Energies

Energy is a property of physical systems that allows them to perform work or produce changes in their state. In physics, energy exists in many forms, and it can be transferred or transformed from one form to another. Kinetic energy depends on an object's motion, while potential energy depends on position or configuration. Other forms include thermal energy, chemical energy, electrical energy, radiant (light) energy, and nuclear energy. In everyday language, energy describes the capacity to do work, such as lifting a load, heating a substance, or powering devices.

Energy is measured in joules in the SI system. The total energy of a closed system is conserved, a principle known as the first law of thermodynamics: energy can be transformed but not created or destroyed, though it may appear in different forms. Transformations occur constantly, such as a falling object converting potential energy to kinetic energy, or a light bulb converting electrical energy into radiant and thermal energy. In living organisms, chemical energy from food is transformed to motion, growth, and heat.

Energy systems in society involve generation, transmission, and consumption. Common sources include fossil fuels, nuclear power, and renewable resources such as wind, solar, and hydro. The choice of energy affects efficiency, reliability, costs, and environmental impact. Energy quality and exergy describe how useful a particular energy form is for a given task. The concept of energy is central to physics, engineering, ecology, and economics, linking microscopic processes to macroscopic phenomena.