Joule
English physicist whose work led to the SI unit of energy.
The joule (symbol: J) is the SI unit of energy. In base units, one joule equals one kilogram-metre squared per second squared. It is the work performed when a one-newton force moves an object one metre in the direction of the force. It also equals the heat dissipated when a one-ampere current flows through a one-ohm resistor for one second. The unit is named after English physicist James Prescott Joule.
The official definition, from the International Bureau of Weights and Measures, states the joule is the work done when a one-newton force moves its point of application one metre in the force’s direction. One joule is also equivalent to the work needed to move one coulomb of charge through a one-volt potential difference (one coulomb-volt), or the work needed to produce one watt of power for one second (one watt-second). As with all SI units named after a person, the symbol is capitalised (J), but the full name is lowercase except at the start of a sentence or in titles.
The joule’s history begins with the CGS system, made official in 1881. The erg became its energy unit in 1882. That same year, Wilhelm Siemens proposed the joule as a heat unit, derived from the ampere and ohm, equal to 10⁷ erg. He suggested naming it after James Prescott Joule, then 63 and retired, for his work on the mechanical theory of heat. The joule was officially adopted at the second International Electrical Congress in 1889, alongside the watt and quadrant (later renamed henry). Joule died that year. In 1893, the “international joule” was defined from the “international ampere” and “international ohm.” In 1935, the International Electrotechnical Commission adopted the Giorgi system, which redefined the joule. By 1946, the International Committee for Weights and Measures approved the joule as the unit of work from one unit of force (later named newton) over one metre, intended for both electromagnetic and mechanical contexts. The 1948 General Conference on Weights and Measures confirmed this definition and preferred the joule over the calorie for heat. The modern SI definition (J = kg⋅m²⋅s⁻²) has remained unchanged since 1946, though the joule as a derived unit has been affected by later redefinitions of the second (1960, 1967), metre (1983), and kilogram (2019).
Practical examples of one joule include: the heat released by a resting person every 1/60 second (about 5,000 kJ per day); the electricity to run a 1 W device for one second; the energy to accelerate a 1 kg mass at 1 m/s² over one metre; the kinetic energy of a 2 kg mass moving at 1 m/s (or a 1 kg mass at about 1.41 m/s); the energy to lift a 101.97 g apple one metre; the heat to raise 0.239 g of water from 0 °C to 1 °C; the kinetic energy of a 50 kg human moving at 0.2 m/s; the kinetic energy of a 56 g tennis ball at 6 m/s; and the food energy in just over half a sugar crystal (0.102 mg).
Multiples include: the zeptojoule (160 zJ is about one electronvolt; the Landauer limit for changing a data bit at room temperature is about 2.75 zJ); the nanojoule (160 nJ is roughly the kinetic energy of a flying mosquito); the microjoule (the Large Hadron Collider produces collisions of about 7 TeV, or microjoules, per particle); the kilojoule (food labels often use kJ; one square metre of Earth receives about 1.4 kJ of solar radiation per second in full daylight; a sprinting human has about 3 kJ of kinetic energy, a cheetah at 122 km/h about 20 kJ; one watt-hour is 3.6 kJ); and the megajoule (roughly the kinetic energy of a one-tonne vehicle at 161 km/h, or the energy to heat 10 litres of water at constant pressure).
- field
- Physics
- nationality
- English
- known_for
- Dynamical theory of heat; namesake of the SI unit of energy, the joule
Lore & Background
The joule, symbolized as J, is the SI unit of energy, defined as the work performed when a force of one newton displaces an object by one meter in the force’s direction. In base SI units, this equates to one kilogram-meter squared per second squared. It also represents the heat dissipated when a current of one ampere flows through a resistance of one ohm for one second. The unit is named after English physicist James Prescott Joule (1818–1889). Its definition was formally established at the second International Electrical Congress in 1889, following a proposal by Wilhelm Siemens, who recommended the name to honor Joule’s contributions to the dynamical theory of heat. The joule replaced the earlier erg from the CGS system. In 1948, the General Conference on Weights and Measures specified that the joule should be the preferred unit for heat in calorimetry, officially deprecating the calorie. The definition as kg·m²·s⁻² has remained unchanged since 1946, though it has inherited updates from redefinitions of the second, meter, and kilogram. Practical examples include the energy needed to lift a 101.97-gram apple one meter, or the heat required to raise 0.239 grams of water from 0°C to 1°C. Multiples range from the microjoule (kinetic energy of a flying mosquito) to the zettajoule (exceeding the energy needed to heat the Earth’s atmosphere).
Reader's Guide
The joule is the SI unit of energy, defined as the work performed when a force of one newton displaces an object by one metre in the direction of that force. It is also equivalent to the heat dissipated when a current of one ampere flows through a resistance of one ohm for one second. In base SI units, it is expressed as one kilogram-metre squared per second squared. The unit was first proposed in 1882 by Wilhelm Siemens as a unit of heat derived from electromagnetic units, and was officially adopted at the second International Electrical Congress in 1889, named after the English physicist James Prescott Joule. Its definition was later refined in 1946 under the Giorgi system, explicitly making it the unit of work in both mechanical and electromagnetic contexts, and in 1948 it was formally preferred over the calorie for calorimetry. The joule is dimensionally equivalent to the newton-metre, though that term is reserved for torque. Practical examples of one joule include the energy required to lift a 101.97-gram apple one metre, the kinetic energy of a two-kilogram mass moving at one metre per second, or the heat needed to raise 0.239 grams of water by one degree Celsius. Multiples range from the microjoule (the kinetic energy of a flying mosquito) to the zettajoule, which exceeds the energy needed to heat the entire Earth’s atmosphere. The joule also relates to the Landauer limit, the minimal energy needed to change a bit of data at room temperature, approximately 2.75 zeptojoules.
Did You Know?
- One joule is the energy dissipated as heat when an electric current of one ampere passes through a resistance of one ohm for one second.
The Principle of Coherence
A coherent system of units is one in which every derived unit is constructed purely as a product of powers of the chosen base units, with no extra proportionality constant. This property guarantees that a physical equation written in terms of abstract quantities takes exactly the same numerical form when expressed in the system's units, including identical numerical factors. For example, kinetic energy is given by E = ½mv². Substituting a mass of 2 kg and a velocity of 3 m/s yields the numerical equation 9 J = ½ × 2 kg × (3 m/s)², with no hidden conversion constants. The joule itself, defined as kg·m²·s⁻², is a textbook case of a coherent derived unit: it is a straightforward product of base units raised to integer powers, and the proportionality factor is exactly one. Because of this, the mathematical structure of physics equations is preserved seamlessly whether one is manipulating symbolic quantities or plugging in concrete numerical values expressed in the system's units.
Historical Roots and the Road to Coherence
The notion that units of different physical quantities should be interrelated without arbitrary scaling factors crystallised in the mid-nineteenth century. Lord Kelvin and James Clerk Maxwell were among the principal developers of the idea, and the British Association for the Advancement of Science played a key role in promoting it. The principle was first put into practice with the centimetre-gram-second system in 1873 and the foot-pound-second system in 1875. Before that, the original metric system of 1795 was decidedly non-coherent: the litre was defined as 0.001 m³ and the are as 100 m², each requiring a numerical factor to connect to base units. A faint precursor to coherence did exist, however, when the gram was tied to the mass of one cubic centimetre of water at its freezing point. The International System of Units, designed in 1960, finally elevated coherence to a central design goal, ensuring that every derived quantity possesses exactly one coherent unit expressible as a product of powers of the base units.
Coherence in Practice: When the Same Unit Changes Status
Whether a particular unit is coherent depends entirely on the chosen set of base units. In the SI, metres per second is a coherent derived unit for speed because it is built solely from the base units metre and second. Kilometres per hour, by contrast, is not coherent: converting 18 km/h to SI requires multiplying by 1000 and dividing by 3600 to obtain 5 m/s, or equivalently 1 km/h = (1/3.6) m/s. The same unit can be coherent in one framework and not in another. Metres per second, for instance, is coherent in SI but not in CGS, where a factor of 100 (centimetres per metre) must be introduced. Likewise, the pascal (kg·m⁻¹·s⁻²) is a coherent SI unit of pressure, while the bar (100,000 kg·m⁻¹·s⁻²) is not. Coherence is also sensitive to the definitions of the base units themselves: if the SI metre were reduced by a factor of 100,000, the bar would become a coherent derived unit without any change in its physical size.
The Joule and the Unification of Energy
One of the most visible consequences of designing a coherent system is the unification of units for a given physical quantity. In the older CGS framework, energy carried two named units: the erg, tied to mechanical work and equal to g·cm²·s⁻², and the calorie, tied to thermal energy. Only the erg could maintain a coherent relationship to the CGS base units. The SI, built with coherence as a guiding principle from the outset, defined a single unit of energy: the joule. This one unit covers both mechanical and thermal energy without requiring a separate conversion constant. More broadly, each physical quantity in the SI has exactly one coherent unit, even when it can be written in several equivalent forms—power, for example, may appear as watts, joules per second, or kg·m²·s⁻³. Yet some units can serve multiple quantities: the joule and the newton-metre are dimensionally equivalent (kg·m²·s⁻²), and nonetheless energy and torque remain physically distinct and cannot be added together.
Frequently Asked Questions
What exactly is a Joule?
A joule (symbol J) is the SI unit for energy, work, or heat. In everyday terms, one joule is the energy transferred when a one-newton force pushes an object one metre along the direction of that force.
How is the Joule expressed in base SI units?
The joule breaks down to one kilogram-metre squared per second squared (kg⋅m²⋅s⁻²). Because it is built only from the kilogram, metre, and second, it is classified as a derived SI unit rather than a base one.
Why is the Joule important in physics?
As the standard measure of energy across the SI system, the joule shows up in mechanics, thermodynamics, electromagnetism, and beyond. It gives researchers a single, consistent yardstick for quantifying work, heat flow, and energy transfer.
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