Lénergie

L'énergie, or energy, is the quantity describing the capacity to perform work or cause change in matter. In physics, energy exists in multiple forms and can be transformed from one form to another. In an isolated system, energy is conserved, though its form may change.

Common forms include kinetic energy, potential energy, thermal energy, chemical energy, electrical energy, radiant energy, and nuclear energy. Energy can be stored in chemical bonds, in raised positions, in fields, or as stored heat, and it can flow as heat, light, or electric current.

The SI unit of energy is the joule; energy is also commonly measured in kilojoules, calories, or kilowatt-hours (1 kWh = 3.6 MJ). Power, the rate of energy transfer, is measured in watts.

Thermodynamics governs energy transformation. The first law, or conservation of energy, states that energy cannot be created or destroyed in an isolated system; the second law introduces limits due to entropy, affecting the direction and efficiency of processes. Heat and work are two modes of energy transfer, with temperature and state variables describing systems.

In practice, energy considerations span science and policy. The energy system includes extraction, conversion, transmission, and end use, with ongoing efforts to increase efficiency and expand renewable sources to address climate change, resource depletion, and energy security.