Fundamentals
Energy is a physical quantity that describes the ability of a system to modify its surroundings or perform work.
In a closed system, where no exchange with the environment occurs, energy is conserved. It can be transformed from one form to another (mechanical, thermal, electrical, chemical, nuclear, etc.), but the total amount of energy remains constant.
Units of Energy
The SI unit of energy is the Joule (J), which corresponds to an amount of energy commonly encountered in everyday life.
🔹 One Joule is enough to:
- Lift a 1 kg mass by approximately 10 cm.
- Heat 1 g of water by about 4.2 °C.
- Accelerate a 2 kg mass to a speed of 1 m/s.
⚛️ The Electron-Volt
In atomic, nuclear and particle physics, a much smaller unit, the electron-volt (eV), is commonly used. It corresponds to the energy gained by an electron when accelerated through a potential difference of 1 Volt.
Other Energy Units
| Unit | Equivalent Energy | Typical Example |
|---|---|---|
| cal (calorie) | ≈ 4.18 J | Heating 1 g of water by 1 °C |
| Cal (kilocalorie) | ≈ 4.18 × 10³ J | Heating 1 kg of water by 1 °C |
| kWh | 3.6 × 10⁶ J | Operating a 1 kW heater for 1 hour |
| toe (tonne of oil equivalent) | ≈ 41.8 × 10⁹ J | Energy released by burning one tonne of oil |
| t TNT | ≈ 4.18 × 10¹² J | Energy released by one tonne of TNT |
Mass–Energy Equivalence
Albert Einstein established the relationship between the mass m of a particle and its energy E.
where c = 299,792,458 m/s is the speed of light in vacuum.
This relation explains why particle masses are often expressed in eV/c² instead of kilograms.