State that there are positive and negative charges

4.2.1 Electric Charge

Learning Objective

State that electric charge exists in two distinct types: positive and negative.

Key Concepts

  • Electric charge is a fundamental property of matter.

  • Only two kinds of charge occur: positive (+) and negative (–).

  • Like charges repel; opposite charges attract.

  • The SI unit of charge is the coulomb (C).  1 C = 6.24 × 1018 elementary charges (e).

  • Charge is conserved – the total charge of an isolated system never changes.

Nature of Positive and Negative Charges

AspectPositive Charge (+)Negative Charge (–)
Typical carrierProtonElectron
Elementary charge+ e = +1.602 × 10⁻¹⁹ C– e = –1.602 × 10⁻¹⁹ C
Electric‑field directionAway from the charge (field lines outward)Toward the charge (field lines inward)
Force on another chargeRepels +, attracts –Repels –, attracts +

Electrostatic Force – Coulomb’s Law

The force between two point charges \(q1\) and \(q2\) separated by a distance \(r\) is

\(F = k \dfrac{|q1 q2|}{r^{2}}\) where \(k = 8.99 \times 10^{9}\,\text{N·m}^2\text{C}^{-2}\)

  • If the charges have the same sign the force is repulsive.

  • If the charges have opposite signs the force is attractive.

Electric Field

An electric field is the region around a charge where another charge would experience a force. By convention the field direction is the direction of the force on a positive test charge.

  • Single positive charge – field lines radiate outward.

  • Single negative charge – field lines converge inward.

Typical field patterns

  • Point charge: radial lines spreading out (positive) or converging (negative).

  • Parallel‑plate capacitor: uniform, parallel lines from the positive plate to the negative plate.

  • Charged conductor: field lines perpendicular to the surface; field inside a conductor is zero.

Frictional (Contact) Charging

When two different materials are rubbed together, electrons are transferred from one material to the other.

  • The material that loses electrons becomes positively charged.

  • The material that gains electrons becomes negatively charged.

  • Thus, frictional charging always involves the movement of negative charge (electrons).

Classic experiments

Glass rod + silk: rubbing removes electrons from the glass; the glass becomes positive, the silk becomes negative.

Rubber rod + wool: rubbing transfers electrons to the rubber; the rubber becomes negative, the wool becomes positive.

Detecting Electrostatic Charge

  • Electroscope: a metal leaf or needle deflects when a charge is placed nearby.

  • Charged paper strips: thin paper pieces are attracted to a charged object.

  • Balloon‑hair test: a negatively charged balloon attracts hair or a thin water stream.

Conductors vs. Insulators (electron model)

  • Conductors (e.g., metals) contain free electrons that move readily, allowing charge to spread quickly over the surface.

  • Insulators (e.g., rubber, glass) have electrons tightly bound to atoms; charge remains where it is deposited.

Quantifying Charge

Elementary charge:

\(e = 1.602 \times 10^{-19}\,\text{C}\) (negative)

A proton carries a charge of \(+e\). Larger amounts are expressed in coulombs (C).

Example Questions (IGCSE style)

  1. Identify the type of charge on a glass rod rubbed with silk and explain the reason.

  2. Two identical rubber balls have been rubbed with wool and then brought close together. Predict what will happen and why.

  3. Explain how an electroscope can be used to determine whether an object is positively or negatively charged.

Suggested diagrams: (a) field lines around a single + charge and a single – charge; (b) uniform field between parallel plates; (c) electroscope showing leaf deflection when charged.