4.2.1 – Electric Charge

Table of Contents

Everything around us is made of tiny particles, and some of these particles carry something called electric charge. Charge is what makes a balloon stick to a wall after you rub it on your hair, and it is the same thing that powers the electricity in our homes. In this topic you will learn that there are two kinds of charge, how charged objects push and pull on each other, and how objects become charged in the first place.

1. Positive and Negative Charges #

There are two types of electric charge:

  • Positive charge (shown with a + sign)
  • Negative charge (shown with a − sign)
Electric Charge Electric charge is a property of matter that can be positive or negative. Charged objects can push or pull on each other without touching.

2. How Charges Push and Pull (Attract and Repel) #

Charged objects can act on each other from a small distance. Whether they push apart or pull together depends on the type of charge each one has.

The golden rule: Like charges repel (push apart). Unlike (opposite) charges attract (pull together).

This gives us three situations:

  • Two positive charges → repel (push apart)
  • Two negative charges → repel (push apart)
  • A positive and a negative charge → attract (pull together)

The bold black arrows below each pair show the direction of the force on each charge.

Two positive: REPEL + + forces point apart
Two negative: REPEL − − forces point apart
Opposite: ATTRACT + − forces point together

3. Making Charge by Friction (Rubbing) #

You can give an object an electric charge by rubbing it with another material. This is called charging by friction. The charge made by rubbing is sometimes called electrostatic charge (static means “not moving”).

Experiment 1 — Producing the charge #

  1. Take a plastic rod (for example a polythene rod) and a dry cloth or duster.
  2. Rub the rod with the cloth several times.
  3. The rubbing gives the rod an electric charge.

Experiment 2 — Detecting the charge #

To show that the rod is now charged, bring it close to small, light objects:

  • Hold the charged rod near tiny pieces of paper — they jump up and stick to the rod.
  • Or hold it near a thin stream of water from a tap — the water bends towards the rod.

If the rod can attract these light objects, it must be charged. This is how we detect electrostatic charge.

4. What Happens When We Rub: Electrons Move #

When two materials are rubbed together, only negative charge moves from one material to the other. This negative charge is carried by tiny particles called electrons.

Very important: Charging by friction is only a transfer of negative charge (electrons). The positive charges do not move — only electrons move from one object to the other.

Because electrons are negative, moving them changes the charge of both objects:

  • The material that gains electrons ends up with extra negative charge → it becomes negatively charged.
  • The material that loses electrons is left with fewer electrons than normal → it becomes positively charged.

Which material gains electrons and which loses them depends on the two materials being rubbed. Here are two common examples, both using a wool cloth:

Polythene rubbed with wool polythene wool − − + + + − electrons more electrons than normal net negative charge (−) fewer electrons than normal net positive charge (+)
The polythene pulls electrons from the wool, so the polythene becomes negative.
Perspex rubbed with wool Perspex wool + + − − − − electrons fewer electrons than normal net positive charge (+) more electrons than normal net negative charge (−)
The wool pulls electrons from the Perspex, so the Perspex becomes positive.
Notice: A rubbed rod does not always become negative. With polythene the rod gains electrons (becomes negative); with Perspex the rod loses electrons (becomes positive). In both cases, only electrons move — the positive charges stay where they are.
Remember: No charge is created or destroyed — the electrons are just moved from one object to the other.

5. Conductors and Insulators #

Some materials let electricity pass through them easily, and some do not.

Electrical Conductor A material that allows electric charge to pass through it easily (for example: copper, iron, and other metals).
Electrical Insulator A material that does not allow electric charge to pass through it (for example: plastic, rubber, glass, wood).

Experiment — Testing if a material is a conductor or an insulator #

You can find out whether a material conducts electricity using a simple test circuit:

  1. Make a circuit with a battery (cell), a lamp, and connecting wires, leaving a gap in the circuit.
  2. Place the material you want to test across the gap.
  3. Watch the lamp:
    • If the lamp lights up, charge can pass through → the material is a conductor.
    • If the lamp stays off, charge cannot pass through → the material is an insulator.
  4. Repeat with different materials and record your results.
Material tested Lamp lights? Conductor or insulator?
Copper wire / iron nail Yes Conductor
Plastic ruler No Insulator
Rubber band No Insulator
Aluminium foil Yes Conductor

6. Explaining Conductors and Insulators (Simple Electron Model) #

We can explain the difference between conductors and insulators by thinking about the electrons inside the material.

  Conductors Insulators
Electrons Have some electrons that are free to move through the material Electrons are not free to move — they are held in place
Can charge flow? Yes — the free electrons carry the charge through No — there are no free electrons to carry charge
Typical examples Copper, iron, aluminium and other metals Plastic, rubber, glass, wood
Conductor − − − − electrons are free to move Insulator − − − − electrons are held in place
In a conductor, some electrons are free to move and can carry charge. In an insulator, the electrons are held in place, so charge cannot flow.

Supplement Topics #

The following sections are for Extended (Supplement) students only.

7. The Unit of Charge Supplement #

Unit of Charge Electric charge is measured in coulombs. The symbol for the coulomb is C.

8. The Electric Field Supplement #

Electric Field An electric field is a region in which an electric charge experiences a force.

This means that if you place a charge anywhere inside an electric field, it will feel a push or a pull.

9. The Direction of an Electric Field Supplement #

Direction of an Electric Field The direction of an electric field at a point is the direction of the force on a positive charge at that point.
How to use this: Imagine placing a small positive charge at a point. The way it would be pushed is the direction of the field. This is why field lines point away from a positive charge and towards a negative charge.

10. Electric Field Patterns Supplement #

We draw electric fields using field lines (also called lines of force). The arrows on the lines show the direction of the field — that is, the direction of the force on a positive charge.

(a) Around a Point Charge #

The field lines are straight and spread out evenly, like the spokes of a wheel. They point away from a positive charge and towards a negative charge.

Positive charge + field points outward (away) Negative charge − field points inward (towards)
Field lines around a point charge: away from positive, towards negative.

(b) Around a Charged Conducting Sphere #

The field around a charged conducting sphere looks just like the field around a point charge: the field lines are straight and spread out evenly, starting from the surface of the sphere. For a positive sphere they point outward.

Positively charged sphere + + + + + + lines start at the surface and point outward
A charged conducting sphere has the same field pattern as a point charge.

(c) Between Two Oppositely Charged Parallel Plates #

Between two flat plates with opposite charges, the field lines are straight, parallel and evenly spaced. They point from the positive plate to the negative plate. This is called a uniform field because it has the same strength and direction everywhere between the plates.

Note: You only need the simple, even pattern of parallel lines between the plates. The behaviour of the field at the edges (end effects) will not be examined.
Two parallel plates (uniform field) + + − − field points from the positive plate to the negative plate
Between parallel plates the field is uniform: straight, parallel, evenly spaced lines from + to −.

Syllabus Reference — 4.2.1 Electric Charge #

Core

  1. State that there are positive and negative charges
  2. State that positive charges repel other positive charges, negative charges repel other negative charges, but positive and negative charges attract each other
  3. Describe simple experiments to show the production of electrostatic charges by friction and to show the detection of electrostatic charges
  4. Explain that charging of solids by friction involves only a transfer of negative charge (electrons)
  5. Describe an experiment to distinguish between electrical conductors and insulators
  6. Recall and use a simple electron model to explain the difference between electrical conductors and insulators and give typical examples

Supplement

  1. State that charge is measured in coulombs
  2. Describe an electric field as a region in which an electric charge experiences a force
  3. State that the direction of an electric field at a point is the direction of the force on a positive charge at that point
  4. Describe simple electric field patterns, including the direction of the field:
    • (a) around a point charge
    • (b) around a charged conducting sphere
    • (c) between two oppositely charged parallel conducting plates (end effects will not be examined)
What are your Feelings

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