What is energy?

Energy is a fundamental quantity used to describe physical systems and changes in them. Different forms are associated with motion, position, temperature, chemical structure and radiation. Its SI unit is the joule. Rather than an invisible fluid, energy is a concept relating measurable changes quantitatively.

Forms and transformations of energy

1. Kinetic and potential energy Moving objects have kinetic energy, which classically depends on mass and speed squared. Potential energy depends on the arrangement of interacting components. Lifting an object changes the gravitational potential energy of the Earth–object system. A reference zero can be chosen freely if energy differences remain consistent. 2. Internal and chemical energy Particle motion and interactions contribute to internal energy. Chemical reactions involve different energies for initial and final arrangements. Breaking a bond requires energy; energy released by an overall reaction must be understood through new bond formation and the other changes involved. 3. Work and heat A force acting through displacement can transfer energy as work. Heat is transfer caused by a temperature difference. Neither is stored as a separate substance inside an object: they describe transfer mechanisms.

Conservation, power and efficiency

4. Conservation Total energy is conserved in an isolated system. A ball losing mechanical energy does not destroy energy; it can transfer energy into heating its surroundings or sound. The chosen system boundary must be clear. 5. Power is different Power measures energy transfer per unit time and is expressed in watts. Completing the same work faster requires greater average power. A kilowatt-hour is an energy unit, not a power unit, and describes an amount of electricity used. 6. Efficiency and availability Efficiency can compare useful output with energy input. Conservation does not mean all energy can become the desired work. Insulation, reduced friction and suitable design reduce losses. Here, loss means dispersal into unwanted forms rather than energy disappearing.