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Energy

EN-er-jee /ˈɛnərdʒi/

nounB1

formaltechnicalgeneral

The capacity to do work or produce heat.

In physics, energy exists in various forms (kinetic, potential, thermal, chemical, electrical, nuclear, etc.) and can be converted from one form to another, but it cannot be created or destroyed (Law of Conservation of Energy).

The story behind the word. The word 'Energy' comes to us from the Greek 'energeia', which literally means 'activity' or 'operation'. It was used by Aristotle to describe the actualization or activity of a potential. It entered English in the late 16th century, initially referring to vigor or power of expression, before its specific scientific meaning was formalized in the 19th century.

Word relationships

power: Power is the rate at which energy is transferred or used; energy is the capacity itself.
vigor: Vigor refers to physical strength and good health, a more human-centric concept than the scientific 'energy'.

lethargy: A state of sluggishness or lack of energy.
exhaustion: Extreme tiredness, often due to depletion of energy.

Commonly confused with

Force: Force is a push/pull; energy is the capacity to do work. A force can exert energy, but they are not the same thing.

Word family

energetic (adjective): possessing or exhibiting energy
energize (verb): to give energy or vigor to

Collocations

renewable energy
kinetic energy
potential energy
conserve energy
release energy

Idioms & expressions

full of energy
burst of energy
save your energy

Example sentences

"Solar panels convert light energy from the sun into electrical energy that can power homes." Discussing the transformation of energy.
"The total energy of an isolated system remains constant, even if it changes form." Referring to a physical property.
"Our bodies derive energy from the food we eat, converting it into fuel for movement and thought." In a biological context.

Memory hook

Think of 'ENergy' as the 'ENgine' that makes things go. Capacity to work

When not to use

While 'energy' is used broadly, in a scientific context, it refers specifically to the capacity for work, not just general enthusiasm or vitality, though those are related everyday meanings.

Fun facts

Albert Einstein's famous equation E=mc² shows the equivalence of mass and energy.
The SI unit for energy is the Joule (J).

Related words

Kinetic, Potential, Thermodynamics

Explore more words

capacity, renewable, metabolism, efficiency, hydroelectric, Renewable Energy

Also appears in

Physical Sciences & Cosmology → Classical Mechanics & Motion