Recall and use a simple electron model to explain the difference between electrical conductors and insulators and give typical examples

4.2.1 Electric Charge

Learning Objective

Recall and use a simple electron model to explain the difference between electrical conductors and insulators and give typical examples.

Syllabus points covered (Cambridge IGCSE 0625)

1. Positive and negative charges
2. Attraction and repulsion of charges
3. Simple experiments for charge production by friction
4. Simple experiments for charge detection
5. Friction transfers electrons (negative charge)
6. Charge is measured in coulombs (C)
7. Definition of electric field and its direction (force on a positive test charge)
8. Simple electric‑field patterns (point charge, charged sphere, parallel plates)

Key Concepts

• Types of charge: Positive (+) and negative (–).
• Attraction & repulsion: Like charges repel; unlike charges attract. (See diagram below.)
• Structure of the atom: Nucleus contains protons (+) and neutrons (0); electrons (–) orbit the nucleus. The elementary charge is

  \$e = 1.6 \times 10^{-19}\ \text{C}\$

• Charge unit: The SI unit of charge is the coulomb (C). One coulomb equals the charge of ≈ \$6.25\times10^{18}\$ electrons.

Production of Charge by Friction

When two different materials are rubbed together, electrons move from the material with lower electron affinity to the one with higher affinity.

• The material that gains electrons becomes negatively charged.
• The material that loses electrons becomes positively charged.

Example (syllabus point 5) – Rub a dry rubber rod with wool. The rod gains electrons (negative charge) and the wool loses electrons (positive charge).

Detection of Charge

A simple electroscope or a pith‑ball can demonstrate the presence of charge. Bringing a charged object near the electroscope causes the leaves (or the pith‑ball) to diverge because like charges repel.

Electric Field

• Definition: The electric field E at a point is the force F experienced by a positive test charge q placed at that point, divided by the magnitude of the test charge.

  \$\mathbf{E} = \frac{\mathbf{F}}{q}\qquad\text{(units: N C}^{-1}\text{)}\$

• Direction: By definition, the field direction is the direction of the force on a *positive* test charge. Consequently, field lines point away from positive charges and toward negative charges.
• Simple field patterns (illustrative diagrams):

  • Point charge – radial lines outward (positive) or inward (negative).
  • Charged conducting sphere – field outside behaves like a point charge at the centre; inside a conductor the field is zero.
  • Parallel‑plate capacitor – uniform field between the plates, directed from the positive plate to the negative plate.

Conductors vs. Insulators (simple electron model)

Conductors

• Electrons are loosely bound to atoms and form a “sea of free electrons”.
• When an electric field is applied, the free electrons drift opposite to the field, producing a measurable current.
• Result: low resistivity (≈ 10⁻⁸ – 10⁻⁶ Ω·m), high electron mobility.
• Typical examples:

  • Metals – copper, aluminium, silver, gold, steel
  • Salt‑water (e.g., sea water)
  • Graphite

Insulators

• Electrons are tightly bound to their atoms; they cannot move freely under ordinary electric fields.
• Result: very high resistivity (≈ 10⁸ – 10¹⁴ Ω·m), negligible current.
• Typical examples:

  • Rubber
  • Glass
  • Plastic (e.g., PVC)
  • Dry wood
  • Porcelain

Quick classroom test

Connect a battery and an LED in series with a short piece of metal – the LED lights (conductor). Replace the metal with a piece of rubber – the LED remains off (insulator).

Comparison Table

Explanation Using the Simple Electron Model

1. Apply a potential difference across the material, creating an electric field E.
2. Conductor: The field exerts a force F = eE on each free electron. The electrons acquire an average drift speed v_d, producing a current

   \$I = n\,e\,A\,v_d\$

   where n is the number of free electrons per unit volume and A the cross‑sectional area.

3. Insulator: Electrons are tightly bound; the same field does not free them, so the drift speed is essentially zero and the current is negligible.

Typical Exam Questions (IGCSE 0625)

1. State two examples of good electrical conductors and two examples of good insulators.
2. Explain why metal wires are used to connect components in a circuit but plastic is used for the outer covering of cables.
3. Using the simple electron model, describe what happens to electrons when a battery is connected across a copper wire.
4. Describe a simple experiment that can be used to show that a material is a conductor or an insulator.
5. Define electric field and state the direction of the field produced by a positive point charge.

Summary

Charges exist as positive and negative entities that attract or repel each other. Friction transfers electrons from one material to another, giving the rubbed material a negative charge (syllabus point 5). The amount of charge is measured in coulombs. An electric field is the force per unit positive charge and its direction is defined by the force on a positive test charge. In the simple electron model, electric current is the flow of electrons. Conductors contain many free electrons, giving them low resistivity and allowing current to flow readily. Insulators have electrons tightly bound, resulting in very high resistivity and preventing current flow. Understanding these ideas is essential for selecting appropriate materials in all electrical and electronic circuits.