Energy, work and power

Energy cannot be created or destroyed, only transferred between stores and forms. Saying energy is used up is a shortcut for saying it has been transferred into a form that is spread out and hard to use, usually thermal energy in the surroundings.

The useful stores are kinetic, gravitational potential, elastic potential, chemical, thermal, nuclear and electrical. Framing a question as which store is emptying and which is filling avoids most conceptual errors.

Work is force times distance moved in the direction of the force, and it is simply energy transferred by a force. If the force is perpendicular to the motion it does no work, which is why carrying a bag horizontally at constant speed does no work on the bag in the physics sense.

Power is the rate of energy transfer, energy divided by time, measured in watts. For a constant force it also equals force times velocity, which is a fast route through many problems and explains why a vehicle at its power limit accelerates less at high speed.

Efficiency is useful output energy divided by total input, expressed as a fraction or percentage. It can never exceed one hundred percent, and any claim otherwise is a definitional error or a measurement mistake.

The losses are mostly thermal, from friction, air resistance and electrical resistance. Because thermal energy is disordered, it cannot be fully converted back into useful work. This one-way tendency is the second law of thermodynamics, usually stated in terms of entropy increasing in an isolated system.

• Conduction: energy passes through a material by particle collisions and, in metals, by free electrons, which is why metals feel cold to touch.
• Convection: warmer fluid becomes less dense, rises, and sets up a circulating current.
• Radiation: energy travels as infrared electromagnetic waves and needs no medium, which is how the sun heats the earth.
• Insulation works by attacking whichever mechanism dominates: trapped air resists conduction and convection, reflective surfaces resist radiation.

Since energy is conserved globally, the practical problem is never running out of energy but running out of concentrated, convenient energy. Fossil fuels are stores that took millions of years to concentrate and are released far faster than that.

Renewable sources tap flows rather than stores: sunlight, wind, moving water, geothermal heat. Their engineering challenge is not availability but density, intermittency and storage, which is why grid-scale storage has become the central technical question.