Energy
Energy is the quantity that measures a system's capacity to do work, and it takes many forms: motion, position in a field, heat, radiation, chemical bonds, and mass itself. Its usefulness comes from the fact that it can be converted between forms but never created or destroyed in a closed system. Physics does not say what energy is made of; it says how much of it a system has and tracks where it goes. Einstein later showed that mass is one of its forms, related by the equation that energy equals mass times the speed of light squared.
Why it matters
Energy accounting works across mechanics, heat, electricity, and nuclear physics alike, which makes it the single most portable idea in the subject.
Where it comes up
Read about Energy on Wikipedia
Sources
- Energy English Wikipedia
- Conservation of energy English Wikipedia
- The Equivalence of Mass and Energy Stanford Encyclopedia of PhilosophyOn what the mass-energy relation does and does not claim.
