Energy Forms and Transfers

Objective: Identify motion, stored, thermal, and other useful energy descriptions.

Energy is a scalar quantity measured in joules (J) and used to account for changes in physical systems. Kinetic energy relates to motion; potential energy describes energy associated with configurations such as height in a gravitational field or deformation of an elastic object. Thermal energy concerns microscopic motion and interactions of particles. Chemical, electrical, and radiant are useful descriptions of energy in particular systems and transfer pathways, not different substances. A book at a height is assigned gravitational potential energy relative to a chosen reference and system. Energy can be transferred by forces doing work, heating, or radiation. Choose system boundaries and reference levels before comparing energy values.

Objective: Distinguish transfer mechanisms from energy stores.

A force can transfer energy through work when its component along displacement acts through a distance. Heating transfers energy because of a temperature difference, and electromagnetic radiation can transfer energy across a vacuum. Calling heat “a substance stored inside a body” confuses a transfer process with internal energy. A warmer cup can warm surrounding air while also radiating energy, so multiple mechanisms can occur simultaneously. Energy-transfer diagrams should label the system and flows and avoid double counting the same joules. A force perpendicular to displacement does no work in the simple dot-product model even though the force exists.

Objective: Use input-output accounting without claiming energy disappears.

An energy flow diagram or Sankey diagram represents input energy and output pathways. For a device, useful-output efficiency is useful energy output divided by energy input, usually expressed as a percentage under matching conditions. A motor receiving 100 J electrically and delivering 70 J of mechanical work has 70% useful efficiency; the remaining 30 J is transferred to other outputs such as thermal energy and sound, not destroyed. “Useful” depends on the device’s intended function. A measured efficiency can vary with load and operating conditions. The total energy balance should include all significant outputs and changes in stored energy.