Joule

The joule is the unit we use to measure energy in the International System of Units (SI). It tells us how much energy is in something or how much work can be done. One joule is a very small amount of energy, but it helps scientists and engineers measure everything from the energy in a falling object to the power used by a light bulb.

We can think of a joule as the energy used when a force of one newton pushes something a distance of one metre. For example, if you push a small box across the floor with a gentle push, the energy you use can be measured in joules.

The unit is named after James Prescott Joule, an English physicist who did important work studying heat and energy in the 1800s. His discoveries helped us understand how different forms of energy are connected, like how moving things can create heat. Today, the joule is used in many areas of science and technology to measure energy in a clear and exact way.

Definition

According to the International Bureau of Weights and Measures, the joule is defined as the work done when the point of applying 1 MKS unit of force newton moves a distance of 1 metre in the direction of the force.

In terms of SI base units and in terms of SI derived units with special names, the joule is defined as

One joule is also equivalent to any of the following:

• The work required to move an electric charge of one coulomb through an electrical potential difference of one volt, or one coulomb-volt (C⋅V). This relationship can be used to define the volt.
• The work required to produce one watt of power for one second, or one watt-second (W⋅s) (compare kilowatt-hour, which is 3.6 megajoules). This relationship can be used to define the watt.

The joule is named after James Prescott Joule. As with every SI unit named after a person, its symbol starts with an upper case letter (J), but when written in full, it follows the rules for capitalisation of a common noun; i.e., joule becomes capitalised at the beginning of a sentence and in titles but is otherwise in lower case.

J =   k g ⋅ m 2 ⋅ s − 2 =   N ⋅ m =   P a ⋅ m 3 =   W ⋅ s =   C ⋅ V {\displaystyle {\begin{alignedat}{3}\mathrm {J} \;&=~\mathrm {kg{\cdot }m^{2}{\cdot }s^{-2}} \\[0.7ex]&=~\mathrm {N{\cdot }m} \\[0.7ex]&=~\mathrm {Pa{\cdot }m^{3}} \\[0.7ex]&=~\mathrm {W{\cdot }s} \\[0.7ex]&=~\mathrm {C{\cdot }V} \\[0.7ex]\end{alignedat}}}

Symbol Name J joule kg kilogram m metre s second N newton Pa pascal W watt C coulomb V volt

History

In 1882, at a meeting of the British Science Association, a leader named Siemens suggested naming a unit of energy after James Prescott Joule. At that time, another unit called the erg was used for energy. Siemens thought this new unit, called the joule, would be a good way to honor Joule’s work on heat.

Later, in 1889, the joule was officially chosen as the unit of energy, along with other units like the watt. Over time, the way the joule was defined changed a little, but it has stayed the same since 1946. Today, the joule is used to measure energy in many different areas of science.

Practical examples

One joule is a small amount of energy. Here are some everyday examples:

• It’s about the energy a person gives off as heat in just over a tenth of a second.
• It’s the energy needed to run a tiny light for one second.
• It’s what you need to push a small object one meter with a gentle force.
• It’s also the energy in picking up a small apple from the ground and holding it up for a meter.

Multiples

For additional examples, see Orders of magnitude (energy).

• Zeptojoule: Very tiny energy, about the energy needed to change one bit of data in a computer.
• Nanojoule: The energy a tiny flying mosquito has while moving.
• Microjoule: The energy produced by big science machines, like the Large Hadron Collider, in each tiny crash.
• Kilojoule: The energy you see on food labels. It’s also the energy the Earth gets from sunlight each second, or what a sprinter or a fast cheetah has while running.
• Megajoule: The energy a big car (like a truck) has when driving fast, or enough to heat a bathtub of water from cold to hot.
• Gigajoule: The energy from burning a barrel of oil, or what a big power plant can make in an hour.
• Terajoule: The energy a huge storm like Hurricane Irma can release, or what the International Space Station has while moving around Earth.
• Petajoule: The energy from the biggest explosion ever made by humans.
• Exajoule: The energy from very big events, like earthquakes, or what the whole world uses in a year.
• Zettajoule: The energy needed to warm up a huge sea like the Baltic Sea by just one degree.
• Yottajoule: The huge amount of energy the Sun sends out every second.

Conversions

Main article: Conversion of units of energy

A joule is a way to measure energy. It can be changed into other units. For example, 1 joule is about the same as 6.24 billion electronvolts, almost three-quarters of a foot-pound, or very close to 24 foot-poundals.

Some units are exactly the same as joules. One calorie is exactly 4.184 joules. One watt-hour equals 3,600 joules, and one kilowatt-hour equals 3.6 million joules. A watt-second is also exactly one joule.

Newton-metre and torque

Main article: Newton-metre

In mechanics, force and torque are similar ideas. Force pushes or pulls something in a straight line, while torque turns something around an axis. Both use the unit called the newton-metre, which looks the same as the joule but means something different.

Energy, measured in joules, is a simple total amount. Torque, measured in newton-metres, is about turning force. Even though they use the same numbers, they describe different things, so we keep the names separate to avoid confusion.

Watt-second

A watt-second (symbol W s or W⋅s) is a special way to measure energy, just like the joule. It tells us the energy used when something with a power of one watt works for one second. Even though a watt-second and a joule mean the same thing, sometimes people use "watt-second" instead of "joule", like when talking about camera flashes.