# Watt (unit)

Physical unit
Unit name watt
Unit symbol ${\ displaystyle \ mathrm {W}}$
Physical quantity (s) power
Formula symbol ${\ displaystyle P}$in mechanics and electrical engineering in thermal engineering
${\ displaystyle {\ dot {Q}}}$
dimension ${\ displaystyle {\ mathsf {M \; L ^ {2} \; T ^ {- 3}}}}$
system International system of units
In SI units ${\ displaystyle \ mathrm {1 \, W = 1 \; {\ frac {J} {s}} = 1 \; VA = 1 \; {\ frac {kg \, m ^ {2}} {s ^ { 3}}}}}$
In CGS units ${\ displaystyle \ mathrm {1 \, W = 10 ^ {7} \; {\ frac {erg} {s}} = 10 ^ {7} \; {\ frac {g \, cm ^ {2}} { s ^ {3}}}}}$
Named after James Watt
Derived from Joule , second
Electrical power meter

The watt is the unit of measurement used in the international system of units (SI) for power ( energy consumption per time period ). It was named after the Scottish scientist and engineer James Watt . The capital letter "W" is used as the unit symbol .

The watt is a derived unit . It can be derived from the basic units kilogram (kg) , meter (m) and second (s) :

${\ displaystyle \ mathrm {1 \, W = 1 \, {\ frac {kg \, m ^ {2}} {s ^ {3}}}} \,}$

As with any SI unit, decimal multiples can be specified using the SI prefixes ("Milli", "Kilo", "Mega" etc.).

## Illustrations

One watt is equal to the power to

• to perform mechanical work of one joule per second ( ), for example to apply the force of one newton over a distance of one meter within one second ,${\ displaystyle \ mathrm {1 \, W = 1 \, {\ tfrac {J} {s}}}}$
• to heat one gram of water per minute by approx. 14.3  K (i.e. from 15 ° C to approx. 29.3 ° C within one minute), or
• to maintain an electrical voltage of one volt via a purely ohmic resistance of one ohm (whereby a current of one ampere flows; ).${\ displaystyle \ mathrm {1 \, W = 1 \, \ mathrm {V \ cdot A}}}$

It should be noted that the different types of power as well as forms of energy cannot always be directly converted into one another.

## history

The unit is named after James Watt, who is known for improving the efficiency of steam engines . It was proposed by the British Science Association in 1882 and recognized as a unit of achievement at its second congress in 1889. Compared to the previously common unit "ampere.volt", the new unit had a clearer relationship to mechanics, so that it was also accepted as a replacement for the unit PS (horse power) in electric motors . In 1946 the unit was redefined by the International Committee for Weights and Measures (CIPM). This resolution was confirmed by the ninth general conference on Weights and Measures in 1948 , and in 1960 the tidal flats were incorporated into the newly created International System of Units.

## Electrical engineering

In electrical engineering, the watt is used exclusively to indicate the active power ; the following units are also used - especially in energy supply technology:

In the field of high-frequency and microwave technology , the signal and energy transmission shifts from the line structures to the electromagnetic fields that develop on the line structures , even with line-based transmission, with increasing frequency . Instead of current and voltage , calculations are made on the so-called “voltage level” with so-called “wave quantities” , which have the unit (“root watt”). ${\ displaystyle {\ sqrt {W}}}$