The SI unit of energy, used here to quantify the heat absorbed per kilogram of material.The joule (symbol J) is the SI unit of energy, work, and heat. It is defined as the energy transferred when a force of one newton acts over a distance of one meter, or equivalently, the energy dissipated as heat when an electric current of one ampere passes through a resistance of one ohm for one second. In thermodynamics and materials science, it is the standard measure for quantifying heat absorbed per unit mass, often expressed in joules per kilogram (J/kg) for specific heat or latent heat.
Named after James Prescott Joule, who in the 1840s demonstrated the mechanical equivalent of heat through his famous paddle-wheel experiments, establishing that mechanical work and heat are interconvertible forms of energy. The unit was formally adopted by the International System of Units (SI) in 1948.
The joule ties mechanical and thermal energy to a single, consistent scale. One joule is a relatively small amount of energy — roughly the energy needed to lift a small apple one meter against Earth's gravity. When a material absorbs heat, the energy in joules increases its internal energy, raising temperature or causing phase changes, and the ratio of joules to mass (and temperature change) defines material properties like specific heat capacity.
Used universally in physics, engineering, and chemistry to specify energy content, heat transfer, and work — from food energy labels (kilojoules) to the thermal properties of building materials.
A single joule is roughly the energy released when a mosquito hits a windshield at highway speed.
