answer: Straight lines pointing in towards the sphere

Working: A small positive charge is attracted to the negative sphere, so the radial field lines point inwards.

Related lessons

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