Watt

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The watt (symbol: W) is the SI derived unit of power, equal to one joule per second. A human climbing a flight of stairs is doing work at the rate of about 200 watts; a highly trained athlete can work at up to approximately 2000 watts for brief periods. An automobile engine produces 25 000 watts (approximately 30 horsepower) while cruising. A typical household incandescent light bulb uses 40 to 100 watts.

Contents 1 Definition 2 Origin 3 SI multiples 4 Derived and qualified units for power distribution 4.1 Kilowatt 4.2 Megawatt 4.3 Watts electrical and thermal 5 See also 6 Popular culture 7 References 8 External links

[edit] Definition

One watt is one joule (the SI unit of energy) per second.

= 1 newton meter per second

In electrical terms, it follows that:

That is, if 1 volt of potential difference is applied to a resistive load, and a current of 1 ampere flows, then 1 watt of power is dissipated. ^[1]

[edit] Origin

The watt is named after James Watt for his contributions to the development of the steam engine, and was adopted by the Second Congress of the British Association for the Advancement of Science in 1889 and by the 11th Conférence Générale des Poids et Mesures in 1960.

[edit] SI multiples

Multiple	Name	Symbol		Multiple	Name	Symbol
100	watt	W
101	decawatt	daW		10–1	deciwatt	dW
102	hectowatt	hW		10–2	centiwatt	cW
103	kilowatt	kW		10–3	milliwatt	mW
106	megawatt	MW		10–6	microwatt	µW
109	gigawatt	GW		10–9	nanowatt	nW
1012	terawatt	TW		10–12	picowatt	pW
1015	petawatt	PW		10–15	femtowatt	fW
1018	exawatt	EW		10–18	attowatt	aW
1021	zettawatt	ZW		10–21	zeptowatt	zW
1024	yottawatt	YW		10–24	yoctowatt	yW

[edit] Derived and qualified units for power distribution

[edit] Kilowatt

The kilowatt (symbol: kW) is a unit for measuring power, equal to one thousand watts. A kilowatt is roughly equivalent to 1.34 horsepower.

[edit] Megawatt

The megawatt (symbol: MW) is equal to one million (10⁶) watts.

Many things can sustain the transfer or consumption of energy on this scale; some of these events or entities include: lightning strikes, large electric motors, naval craft (such as aircraft carriers and submarines), engineering hardware, and some scientific research equipment (such as the supercollider and large lasers). A large residential or retail building may consume several megawatts in electric power and heating energy.

The productive capacity of electrical generators operated by utility companies is often measured in MW. Modern high-powered diesel-electric railroad locomotives typically have a peak power output of (3 to 5) MW, whereas a typical modern nuclear power plant produces a peak output on the order of 500 to 2000 MW.

According to the Oxford English Dictionary, the earliest citing for "megawatt" is a reference in the 1900 Webster's International Dictionary of English Language. The OED also says "megawatt" appeared in a 28 November 1847, article in Science (506:2).

[edit] Watts electrical and thermal

Watt electrical (abbreviation: We) is a term that refers to power produced as electricity. SI prefixes can be used, for example megawatt electrical (MWe) and gigawatt electrical (GWe). Watt thermal (abbreviation: Wt or Wth) refers to thermal power produced; again SI prefixes can be used, for example megawatt thermal (MWt) and gigawatt thermal (GWt). These terms are used to disambiguate the electric output of a thermal power station versus the (larger) thermal output. For example, the Embalse nuclear power plant in Argentina uses a fission reactor to generate 2109 MWt of heat, which creates steam to drive a turbine, which generates 648 MWe of electricity.

This SI unit is named after James Watt. As for all SI units whose names are derived from the proper name of a person, the first letter of its symbol is uppercase (W). But when an SI unit is spelled out, it should always be written in lowercase (watt), unless it begins a sentence or is the name "degree Celsius". — Based on The International System of Units, section 5.2.

[edit] See also

• Watt-hour
• Conversion of units
• James Watt
• Orders of magnitude (power)
• Root mean square (RMS)
• Watt balance
• Power factor
• Declared net capacity (power plants)
• 2000 Watt society

[edit] Popular culture

In Back to the Future the character Dr Emmett Brown is astonished (and horrified) to discover the amount of power required by the time machine.

"1.21 gigawatts!"

[edit] References

[edit] External links

• Nelson, Robert A., "The International System of Units Its History and Use in Science and Industry". Via Satellite, February 2000.