Description:
Unit of Energy
The SI unit of energy is the Joule (J), named after the English physicist James Prescott Joule.
1 Joule is defined as the work done when a force of one Newton is applied to an object and the object is displaced by 1 metre in the direction of the applied force:
$1 J = 1 N·m = 1 kg·m^{2}·s^{-2}$
Other Common Units of Energy
| Unit | Symbol | Equivalent in Joules | Used In |
|---|---|---|---|
| Electronvolt | eV | $1.6 × 10^{-19} J$ | Atomic & nuclear physics |
| Calorie | cal | $4.184 J$ | Thermodynamics, food |
| Kilocalorie | kcal | $4184 J$ | Nutrition |
| Kilowatt-hour | kWh | $3.6 × 10^{6} J$ | Electrical energy |
| Erg | erg | $10^{-7} J$ | CGS system |
| British Thermal Unit | BTU | $1055 J$ | Engineering (UK/US) |
Scale of Energy — From Smallest to Largest
| Scale | Energy (Joules) | Example |
|---|---|---|
| $10^{-34} J$ | Quantum scale | Energy of a photon of a radio wave |
| $10^{-19} J$ | Atomic scale | 1 electronvolt (ionisation energy) |
| $10^{-18} J$ | Molecular scale | Energy of a chemical bond |
| $10^{-3} J$ | Millijoule | Energy in SHM of a spring (small oscillation) |
| $1 J$ | Joule | KE of a 1 kg mass moving at √2 m/s |
| $10^{3} J$ | Kilojoule | Energy in a food biscuit |
| $10^{6} J$ | Megajoule | Energy of a 1-tonne car at 160 km/h |
| $10^{9} J$ | Gigajoule | Daily energy use of a household |
| $10^{15} J$ | Petajoule | Energy released by a small nuclear bomb |
| $10^{22} J$ | Astronomical scale | Kinetic energy of Earth's rotation |
| $10^{41} J$ | Stellar scale | Energy released by a supernova explosion |
