joules

Joules are the SI unit of energy, work, and the amount of heat transferred. The symbol is J. A joule is defined as the work done when a force of one newton displaces an object one meter in the direction of the force, or equivalently as one newton meter. In base SI units, a joule is kg·m^2·s^-2. The unit is named after James Prescott Joule, a 19th-century English physicist who studied the relationship between mechanical work and heat.

Joules are used to quantify energy in physics, engineering, and everyday contexts. They measure mechanical work, electrical energy, chemical energy, and thermal energy. Common conversions include 1 calorie ≈ 4.184 J; 1 kilowatt-hour = 3.6×10^6 J; 1 electronvolt ≈ 1.602×10^-19 J. The watt is a unit of power, defined as one joule per second.

Examples help illustrate the concept. Lifting a 10-newton force by 2 meters performs about 20 J of work. A 100-watt light bulb consumes about 100 J of electrical energy each second. A moving object with mass m and speed v has kinetic energy equal to (1/2) m v^2, measured in joules. Energy transfer can be described as work when a force causes displacement, while power describes the rate of energy transfer.

Joules are the standard measure of energy across physics and engineering. They underpin thermodynamics, mechanics, and electrical calculations, with many derived units and scales built around this fundamental quantity.