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 it can do. One joule is a small amount of energy, but it helps scientists and engineers measure many things, like the energy in a falling object or the power of 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 studied heat and energy in the 1800s. His work helped us understand how different forms of energy are connected. Today, the joule is used in many areas of science and technology to measure energy clearly and exactly.

Definition

The joule is a unit of work or energy. It is defined as the work done when a force of 1 newton moves something a distance of 1 metre.

In SI units, the joule can also be described in other ways:

• It is the work needed to move an electric charge of one coulomb through a voltage of one volt.
• It is the work needed to produce one watt of power for one second.

The joule is named after James Prescott Joule. Its symbol is J. When we write the word "joule," we use a lower case "j" unless it starts a sentence.

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, a leader named Siemens suggested naming a unit of energy after James Prescott Joule at a meeting of the British Science Association. At that time, another unit called the erg was used for energy. Siemens thought 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. The way the joule was defined changed a little over time, but it has stayed the same since 1946. Today, the joule is used to measure energy in many 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 a moment.
• 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, like changing one bit of data in a computer.
• Nanojoule: The energy a tiny flying mosquito has while moving.
• Microjoule: The energy made 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 one of the biggest explosions 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.

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. Torque turns something around an axis. Both use the unit called the newton-metre. It looks the same as the joule but means something different.

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

Watt-second

A watt-second (symbol W s or W⋅s) is a way to measure energy, like the joule. It shows 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, people sometimes say "watt-second" instead of "joule", for example when talking about camera flashes.