energij

Energij is a physical quantity that describes the capacity to do work or to produce heat. In physics it is treated as a scalar quantity, though its exact meaning depends on the context. Energy can exist in many forms and can be stored, transferred, or transformed between forms without changing the total amount in a closed system.

Common forms include kinetic energy (motion), potential energy (position in a field), thermal energy (random molecular motion), chemical energy (stored in chemical bonds), electrical energy (flow of electric charges), radiant energy (light and other electromagnetic radiation), and nuclear energy (strong-force interactions within atomic nuclei).

Energy is conserved in isolated systems, meaning it can change form but the total amount remains constant. The SI unit of energy is the joule (J); power, the rate at which energy is transferred, is measured in watts (W), where 1 W = 1 J/s. Energy can be computed from work done or from integrating power over time.

Energy sources are categorized as primary sources, such as fossil fuels, sunlight, wind, or uranium, which are harnessed to produce usable energy. Energy systems may store energy (batteries, pumped hydro) and transmit it (electrical grids). The environmental and economic characteristics of energy depend on the source, with renewable energy and energy efficiency receiving increasing emphasis in policy.

In everyday use, energy also describes dietary energy in humans and animals, typically expressed in kilocalories. Understanding energij informs science, engineering, and policy, including climate, infrastructure, and technology development.