Physics · Glossary

What is Energy and the joule?

Also known as: energy · joule

Definition 9.1 High School Physics · Chapter 9 — Energy: Forms and Conservation

Energy measures a system’s capacity to produce changes: set matter in motion, lift it, heat it, light it. Its unit is the joule (J\mathrm{J}). Stored, transferred, converted — every branch of physics counts in this one currency.

Examples

Example 9.5 (Water tower)

Each cubic metre (1000kg1000\,\mathrm{kg}) of water stored 30m30\,\mathrm{m} up a water tower holds 1000×9.81×302.9×105J1000 \times 9.81 \times 30 \approx 2.9 \times 10^{5}\,\mathrm{J} — about six tenths of the car’s kinetic energy above. Cities stockpile energy overhead.

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