Kilojoule (kJ)

Definition

A kilojoule (kJ) is a unit of energy in the International System of Units (SI). It is defined as 1,000 joules (J), where 1 joule is the energy transferred when one newton of force is applied over one meter.

History

The concept of energy was formalized in the 19th century. The joule was named after James Prescott Joule, who conducted experiments on the relationship between heat and mechanical work in the 1840s. The kilojoule became prevalent in the late 20th century as the metric system gained acceptance globally.

Uses

Kilojoules are commonly used to measure energy in food, exercise, and various scientific applications. Nutritional labels in many countries, including Australia and New Zealand, use kilojoules to indicate energy content. In engineering, kJ is used to quantify work and heat transfer.

Conversions

• 1 kJ = 0.239 kilocalories (kcal)
• 1 kJ = 1,000 joules (J)
• 1 kJ = 0.278 watt-hours (Wh)

Fun Facts

• A common misconception is that kilojoules and calories are interchangeable without context; they measure energy but in different systems.
• In some regions, especially in Europe, energy in food is primarily expressed in kilojoules rather than calories.