Energy

Energy is a physical quantity that measures the capacity of a system to perform work or produce heat. It is a scalar, having no direction. The standard unit in the SI system is the joule (J), abbreviated J; kilowatt-hour (kWh) is also used, and in food science, the calorie.

The law of energy conservation states that a closed system cannot create or destroy energy, only transform it from one form to another. For example, an object raised to a height possesses potential energy; when it falls, this converts to kinetic energy, and some of it transforms into heat when it strikes the ground.

Types of energy

Energy is commonly divided into types. Kinetic energy depends on an object's motion and mass. Potential energy depends on an object's position within a force field, such as gravity or electric charge. There is also thermal energy, chemical energy (stored in atomic bonds), electromagnetic energy, and nuclear energy stored in the nucleus of an atom.

Energy and mass

Albert Einstein's theory of special relativity established a relationship between energy and mass, expressed in the equation E = mc², where c is the speed of light. This means that mass is a form of energy, which explains nuclear reactions such as those occurring in the sun.

Energy, work, and power

Work is the transfer of energy when a force moves an object over a distance. Power is the rate of energy transfer over time, measured in watts (W), where one watt equals one joule per second. Household electricity consumption is therefore measured in kilowatt-hours, representing total energy used.

Energy in industry and society

In economic and policy contexts, the term energy typically refers to sources such as fossil fuels (petroleum, gas, coal), nuclear energy, and renewable sources such as solar, wind, and hydroelectric power. Changes to energy systems affect electricity prices, resource consumption, and carbon emissions.