What is joule?
The joule is the SI base unit of energy. It is the standard unit for work, heat, and electrical energy in physics, engineering, and scientific measurement worldwide.
Real-world uses
The joule is the SI unit of energy used in physics, chemistry, and nutrition (food energy in some countries is labelled in kilojoules). Electrical energy, mechanical work, and heat transfer are all measured in joules. One joule is the energy to lift a small apple about 1 metre.
History
Named after James Prescott Joule, the English physicist who demonstrated the mechanical equivalent of heat in the 1840s, establishing that heat and mechanical work are interchangeable forms of energy. The joule was adopted as the SI energy unit in 1960.
Common mistakes
Confusing joules with watts—a joule is a unit of energy, while a watt is a unit of power (1 W = 1 J/s). Also, assuming "calorie" and "joule" are equivalent without conversion: 1 cal = 4.184 J.
What is electronvolt?
An electronvolt is a unit of energy equal to the work done on an electron by a potential difference of one volt, approximately 1.602 × 10⁻¹⁹ joules. It is used in atomic physics, particle physics, and quantum mechanics.
Real-world uses
The electronvolt is the standard energy unit in particle physics, atomic physics, and semiconductor science. X-ray photon energies are measured in keV; visible light photons in eV (about 1.8–3.1 eV); nuclear reactions in MeV; and particle accelerator energies in GeV or TeV.
History
The electronvolt came into use in the early 20th century with the development of quantum mechanics and experimental atomic physics. As physicists began measuring energies of electrons and photons, the eV provided a convenient unit far smaller than the joule but appropriate for subatomic-scale phenomena.
Common mistakes
Confusing the electronvolt with a unit of voltage—it is a unit of energy equal to the kinetic energy gained by one electron accelerated through a potential difference of one volt (1 eV = 1.602 × 10⁻¹⁹ J). Also mixing up the scale prefixes (keV, MeV, GeV, TeV) when discussing different physical phenomena.
When is this conversion used?
Electricity bills use kilowatt-hours while physics calculations use joules. Converting between them is essential for energy audits, comparing appliance efficiency, and understanding power consumption.
Worked examples
1 joule = 6.241509e+18 electronvolt
1 electronvolt = 1.602177e-19 joule
How to convert joule to electronvolt
To convert joule to electronvolt, multiply the value by 6.241509e+18.
To convert electronvolt back to joule, multiply by 1.602177e-19.
Measurement standards
The joule is the SI derived unit of energy, defined as one kilogram-metre-squared per second-squared (kg·m²/s²). The kilowatt-hour, equal to exactly 3.6 megajoules, is accepted for use with the SI by the BIPM for practical electricity metering.
Did you know?
A single bolt of lightning releases roughly 1 to 5 gigajoules of energy, yet only a small fraction reaches the ground as electrical current. Most of the energy dissipates as light, heat, and the thunder shockwave.
Quick reference: joule to electronvolt
| joule | electronvolt |
|---|---|
| 0.1 | 6.241509e+17 |
| 0.5 | 3.120755e+18 |
| 1 | 6.241509e+18 |
| 2 | 1.248302e+19 |
| 5 | 3.120755e+19 |
| 10 | 6.241509e+19 |
| 25 | 1.560377e+20 |
| 50 | 3.120755e+20 |
| 100 | 6.241509e+20 |
| 250 | 1.560377e+21 |
| 500 | 3.120755e+21 |
| 1,000 | 6.241509e+21 |
Common values
| joule | electronvolt | |
|---|---|---|
| A food calorie (1 kcal) | 4,184 joule | 2.611447e+22 electronvolt |
| A AA battery | 9,360 joule | 5.842052e+22 electronvolt |
| Boiling a kettle | 420,000 joule | 2.621434e+24 electronvolt |
| Daily human diet (2000 kcal) | 8,368,000 joule | 5.222895e+25 electronvolt |
| 1 gallon of gasoline | 131,760,000 joule | 8.223812e+26 electronvolt |
Available Energy units
More joule conversions
- Convert joule to kilojoule
- Convert joule to kilowatt hour
- Convert joule to calorie (thermochemical)
- Convert joule to kilocalorie (dietary)
- Convert joule to watt hour
- Convert joule to megajoule
- Convert joule to electronvolt
- Convert joule to British thermal unit
- Convert joule to therm (US)
Assumption: calorie means thermochemical calorie, and therm means US therm.