Ват

Ват
Систем	СИ изведена јединица снаге
Јединица	Снага
Симбол	W
Именован по	Џејмс Ват
Јединична претварања
1 W у ...	... је једнак са ...
СИ основне јединице	1 kg⋅m2⋅s−3
ЦГС јединице	7007100000000000000♠107 erg⋅s−1
Енглеске инжењерске јединице	0,7375621 ft⋅lbf/s = 0,001341022 КС

Ват (енгл. watt; симбол: W) је СИ изведена јединица за снагу. 1 kg⋅m²⋅s⁻³ or, equivalently, Једнак је једном џулу у секунди (1 J/s), или у електричним јединицама, једном волт амперу (1 V·A). Ват је брзина, у џулима по секунди, којом се енергија претвара, користи или шири.

${\mbox{W}}={\frac {\mbox{J}}{\mbox{s}}}={\frac {\mbox{Nm}}{\mbox{s}}}={\frac {{\mbox{kgm}}^{2}}{{\mbox{s}}^{3}}}={\mbox{VA}}$

Јединица ват је добила име по Џејмсу Вату због његових доприноса у развоју парне машине и усвојена је на Другом Конгресу Британске асоцијације за напредак науке 1889. године и на једанаестој Генералној конференцији тежина и мера 1960. године.

Преглед

When an object's velocity is held constant at one metre per second against a constant opposing force of one newton, the rate at which work is done is one watt.

$\mathrm {1~W=1~{\frac {J}{s}}=1~{\frac {N{\cdot }m}{s}}=1~{\frac {kg{\cdot }m^{2}}{s^{3}}}}$

In terms of electromagnetism, one watt is the rate at which electrical work is performed when a current of one ampere (A) flows across an electrical potential difference of one volt (V), meaning the watt is equivalent to the volt-ampere (the latter unit, however, is used for a different quantity from the real power of an electrical circuit).

$\mathrm {1~W=1~V\cdot 1~A}$

Two additional unit conversions for watt can be found using the above equation and Ohm's law.

$\mathrm {1~W=1~{\frac {V^{2}}{\Omega }}=1~A^{2}{\cdot }\Omega }$

where ohm ($\Omega$) is the SI derived unit of electrical resistance.

Examples

• A person having a mass of 100 kg who climbs a 3-metre-high ladder in 5 seconds is doing work at a rate of about 600 watts. Mass times acceleration due to gravity times height divided by the time it takes to lift the object to the given height gives the rate of doing work or power.
• A laborer over the course of an eight-hour day can sustain an average output of about 75 watts; higher power levels can be achieved for short intervals and by athletes.

Настанак и усвајање као јединица СИ

The watt is named after the Scottish inventor James Watt. The unit name was proposed initially by C. William Siemens in August 1882 in his President's Address to the Fifty-Second Congress of the British Association for the Advancement of Science. Noting that units in the practical system of units were named after leading physicists, Siemens proposed that watt might be an appropriate name for a unit of power. Siemens defined the unit consistently within the then-existing system of practical units as "the power conveyed by a current of an Ampère through the difference of potential of a Volt".

In October 1908, at the International Conference on Electric Units and Standards in London, so-called "international" definitions were established for practical electrical units. Siemens' definition was adopted as the "international" watt. (Also used: 1 A² × 1 Ω.) The watt was defined as equal to 10⁷ units of power in the "practical system" of units. The "international units" were dominant from 1909 until 1948. After the 9th General Conference on Weights and Measures in 1948, the "international" watt was redefined from practical units to absolute units (i.e., using only length, mass, and time). Concretely, this meant that 1 watt was now defined as the quantity of energy transferred in a unit of time, namely 1 J/s. In this new definition, 1 "absolute" watt = 1.00019 "international" watts. Texts written before 1948 are likely to be using the "international" watt, which implies caution when comparing numerical values from this period with the post-1948 watt. In 1960, the 11th General Conference on Weights and Measures adopted the "absolute" watt into the International System of Units (SI) as the unit of power.

Умношци

Microwatt

The microwatt (µW) is equal to one millionth of a watt (10⁻⁶ W). Important powers that are measured in microwatts are typically stated in medical instrumentation systems such as the electroencephalograph (EEG) and the electrocardiograph (ECG), in a wide variety of scientific and engineering instruments and also in reference to radio and radar receivers. Compact solar cells for devices such as calculators and watches are typically measured in microwatts.

Milliwatt

The milliwatt (mW) is equal to one thousandth of a watt (0.001 W or 10⁻³ W). A typical laser pointer outputs about 5 milliwatts of light power, whereas a typical hearing aid uses less than 1 milliwatt. Audio signals and other electronic signal levels are often measured in dBm, referenced to 1 milliwatt.

Kilowatt

The kilowatt (kW) is equal to one thousand watts (1000 W or 10³ W). This unit is typically used to express the output power of engines and the power of electric motors, tools, machines, and heaters. It is also a common unit used to express the electromagnetic power output of broadcast radio and television transmitters.

One kilowatt is approximately equal to 1.34 horsepower. A small electric heater with one heating element can use 1 kilowatt. The average electric power consumption of a household in the United States is about 1 kilowatt.

A surface area of 1 square meter on Earth receives typically about 1 kilowatt of sunlight from the Sun (the solar irradiance) (on a clear day at midday, close to the equator).

Megawatt

The megawatt (MW) is equal to one million watts (10⁶ W). Many events or machines produce or sustain the conversion of energy on this scale, including large electric motors; large warships such as aircraft carriers, cruisers, and submarines; large server farms or data centers; and some scientific research equipment, such as supercolliders, and the output pulses of very large lasers. A large residential or commercial building may use several megawatts in electric power and heat. On railways, modern high-powered electric locomotives typically have a peak power output of 7000500000000000000♠5 or 7006600000000000000♠6 MW, while some produce much more. The Eurostar, for example, uses more than 7007120000000000000♠12 MW, while heavy diesel-electric locomotives typically produce/use 7000300000000000000♠3 and 7006500000000000000♠5 MW. U.S. nuclear power plants have net summer capacities between about 7002500000000000000♠500 and 7009130000000000000♠1300 MW.

Gigawatt

The gigawatt (GW) is equal to one billion watts (10⁹ W) or 1 gigawatt = 1000 megawatts. This unit is often used for large power plants or power grids. For example, by the end of 2010, power shortages in China's Shanxi province were expected to increase to 5–6 GW and the installed capacity of wind power in Germany was 25.8 GW. The largest unit (out of four) of the Belgian Doel Nuclear Power Station has a peak output of 1.04 GW. HVDC converters have been built with power ratings of up to 2 GW.

Terawatt

The terawatt (TW) is equal to one trillion watts (10¹² W). The primary energy used by humans worldwide was about 160,000 terawatt-hours in 2019, corresponding to an average continuous power consumption of 18 TW that year. The most powerful lasers from the mid-1960s to the mid-1990s produced power in terawatts, but only for nanosecond intervals. The average lightning strike peaks at 1 TW, but these strikes only last for 30 microseconds.

Petawatt

The petawatt (PW) is equal to one quadrillion watts (10¹⁵ W) and can be produced by the current generation of lasers for time scales on the order of picoseconds (10⁻¹² s). One such laser is the Lawrence Livermore's Nova laser, which achieved a power output of 1.25 PW (7015125000000000000♠1,25×10¹⁵ W) by a process called chirped pulse amplification. The duration of the pulse was roughly 0.5 ps (6987500000000000000♠5×10⁻¹³ s), giving a total energy of 600 J. Another example is the Laser for Fast Ignition Experiments (LFEX) at the Institute of Laser Engineering (ILE), Osaka University, which achieved a power output of 2 PW for a duration of approximately 1 ps.

Based on the average total solar irradiance of 1.366 kW/m², the total power of sunlight striking Earth's atmosphere is estimated at 174 PW. The planet's average rate of global warming, measured as Earth's energy imbalance, reached about 0.5 PW (0.3% of incident solar power) by 2019.

Киловат-час

Јединица за снагу помножена јединицом за време је чест начин исказивања енергије. На пример, један киловат час, количина енергије коју утроши направа од једног киловата за један сат (1 сат = 3600 секунди) и износи 3,6 мегаџула.

$1\,{\mbox{kWh}}=3.6\,{\mbox{MJ}}$

Ова јединица се често користи у контексту електрана и рачуна за струју. Мегават дан (MWd или MW·d) је једнак 86,4 GJ.

Остале јединице снаге

• 1 ват ≈ 3.412141630 BTU/h
• 1 ват = 1 волт × 1 ампер
• 1 коњска снага ≈ 745.700 W (заокружено са 745.69987158227022 W)
• 1 коњска снага (електрична, Велика Британија) = 746 W
• 1 коњска снага (електрична, Европа) = 736 W
• 1 коњска снага ("метрична") = 735.498 75 W

Види још

• Киловат час
• Конверзија мерних јединица

Литература

• E Richard Cohen; Tom Cvitas; Jeremy G Frey; Bertil Holstrom; John W Jost, ур. (2007). Quantities, Units and Symbols in Physical Chemistry (PDF). International Union of Pure and Applied Chemistry (3. изд.). Royal Society of Chemistry; 3rd edition. ISBN 0854044337. 

• International Union of Pure and Applied Chemistry (1993). Quantities, Units and Symbols in Physical Chemistry, 2nd edition, Oxford: Blackwell Science. ISBN 0-632-03583-8. Electronic version.

Nelson, Robert A. (фебруар 2000). The International System of Units: Its History and Use in Science and Industry. Via Satellite. ATI courses. CS1 одржавање: Формат датума (веза)

Спољашње везе

• Borvon, Gérard. „History of the electrical units”.