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Joule

Joule

The joule (symbol: J) is the SI unit of energy, or work. It is named in honour of the physicist James Prescott Joule (1818–1889).

Definition

The joule is a derived unit defined as the work done, or energy required, to exert a force of one newton for a distance of one metre, so the same quantity may be referred to as a newton metre or newton-metre (also with meter spelling), with the symbol N·m or N m. It can also be written as kg·m²·s⁻². However, the newton metre is usually used as a measure of torque, not energy. One joule is also:
- The work required to move an electric charge of one coulomb through an electrical potential difference of one volt; or one coulomb volt, with the symbol C·V.
- The work done to produce power of one watt continuously for one second; or one watt second (compare kilowatt-hour), with the symbol W·s

Conversions

1 joule is exactly 10⁷ erg. 1 joule is approximately equal to:
- 6.241506363 eV (electron-volts)
- 0.239 cal (calorie) (small calories)
- 2.390 Calorie or kilocalorie (food)
- 9.48 BTU (British thermal unit)
- 0.738 ft·lbf (foot pound force)
- 23.7 ft·pdl (foot poundals)
- 2.7778 kilowatt-hour
- 2.7778 watt-hour
- 9.8692 litre-atmosphere
- the energy required to lift a small apple (102 g) one metre against Earth's gravity Units defined in terms of the joule include:
- 1 thermochemical calorie = 4.184 J (exact)
- 1 International Table calorie = 4.1868 J (exact)
- 1 watt-hour = 3600 J (exact)

SI

The International System of Units (abbreviated SI from the French language name Système International d'Unités) is the modern form of the metric system. It is the world's most widely used system of units, both in everyday commerce and in science. The older metric system included several groupings of units. The SI was developed in 1960 from one of these, the metre-kilogram-second (MKS) system, rather than the centimetre-gram-second (CGS) system, which, in turn, had many variants. The SI introduced several newly named units. The SI is not static; it is a living set of standards where units are created and definitions are modified with international agreement as measurement technology progresses. With few exceptions (such as draught beer sales in the United Kingdom), the system is legally being used in every country in the world, and many countries do not maintain official definitions of other units. In the United States, industrial use of SI is increasing, but popular use is still limited. In the United Kingdom, conversion to metric units is official policy but not yet complete. Those countries that still recognize non-SI units (e.g. the US and UK) have redefined most of their traditional, non-SI units in terms of SI units.

History

:See main articles: metre, kilogram, second, ampere, Kelvin, and candela. The metric system was officially adopted in France after the French Revolution. During the history of the metric system a number of variations have evolved and their use spread around the world replacing many traditional measurement systems. By the end of World War II a number of different systems of measurement were still in use throughout the world. Some of these systems were metric system variations whilst others were based on the Imperial and American systems. It was recognised that additional steps were needed to promote a worldwide measurement system. As a result the 9th General Conference on Weights and Measures (CGPM), in 1948, asked the International Committee for Weights and Measures (CIPM) to conduct an international study of the measurement needs of the scientific, technical, and educational communities. Based on the findings of this study, the 10th CGPM in 1954 decided that an international system should be derived from six base units to provide for the measurement of temperature and optical radiation in addition to mechanical and electromagnetic quantities. The six base units recommended were the metre, kilogram, second, ampere, Kelvin degree (later renamed the kelvin), and the candela. In 1960, the 11th CGPM named the system the International System of Units, abbreviated SI from the French name: Le Système International d'Unités. The seventh base unit, the mole, was added in 1970 by the 14th CGPM. The International System is now either obligatory or permissible throughout the world. It is administered by the standards organisation: the Bureau International des Poids et Mesures (International Bureau of Weights and Measures).
Units
:Main articles: SI base unit, SI derived unit, SI prefix The international system of units consists of a set of units together with a set of prefixes. The units of SI can be divided into two subsets. There are the seven base units. Each of these base units are dimensionally independent. From these seven base units several other units are derived. In addition to the SI units there are also a set of non-SI units accepted for use with SI. A prefix may be added to units to produce a multiple of the original unit. All multiples are integer powers of ten. For example, kilo- denotes a multiple of a thousand and milli- denotes a multiple of a thousandth hence there are one thousand millimetres to the metre and one thousand metres to the kilometre. The prefixes are never combined: a millionth of a kilogram is a milligram not a microkilogram.
SI writing style

- Symbols are written in lower case, except for symbols derived from the name of a person. For example, the unit of pressure is named after Blaise Pascal, so its symbol is written "Pa" whereas the unit itself is written "pascal". The one exception is the litre, whose original abbreviation "l" is dangerously similar to "1". The NIST recommends that "L" be used instead, a usage which is common in the U.S., Canada and Australia, and has been accepted as an alternative by the CGPM. The cursive "ℓ" is occasionally seen, especially in Japan, but this is not currently recommended by any standards body. For more information, see Litre.
- Symbols are written without grammatical markers when used with singular numerals: i.e. "25 kg", not "25 kgs". Pluralization would be language dependent; "s" plurals (as in French and English) are particularly undesirable since "s" is the symbol of the second. Other cases may be marked in a language-dependent manner, e.g. Finnish 25 kg:lla = 25 kilogrammalla "with 25 kg".
- Symbols do not have an appended period (.).
- It is preferable to write symbols in upright Roman type (m for metres, L for litres), so as to differentiate from the italic type used for mathematical variables (m for mass, l for length).
- A space should separate the number and the symbol, e.g. "2.21 kg", "7.3×10² m²", "22 °C" [http://physics.nist.gov/Pubs/SP811/sec07.html]. Exceptions are the symbols for plane angular degrees, minutes and seconds (°, ′ and ″), which are placed immediately after the number with no intervening space.
- Spaces should be used to group decimal digits in threes, e.g. 1 000 000 or 342 142 (in contrast to the commas or dots used in other systems, e.g. 1,000,000 or 1.000.000).
- The 10th resolution of CGPM in 2003 declared that "the symbol for the decimal marker shall be either the point on the line or the comma on the line". In practice, the full stop is used in English, and the comma in most other European languages.
- Symbols for derived units formed from multiple units by multiplication are joined with a space or centre dot (·), e.g. N m or N·m.
- Symbols formed by division of two units are joined with a solidus (/), or given as a negative exponent. For example, the "metre per second" can be written "m/s", "m s^-1", "m·s^-1" or $\frac$ . A solidus should not be used if the result is ambiguous, i.e. "kg·m^-1·s^-2" is preferable to "kg/m/s²".
Spelling variations

- Several nations, notably the United States, typically use the spellings 'meter' and 'liter' instead of 'metre' and 'litre' in keeping with standard American English spelling. In addition, the official US spelling for the SI prefix 'deca' is 'deka'.
- The unit 'gram' is also sometimes spelled 'gramme' in English-speaking countries other than the United States, though that is an older spelling and its use is declining.
Cultural issues
The swift worldwide adoption of the metric system as a tool of economy and everyday commerce was based mainly on the lack of customary systems in many countries to adequately describe some concepts, or as a result of an attempt to standardize the many regional variations in the customary system. International factors also affected the adoption of the metric system, as many countries increased their trade. Scientifically, it provides ease when dealing with very large and small quantities because it lines up so well with our decimal numeral system. Cultural differences can be represented in the local everyday uses of metric units. For example, bread is sold in one-half, one or two kilogram sizes in many countries, but you buy them by multiples of one hundred grams in the former USSR. In some countries, the informal cup measurement has become 250 mL, and prices for items are sometimes given per 100 g rather than per kilogram. A profound cultural difference between physicists and engineers, especially radio engineers, existed prior to the adoption of the metre-kilogram-second (MKS) system and hence its descendent, SI. Engineers work with volts, amperes, ohms, farads, and coulombs, which are of great practical utility, while the centimetre-gram-second (CGS) units, which, though appropriate for theoretical physics, can be inconvenient for electrical engineering usage and are largely unfamiliar to householders using appliances rated in volts and watts. People with diabetes test their plasma glucose level regularly. In the U.S., measurement are recorded in milligrams per deciliter (mg/dL); in Europe, the standard is millimole/liter (mmol/L). The fine-tuning that has happened to the metric base units over the past 200 years, as experts have tried periodically to refine the metric system to fit the best scientific research do not affect the everyday use of metric units. Since most non-SI units, such as the U.S. customary units, are nowadays defined in terms of SI units, any change in the definition of the SI units results in a change of the definition of the older units as well.
See also

- Units of measurement
- Weights and measures
- Mesures usuelles
- Metrified English unit
- History of measurement
- Other systems of measurement:
- Imperial units
- U.S. customary units
- Metre-tonne-second system of units
- Chinese system of units
- Planck units
- Atomic units
- Geometrized units
- CODATA
- Metrication
- Metric system in the United States
- Metrology
- UTC (Coordinated Universal Time)
- Binary prefixes - used to quantify large amounts of computer data
- Orders of magnitude
- ISO 31
External links
Official
- [http://www.bipm.fr/en/si/ BIPM (SI maintenance agency)] (home page)
- [http://www.bipm.org/en/si/si_brochure/ BIPM brochure] (SI reference)
- [http://www.iso.ch/iso/en/CatalogueDetailPage.CatalogueDetail?CSNUMBER=5448&ICS1=1 ISO 1000:1992 SI units and recommendations for the use of their multiples and of certain other units], with its price tag of 99 Swiss francs for a 22 page, coverless pamphlet showing why the public is sometimes a little slow to pick up on their recommendations. Information
- [http://physics.nist.gov/cuu/Units/index.html US NIST reference on SI]
- [http://ts.nist.gov/ts/htdocs/200/202/pub814.htm#chart chart]
- [http://www.aticourses.com/international_system_units.htm SI - Its history and use in science and industry]
- [http://www.unc.edu/~rowlett/units/ A Dictionary of Units of Measurement]
- [http://www.unics.uni-hannover.de/ntr/russisch/si-einheiten.html5 Cyrillic transcription of SI symbols]
- Judson, Lewis B., Weights and Measures Standards of the United States: A brief history, NBS Special Publication 447, orig. iss. October 1963, updated March 1976 ([http://ts.nist.gov/ts/htdocs/200/202/SP%20447.pdf 46 page PDF file])
- [http://www.france-property-and-information.com/metric_conversion_table.htm Metric system and conversion tables (courtesy French property advice)]
- [http://www.metre.info metre-info - an encyclopaedia of all metric units] Pro-metric pressure groups
- [http://www.ukma.org.uk/ The UK Metric Association]
- [http://www.metric.org/ The US Metric Association] Pro-customary measures pressure groups
- [http://www.bwmaonline.com/ The British Weights and Measures Association]
Further reading

- I. Mills, Tomislav Cvitas, Klaus Homann, Nikola Kallay, IUPAC: Quantities, Units and Symbols in Physical Chemistry, 2nd ed., Blackwell Science Inc 1993, ISBN 0632035838. Category:SI units Category:Systems of units Category:International standards Category:Dimensional analysis ko:SI 단위계 ja:国際単位系 simple:SI th:หน่วยเอสไอ

Mechanical work

Work (abbreviated W) is the energy transferred by a force to a moving object. Work is a scalar quantity, but it can be positive or negative. It is associated with a change in energy, but not all changes in energy can be readily analysed in terms of work.

Definition

Note: Readers not familiar with multivariate calculus or vectors, please see "Simpler formulae" below Work is defined as the following line integral :

W = \int_ \vec F \cdot \vec \,\!

where :C is the path or curve traversed by the object; :

\vec F

is the force vector; :

\vec s

is the position vector.

Units

The SI derived unit of work is the joule (J), which is defined as the work done by a force of one newton acting over a distance of one metre. The dimensionally equivalent newton-meter (N·m) is sometimes used instead; however, it is also sometimes reserved for torque to distinguish its units from work or energy. Non-SI units of work include the erg, the foot-pound, and the foot-poundal.

Simpler formulae

In the simplest case, that of a body moving in a steady direction, and acted on by a force parallel to that direction, the work is given by the formula :

W = \mathbf s \,\!

where : F is the force and : s is the distance traveled by the object. The work is taken to be negative when the force opposes the motion. More generally, the force and distance are taken to be vector quantities, and combined using the dot product: :

W = \vec F \cdot \vec \mathbf = |\mathbf| |\mathbf| \cos\phi \,\!

where

\phi

is the angle between the force and the displacement vector. This formula holds true even when the force acts at an angle to the direction of travel. To further generalize the formula to situations in which the force and the object's direction of motion changes over time, it is necessary to use differentials, d, to express the infinitesimal work done by the force over an infinitesimal displacement, thus: :

dW = \vec F \cdot \vec \,\!

The integration of both sides of this equation yields the most general formula, as given above.

Types of work

Forms of work that are not evidently mechanical, such as electrical work, can be considered as special cases of this principle; for instance, in the case of electricity, work is done on charged particles moving through a medium. Heat conduction from a warmer body to a colder one is not normally considered to be a form of mechanical work, because at the macroscopic level, there is no measurable force. At the atomic level, there are forces as the atoms collide, but they average to nearly zero in bulk. Not all forces do work. For instance, a centripetal force in uniform circular motion does not transfer energy; the kinetic energy of the object undergoing the motion remains constant. This fact is confirmed by the formula: if the vectors of force and displacement are perpendicular, their dot product is zero. This does not mean the centripetal force is doing nothing; the velocity, and hence the momentum of the object undergoing circular motion, is changing continuously. Momentum is a vector quantity, change of direction is a change of momentum. A type of work in chemical thermodynamics is PV work, which involves a change in volume. PV work can be applied to the conservation of energy. Like all work functions, PV work is not a state function; it is path-dependent. PV work is represented by the following equation: w = -PΔV w = work P = external pressure ΔV = change in volume The units of PV work are Litre-atmosphere, where 1 L-atm = 101.3 J.

Mechanical energy

The mechanical energy of a body is that part of its total energy which is subject to change by mechanical work. It includes kinetic energy and potential energy. Some notable forms of energy that it does not include are thermal energy (which can be increased by frictional work, but not easily decreased) and rest energy (which is constant so long as the rest mass remains the same).

Conservation of mechanical energy

The principle of conservation of mechanical energy states that, if a system is subject only to conservative forces (e.g. only to gravitational force), its mechanical energy remains constant. For instance, if an object with constant mass is in free fall, the total energy of position 1 will be equal position 2. :

(E_k + E_p)_1 = (E_k + E_p)_2 \,\!

where
-

E_k

is the kinetic energy, and
-

E_p

is the potential energy.

References

-
- ms:Kerja Dalam Mekanik ja:仕事 (物理学) Work, Mechanical Category:Introductory physics

Force

:For other senses of this word, see force (disambiguation). In physics, a force is an external cause responsible for any change of a physical system. For instance, a person holding a dog by a rope is experiencing the force applied by the rope on their hand, and the cause for its pulling forward is the force exercised by the rope. The kinetic expression of this change is, according to Newton's second law, acceleration, non kinetic expressions such as deformation can also occur. The SI unit for force is the newton.

Elementary concepts

Force in its most primitive definition can be thought of as that which when acting alone causes an object to accelerate. In a practical sense forces can be divided into two groups: contact forces and field forces. Contact forces require the physical contact of one object with other such as a hammer striking a nail or the force exerted by a gas under pressure - gas produced by exploding gunpowder forces a heavy ball out of a cannon. Field forces on the other hand need no physical medium of contact. Gravity and magnetism are examples of such forces. It should be noted however, that fundamentally all forces are in fact field forces. The force of hammer striking the nail in the previous example turns out to be a clash of the electric forces in both hammer and nail. Nevertheless it is appropriate in some cases to maintain these two classifications for ease of understanding.

Quantitative definition

In physics models, the point-like system is used, where objects are represented as one-dimensional points at their centre of mass. The only change the system can experience is a change of its momentum (its speed). Since the rise of the atomic theory, any physical system has been considered in classical physics as composed of point-like systems called atoms or molecules. Therefore, all forces can be defined by their effect; that is, by the change of movement they induce on point-like systems. This change of movement can be quantified by the acceleration (the derivative of velocity). The discovery by Isaac Newton that a given force will induce an acceleration in inverse proportion to a quantity called the mass of inertia or inertial mass which is independent of the speed of the system is Newton's second law. This law allows us to predict the effect of a force on any point-like system whose mass is known. It is usually written as: :F = dp/dt = d(m·v)/dt = m·a (in the case where m does not depend on t) where :F is the force (a vector quantity), :p is the momentum, :t is the time, :v is the velocity, :m is the mass, and :a=d²x/dt² is the acceleration, the second derivative with respect to t of the position vector x. If the mass m is measured in kilograms and the acceleration a is measured in metres per second squared, then the unit of force is kilogram × metre/second squared. This unit is called the newton: 1 N = 1 kg x 1 m/s². This equation is a system of three second-order differential equations with respect to the three-dimensional position vector which is an unknown function of time. This equation can be solved if F is a known function of x and some of its derivatives and if the mass m is known. Morevover the boundary conditions are required; for example, the values of the position vector and x and the velocity v at the starting time, say t=0. Of course, this formula is only useful if one knows the numerical values of F and m. The definition above is an implicit definition, arrived at as follows. One defines a reference system (one litre of water) and a reference force (the gravitational force applied by the Earth on it at the altitude of Paris). One takes Newton's second law for granted (one postulates that it is true) and measures the acceleration induced by the reference force on the reference system. This gives us a mass unit (1 kg) and a force unit (the older unit of 1 kilogram-force = 9.81 N). Once this is done, one can measure any force by the acceleration it induces on the reference system and measure the inertial mass of any system by measuring the acceleration induced on this system by the reference force. Force is often considered a fundamental quantity in physics, but there are more fundamental quantities, such as momentum (p = mass m x velocity v). Energy, measured in joules, is still less fundamental than force and momentum, because it is defined as work, and work is defined in terms of force. The two most fundamental theories of nature - quantum electrodynamics and general relativity - do not contain the concept of force at all. Although not the most fundamental quantity in physics, force is an important basic mathematical concept from which other concepts, such work and pressure (measured in pascals), are derived. Force is sometimes confused with stress.

Types of force

There are four known fundamental forces in nature.
- Nuclear forces acting between subatomic particles
- Electromagnetic forces between electric charges
- Weak forces arising from radioactive decay
- Gravitational forces between masses Quantum field theory accurately models the first three fundamental forces, but does not model quantum gravity. Quantum gravity on a large scale can, however, be described by general relativity. The four fundamental forces describe every observable phenomenon including the many other forces observed such as: Coulomb's force (the force between electrical charges), gravitational force (force between masses), magnetic force, frictional forces, centripetal, centrifugal, impact force, and spring force, to name a few. Forces can also be classified into conservative forces and nonconservative forces. Conservative forces are equivalent to the gradient of a potential, and include gravity, electromagnetic force, and spring force. Nonconservative forces include friction and drag.

Properties of force

Because momentum is a vector, then force, being its time derivative, is also a vector - it has magnitude and direction. Forces can be added together using the parallelogram of force. When two forces act on an object, the resulting force, the resultant, is the vector sum of the original forces. This is called the principle of superposition. The magnitude of the resultant varies from zero to the sum of the magnitudes of the two forces, depending on the angle between their lines of action. If the two forces are equal, but opposite, the resultant is zero. This condition is called static equilibrium, with the result that the object remains at rest or moves with a constant velocity. As well as being added, forces can be can also be broken down (or 'resolved'). For example, a horizontal force pointing northeast can be split into two forces, one pointing north, and one pointing east. Summing these component forces using vector addition yields the original force. Force vectors can also be three-dimensional, with the third (vertical) component at right-angles to the two horizontal components.

Forces in theory

The total (Newtonian) force, in newtons, on an object at any given time is defined as the rate of change of the object's velocity multiplied by the object's mass: :

\mathbf = \lim_ \frac

where :m is the inertial mass of the particle (measured in kilograms) :v_o is its initial velocity (measured in metres per second) :v is its final velocity (measured in metres per second) :T is the time from the initial state to the final state (measured in seconds); :Lim T→0 is the limit as T tends towards zero. Force was so defined to explain the effects of superimposing situations: if in one situation, a force is experienced by a particle, and if in another situation another force is experienced by that particle, then in a third situation, which (according to standard physical practice) is taken to be a combination of the two individual situations, the force experienced by the particle will be the vector sum of the individual forces experienced in the first two situations. This superposition of forces, and the definition of inertial frames and inertial mass, are the empirical content of Newton's laws of motion. There are other ways to look at the above definition of force. First, the mass of a body multiplied by its velocity is called its momentum, p, so the above definition is equivalent to: :

\textbf=

If F is not constant over Δt, then this is the definition of average force over the time interval. To apply it at an instant we apply an idea from calculus. If we graph p as a function of time, the average force will be the slope of the line connecting the momentum at two times. Taking the limit as the two times get closer together gives the slope at an instant, which is called the derivative: :

\textbf=

Many forces are associated with a potential energy field. For instance, the gravitational force acting upon a body can be seen as the action of the gravitational field that is present at the body's location. The potential field is defined as that field whose gradient is equal and opposite to the force produced at every point: :

\textbf=-\nabla U

The derivative of force with respect to time is called yank. Higher order derivatives are sometimes used, but they lack names because of their rarity. In most explanations of mechanics, force is usually defined only implicitly, in terms of the equations that work with it. Some physicists, philosophers and mathematicians, such as Ernst Mach, Clifford Truesdell and Walter Noll, have found this problematic and sought a more explicit definition of force. According to the Special theory of relativity the mass of an object increases as it's velocity and therefore it's energy increases. The law of force must then be modified to the following: :

\textbf=

where :v is the mass's velocity :c is the speed of light. Note that the equation is undefined if the mass's speed is equal to c because one then has to divide by zero. This is one reason most physicists believe an object with nonzero rest mass can not be accelerated to the speed of light, as this would require an infinite force.

Units of measurement

The SI unit used to measure force is the newton (symbol N), which is equivalent to kg·m·s⁻².

Non-SI units of force and mass

The F=m·a relationship can be used with any consistent units (SI or CGS). If these units are not consistent, a more general form, F=k·m·a, can be used, where the constant k is a conversion factor dependent upon the units being used. For example, in imperial engineering units, F is measured in "pounds force" or "lbf", m in "pounds mass" or "lb", and a in feet per second squared. In this particular system, one needs to use the more general form above, usually written F=m·a/g_c with the constant normally used for this purpose g_c = 32.174 lb·ft/(lbf·s²) equal to the reciprocal of the k above. As with the kilogram, the pound is colloquially used as both a unit of mass and a unit of force. 1 lbf is the force required to accelerate 1 lb at 32.174 ft per second squared, since 32.174 ft per second squared is the standard acceleration due to terrestrial gravity. Another imperial unit of mass is the slug, defined as 32.174 lb. It is the mass that accelerates by one foot per second squared when a force of one lbf is exerted on it. When the standard gee (an acceleration of 9.80665 m/s²) is used to define pounds force, the mass in pounds is numerically equal to the weight in pounds force. However, even at sea level on Earth, the actual acceleration of free fall is quite variable, over 0.53% more at the poles than at the equator. Thus, a mass of 1.0000 lb at sea level at the equator exerts a force due to gravity of 0.9973 lbf, whereas a mass of 1.000 lb at sea level at the poles exerts a force due to gravity of 1.0026 lbf. The normal average sea level acceleration on Earth (World Gravity Formula 1980) is 9.79764 m/s², so on average at sea level on Earth, 1.0000 lb will exerts a force of 0.9991 lbf. The equivalence 1 lb = 0.453 592 37 kg is always true, by definition, anywhere in the universe. If you use the standard gee which is official for defining kilograms force to define pounds force as well, then the same relationship will hold between pounds-force and kilograms-force (an old non-SI unit is still used). If a different value is used to define pounds force, then the relationship to kilograms force will be slightly different—but in any case, that relationship is also a constant anywhere in the universe. What is not constant throughout the universe is the amount of force in terms of pounds-force (or any other force units) which 1 lb will exert due to gravity. By analogy with the slug, there is a rarely used unit of mass called the "metric slug". This is the mass that accelerates at one metre per second squared when pushed by a force of one kgf. An item with a mass of 10 kg has a mass of 1.01972661 metric slugs (= 10 kg divided by 9.80665 kg per metric slug). This unit is also known by various other names such as the hyl, TME (from a German acronym), and mug (from metric slug). Another unit of force called the poundal (pdl) is defined as the force that accelerates 1 lbm at 1 foot per second squared. Given that 1 lbf = 32.174 lb times one foot per second squared, we have 1 lbf = 32.174 pdl. The kilogram-force is a unit of force that was used in various fields of science and technology. In 1901, the CGPM improved the definition of the kilogram-force, adopting a standard acceleration of gravity for the purpose, and making the kilogram-force equal to the force exerted by a mass of 1 kg when accelerated by 9.80665 m/s². The kilogram-force is not a part of the modern SI system, but is still used in applications such as:
- Thrust of jet and rocket engines
- Spoke tension of bicycles
- Draw weight of bows
- Torque wrenches in units such as "meter kilograms" or "kilogram centimetres" (the kilograms are rarely identified as units of force)
- Engine torque output (kgf·m expressed in various word orders, spellings, and symbols)
- Pressure gauges in "kg/cm²" or "kgf/cm²" In colloquial, non-scientific usage, the "kilograms" used for "weight" are almost always the proper SI units for this purpose. They are units of mass, not units of force. The symbol "kgm" for kilograms is also sometimes encountered. This might occasionally be an attempt to disintinguish kilograms as units of mass from the "kgf" symbol for the units of force. It might also be used as a symbol for those obsolete torque units (kilogram-force metres) mentioned above, used without properly separating the units for kilogram and metre with either a space or a centered dot.

Conversions

Below are several coversion factors between various mesurements of force:
- 1 kgf (kilopond kp) = 9.80665 newtons
- 1 metric slug = 9.80665 kg
- 1 lbf = 32.174 poundals
- 1 slug = 32.174 lb
- 1 kgf = 2.2046 lbf

Forces in everyday life

Forces are part of everyday life, with examples such as:
- gravity: objects fall, even after being thrown upwards, or slide and roll down
- friction: floors and objects that are not extremely slippery
- spring force, objects resist tensile stress, compressive stress and/or shear stress, objects bounce back.
- electromagnetic force: attraction of magnets
- movement created by force: the movement of objects when force is applied.

Forces in the laboratory

Founding experiments

- Galileo Galilei used rolling balls to disprove the Aristotelian theory of motion (1602 - 1607)
- Henry Cavendish's torsion bar experiment measured the force of gravity between two masses (1798)

Instruments to measure forces

- spring balance
- pivot balance
- forcemeter

History

Force was first described by Archimedes.

References

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-
-

External links

- [http://jumk.de/calc/force.shtml Calculation: force F - English and American units to metric units]
- [http://calc.skyrocket.de/en/ Online Unit Converter - Conversion of many different units]
- [http://www.patbelford.com/gallery/web3d/education/forceworkpower/index.html Interactive demonstration of Force-Work-Power Relationship] Category:Introductory physics ko:힘 ms:Daya (fizik) ja:力 simple:Force (physics)

Metre

:This article is about the unit of length. For other uses of metre or meter, see meter (disambiguation). The metre (Commonwealth English) or meter (American English) (symbol: m) is the SI base unit of length. It is defined as the length of the path travelled by light in absolute vacuum during a time interval of 1/299,792,458 of a second. Adding SI prefixes to metre creates multiples and submultiples; for example kilometre (1000 metres; kilo- = 1000) and millimetre (one thousandth of a metre; milli- = 1 / 1 000).

Conversions

1 metre is equivalent to:
- exactly 1/0.9144 yards (approximately 1.0936 yards)
- exactly 1/0.3048 feet (approximately 3.2808 feet)
- exactly 10000/254 inches (approximately 39.370 inches)

History

The word metre is from the Greek metron (μετρον), "a measure" via the French mètre. Its first recorded usage in English is from 1797. In the 18th century, there were two favoured approaches to the definition of the standard unit of length. One suggested defining the metre as the length of a pendulum with a half-period of one second. The other suggested defining the metre as one ten-millionth of the length of the earth's meridian along a quadrant (one-fourth the polar circumference of the earth). In 1791, the French Academy of Sciences selected the meridional definition over the pendular definition because of the slight variation of the force of gravity over the surface of the earth, which affects the period of a pendulum. In 1793, France adopted the metre, with this definition, as its official unit of length. Although it was later determined that the first prototype metre bar was short by a fifth of a millimetre due to miscalculation of the flattening of the earth, this length became the standard. So, the circumference of the Earth through the poles is approximately forty million metres. Earth in a vacuum.]] In the 1870s and in light of modern precision, a series of international conferences were held to devise new metric standards. The Metre Convention (Convention du Mètre) of 1875 mandated the establishment of a permanent International Bureau of Weights and Measures (BIPM: Bureau International des Poids et Mesures) to be located in Sèvres, France. This new organisation would preserve the new prototype metre and kilogram when constructed, distribute national metric prototypes, and would maintain comparisons between them and non-metric measurement standards. This organisation created a new prototype bar in 1889 at the first General Conference on Weights and Measures (CGPM: Conférence Générale des Poids et Mesures), establishing the International Prototype Metre as the distance between two lines on a standard bar of an alloy of ninety percent platinum and ten percent iridium, measured at the melting point of ice. In 1893, the standard metre was first measured with an interferometer by Albert A. Michelson, the inventor of the device and an advocate of using some particular wavelength of light as a standard of distance. By 1925, interferometry was in regular use at the BIPM. However, the International Prototype Metre remained the standard until 1960, when the eleventh CGPM defined the metre in the new SI system as equal to 1,650,763.73 wavelengths of the orange-red emission line in the electromagnetic spectrum of the krypton-86 atom in a vacuum. The original international prototype of the metre is still kept at the BIPM under the conditions specified in 1889. To further reduce uncertainty, the seventeenth CGPM of 1983 replaced the definition of the metre with its current definition, thus fixing the length of the metre in terms of time and the speed of light: :The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second. Note that this definition exactly fixes the speed of light in a vacuum at 299,792,458 metres per second. Definitions based on the physical properties of light are more precise and reproducible because the properties of light are considered to be universally constant.

Timeline of definition

- 1790 May 8 — The French National Assembly decides that the length of the new metre would be equal to the length of a pendulum with a half-period of one second.
- 1791 March 30 — The French National Assembly accepts the proposal by the French Academy of Sciences that the new definition for the metre be equal to one ten-millionth of the length of the earth's meridian along a quadrant (one-fourth the polar circumference of the earth).
- 1795 — Provisional metre bar constructed of brass.
- 1799 December 10 — The French National Assembly specifies that the platinum metre bar, constructed on 23 June 1799 and deposited in the National Archives, as the final standard.
- 1889 September 28 — The first CGPM defines the length as the distance between two lines on a standard bar of an alloy of platinum with ten percent iridium, measured at the melting point of ice.
- 1927 October 6 — The seventh CGPM adjusts the definition of the length to be the distance, at 0 °C, between the axes of the two central lines marked on the prototype bar of platinum-iridium, this bar being subject to one standard atmosphere of pressure and supported on two cylinders of at least one centimetre diameter, symmetrically placed in the same horizontal plane at a distance of 571 millimetres from each other.
- 1960 October 20 — The eleventh CGPM defines the length to be equal to 1,650,763.73 wavelengths in vacuum of the radiation corresponding to the transition between the 2p¹⁰ and 5d⁵ quantum levels of the krypton-86 atom.
- 1983 October 21 — The seventeenth CGPM defines the length to be distance travelled by light in vacuum during a time interval of 1/299 792 458 of a second.

External links

- [http://www.unitconversion.org/unit_converter/length.html?unit=meter&value=1 Length Converter: convert metre to other units, such as yard, mile, and so on]
- [http://physics.nist.gov/cuu/Units/meter.html History of the metre at the U.S. National Institute of Standards and Technology (NIST)]
- [http://www.mel.nist.gov/div821/museum/timeline.htm Timeline of history of the metre at the NIST]
- [http://www1.bipm.org/en/scientific/length/ Bureau International des Poids et Measures - Lengths] Category:SI base units Category:Units of length ko:미터 ms:Meter ja:メートル simple:Metre th:เมตร

Metre

Conversions

1 metre is equivalent to:
- exactly 1/0.9144 yards (approximately 1.0936 yards)
- exactly 1/0.3048 feet (approximately 3.2808 feet)
- exactly 10000/254 inches (approximately 39.370 inches)

History

Timeline of definition

External links

Torque

: For the game engine, see Torque Game Engine. For the item of jewelry, see torc. In physics, torque can be thought of informally as "rotational force". Torque is measured in units of newton metres. The concept of Torque, also called moment or couple, originated with the work of Archimedes on levers. The rotational analogues of force, mass and acceleration are torque, moment of inertia and angular acceleration respectively. The force applied to a lever, multiplied by its distance from the lever's fulcrum, is the torque. For example, a force of three newtons applied two metres from the fulcrum exerts the same torque as one newton applied six metres from the fulcrum. This assumes the force is in a direction at right angles to the straight lever. More generally, one may define torque as the cross product: :

\boldsymbol = \mathbf \times \mathbf

where r is the vector from the axis of rotation to the point on which the force is acting F is the vector of force.

Units

Torque has dimensions of distance × force and the SI units of torque are stated as "newton-metres". Even though the order of "newton" and "metre" are mathematically interchangeable, the BIPM (Bureau International des Poids et Mesures) specifies that the order should be N·m not m·N[http://www1.bipm.org/en/si/derived_units/2-2-2.html]. The joule, the SI unit for energy or work, is also defined as 1 N·m, but this unit is not used for torque. Of course, the dimensional equivalence of these units is not simply a coincidence; a torque of 1 N·m applied through a full revolution will require an energy of exactly 2π joules. Mathematically, :

E=\tau\theta

where E is the energy τ is torque θ is the angle moved, in radians. Other non-SI units of torque include "pound-force-feet" or "foot-pounds-force" or "ounce-force-inches or meter-kilograms-force.

Special cases and other facts

Moment arm formula

kilograms-force A very useful special case, often given as the definition of torque in fields other than physics, is as follows: :

\boldsymbol = (\textrm) \times \textrm

The construction of the "moment arm" is shown in the figure below, along with the vectors r and F mentioned above. The problem with this definition is that it does not give the direction of the torque but only the magnitude, and hence it is difficult to use in three-dimensional cases. If the force is perpendicular to the displacement vector r, the moment arm will be equal to the distance to the centre, and torque will be a maximum for the given force. The equation for the magnitude of a torque arising from a perpendicular force: :

\boldsymbol = (\textrm) \times \textrm

For example, if a person places a force of 10 N on a spanner which is 0.5m long, the torque will be 5 N·m, assuming that the person pulls the spanner in the direction best suited to turning bolts.

Force at an angle

If a force of magnitude F is at an angle θ from the displacement arm of length r (and within the plane perpendicular to the rotation axis), then from the definition of cross product, the magnitude of the torque arising is: :

\boldsymbol \tau=rF \sin \theta

Static equilibrium

For an object to be at static equilibrium, not only must the sum of the forces be zero, but also the sum of the torques (moments) about any point. For a two-dimensional situation with horizontal and vertical forces, the sum of the forces requirement is two equations: ΣH = 0 and ΣV = 0, and the torque a third equation: Στ = 0. That is, to solve statically determinate equilibrium problems in two-dimensions, we use three equations.

Torque as a function of time

Torque is the time-derivative of angular momentum, just as force is the time derivative of linear momentum. For multiple torques acting simultaneously: :

\sum\boldsymbol =

where L is angular momentum. Angular momentum on a rigid body can be written in terms of its moment of inertia

\boldsymbol I

and its angular velocity

\boldsymbol

: :

\mathbf=I\,\boldsymbol

so if

\boldsymbol I

is constant, :

\boldsymbol=I=I\boldsymbol

where α is angular acceleration, a quantity usually measured in rad/s²

Machine torque

Torque is part of the basic specification of an engine: the power output of an engine is expressed as its torque multiplied by its rotational speed. Internal-combustion engines produce useful torque only over a limited range of rotational speeds (typically from around 1,000–6,000 rpm for a small car). The varying torque output over that range can be measured with a dynamometer, and shown as a torque curve. The peak of that torque curve usually occurs somewhat below the overall power peak. The torque peak cannot, by definition, appear at higher rpm than the power peak. Understanding the relationship between torque, power and engine speed is vital in automotive engineering, concerned as it is with transmitting power from the engine through the drive train to the wheels. The gearing of the drive train must be chosen appropriately to make the most of the motor's torque characteristics. Steam engines and electric motors tend to produce maximum torque at or around zero rpm, with the torque diminishing as rotational speed rises (due to increasing friction and other constraints). Therefore, these types of engines usually have quite different types of drivetrains from internal combustion engines. Torque is also the easiest way to explain mechanical advantage in just about every simple machine.

Relationship between torque and power

If a force is allowed to act through a distance, it is doing mechanical work. Similarly, if torque is allowed to act through a rotational distance, it is doing work. Power is the work per unit time. However, time and rotational distance are related by the angular speed where each revolution results in the circumference of the circle being travelled by the force that is generating the torque. This means that torque that is causing the angular speed to increase is doing work and the generated power may be calculated as: :

\mbox=\mbox \times \mbox

Mathematically, the equation may be rearranged to compute torque for a given power output. However in practice there is no direct way to measure power whereas torque and angular speed can be measured directly. Consistent units must be used. For metric SI units power is watts, torque is newton-metres and angular speed is radians per second (not rpm and not even revolutions per second).

Conversion to other units

For different units of power, torque or angular speed, a conversion factor must be inserted into the equation. For example, if the angular speed is measured in revolutions instead of radians, a conversion factor of

2 \pi

must be added because there are

2 \pi

radians in a revolution: :

\mbox=\mbox \times 2 \pi \times \mbox

, where rotational speed is in revolutions per unit time Some people (e.g. American automotive engineers) use horsepower (imperial mechanical) for power, foot-pounds (lbf·ft) for torque and rpm's (revolutions per minute) for angular speed. This results in the formula changing to: :

\mbox = \frac

This conversion factor is approximate because the transcendental number π appears in it; a more precise value is 5252.113 122 032 55... It also changes with the definition of the horsepower, of course; for example, using the metric horsepower, it becomes ~5180. Use of other units (e.g. BTU/h for power) would require a different custom conversion factor.

Derivation

For a rotating object, the linear distance covered at the circumference in a radian of rotation is the product of the radius with the angular speed. That is: linear speed = radius x angular speed. By definition, linear distance=linear speed x time=radius x angular speed x time. By the definition of torque: torque=force x radius. We can rearrange this to determine force=torque/radius. These two values can be substituted into the definition of power: :

\mbox = \frac=\frac  = \mbox \times \mbox

The radius r and time t have dropped out of the equation. However angular speed must be in radians, by the assumed direct relationship between linear speed and angular speed at the beginning of the derivation. If the rotational speed is measured in revolutions per unit of time, the linear speed and distance are increased proportionately by

2 \pi

in the above derivation to give: :

\mbox=\mbox \times 2 \pi \times \mbox

If torque is in lbf·ft and rotational speed in revolutions per minute, the above equation gives power in ft·lbf/min. The horsepower form of the equation is then derived by applying the conversion factor 33,000 ft·lbf/min per horsepower: :

\mbox=\mbox \times 2 \pi \times \mbox \frac \times \frac = \frac

Because

5252.113... = 33,000/2 \pi

References

-
- Category:Mechanical engineering Category:Physical quantity Category:Introductory physics Category:Rotational symmetry ms:Tork ja:力のモーメント

Electric charge

Electric charge is a fundamental conserved property of some subatomic particles, which determines their electromagnetic interactions. Electrically charged matter is influenced by, and produces, electromagnetic fields. The interaction between charge and field is the source of one of the four fundamental forces, the electromagnetic force.

Overview

Electric charge is a characteristic of subatomic particles, and is quantized. For example, electrons have a charge, by convention, of −1. Protons have the opposite charge of +1. Quarks have a fractional charge of −1/3 or +2/3. The antiparticle equivalents of these have the opposite charge. There are other charged particles. The SI unit of electric charge is the coulomb, which represents approximately 6.24 x 10¹⁸ elementary charges (the charge on a single electron or proton). The coulomb is defined as the quantity of charge that has passed through the cross-section of a conductor carrying one ampere within one second. The symbol Q is used to denote a quantity of electric charge. Electric charge can be directly measured with an electrometer. The discrete nature of electric charge was demonstrated by Robert Millikan in his oil-drop experiment. Formally, a measure of charge should be a multiple of the elementary charge e (charge is quantized), but since it is an average, macroscopic quantity, many orders of magnitude larger than a single elementary charge, it can effectively take on any real value.

History

As reported by the Ancient Greek philosopher Thales of Miletus around 600 BC, charge (or electricity) could be accumulated by rubbing fur on various substances, such as amber. The Greeks noted that the charged amber buttons could attract light objects such as hair. They also noted that if they rubbed the amber for long enough, they could even get a spark to jump. This property derives from the triboelectric effect. The word electricity derives from ηλεκτρον (electron), the Greek word for amber. C. F. Du Fay proposed in 1733 [http://www.sparkmuseum.com/BOOK_DUFAY.HTM] that electricity came in two varieties which cancelled each other, and expressed this in terms of a two-fluid theory. When glass was rubbed with silk, DuFay said that the glass was charged with vitreous electricity, and when amber was rubbed with fur, the amber was said to be charged with resinous electricity. By the 18th century, the study of electricity had become popular. One of the foremost experts was Benjamin Franklin, who argued in favor of a one-fluid theory of electricity. Franklin imagined electricity as being a type of invisible fluid present in all matter; for example he believed that it was the glass in a Leyden jar that held the accumulated charge. He posited that rubbing insulating surfaces together caused this fluid to change location, and that a flow of this fluid constitutes an electric current. He also posited that when matter contained too little of the fluid it was "negatively" charged, and when it had an excess it was "positively" charged. Arbitrarily (or for a reason that was not recorded) he identified the term "positive" with vitreous electricity and "negative" with resinous electricity. William Watson arrived at the same explanation at about the same time. We now know that the Franklin/Watson model was close, but too simple. Matter is actually composed of several kinds of electrically charged particles, the most common being the positively charged proton and the negatively charged electron. Rather than one possible electric current there are many: a flow of electrons, a flow of electron "holes" which act like positive particles, or in electrolytic solutions, a flow of both negative and positive particles called ions moving in opposite directions. To reduce this complexity, electrical workers still use Franklin's convention and they imagine that electric current (known as conventional current) is a flow of exclusively positive particles. The conventional current simplifies electrical concepts and calculations, but it ignores the fact that within some conductors (electrolytes, semiconductors, and plasma), two or more species of electric charges flow in opposite directions. The flow direction for conventional current is also backwards compared to the actual electron drift taking place during electric currents in metals, the typical conductor of electricity, which is a source of confusion for beginners in electronics.

Properties

Aside from the properties described in articles about electromagnetism, charge is a relativistic invariant. This means that any particle that has charge q, no matter how fast it goes, always has charge q. This property has been experimentally verified by showing that the charge of one helium nucleus (two protons and two neutrons bound together in a nucleus and moving around at incredible speeds) is the same as two deuterium nuclei (one proton and one neutron bound together, but moving much more slowly than they would if they were in a helium nucleus).

Conservation of charge

The total electric charge of isolated systems remains constant regardless of changes within the system itself. This law is inherent to all processes known to physics and can be derived in a local form from Maxwell's equation as a continuity equation. More generally, the net change in charge density

\rho

within a volume of integration

V

is equal to the area integral over the current density

J

on the surface of the volume

S

, which is in turn equal to the net current

I

: :

- \frac \int_V \rho dV = \int_S \mathbf \cdot \mathbf = I

External links

- [http://www.unitconversion.org/unit_converter/charge.html Online Charge Converter] - convert between various units of charge, such as coulomb, EMU of charge, franklin, ampere-hour, faraday, and so on
- [http://www.unitconversion.org/unit_converter/charge-v.html Interactive Charge Conversion Table] - convert selected unit to all other units of charge
- [http://www.ce-mag.com/archive/2000/marapril/mrstatic.html How fast does a charge decay?]
- Category:Electricity Category:Physical quantity Category:Chemical properties Category:Introductory physics Category:Fundamental physics concepts ko:전하 ja:電荷

Coulomb

The coulomb (symbol: C) is the SI unit of electric charge. It is named after Charles-Augustin de Coulomb (1736 to 1806).

Definition

1 coulomb is the amount of electric charge carried by a current of 1 ampere flowing for 1 second. :

C = A \cdot s

Explanation

The coulomb is also the unit of electric flux. (See Gauss Law). The coulomb could in principle be defined in terms of the charge of an electron or elementary charge. Since the values of the Josephson (CIPM (1988) Recommendation 1, PV 56; 19) and von Klitzing (CIPM (1988), Recommendation 2, PV 56; 20) constants have been given conventional values (K_J ≡ 4.835 979 Hz/V and R_K ≡ 2.5812807 Ω), it is possible to combine these values to form an alternative (not yet official) definition of the coulomb. A coulomb is then equal to exactly 6.24150962915265 elementary charges. Combined with the current definition of the ampere, this proposed definition would make the kilogram a derived unit.

SI multiples

Conversions

- One mole of electrons (approximately 6.022, or Avogadro's number) is known as a faraday (actually -1 faraday, since electrons are negatively charged). One faraday equals 96.4853415 kC (the Faraday constant). In terms of Avogadro's number (N_A), one coulomb is equal to approximately 1.036 × N_A elementary charges.
- one ampere-hour = 3600 C
- The elementary charge is approximately 160.2176 zC.
- One statcoulomb (statC), the CGS electrostatic unit of charge (esu), is approximately 3.3356 C or about 1/3 nC.

Volt

The volt (symbol: V) is the SI derived unit of electric potential difference. The number of volts is a measure of the strength of an electrical source in the sense of how much power is produced for a given current level. It is named in honor of Alessandro Volta (1745–1827), who invented the voltaic pile, the first chemical battery. battery

Definition

battery The volt is defined as the potential difference across a conductor when a current of one ampere dissipates one watt of power. Hence, it is the base SI representation m² · kg · s^-3 · A^-1, which can be equally represented as one joule of energy per coulomb of charge, J/C. :1 V = 1 W/A = 1 m²·kg·s^–3·A^–1 Since 1990 the volt is maintained internationally for practical measurement using the Josephson effect, where a conventional value is used for the Josephson constant, fixed by the 18th General Conference on Weights and Measures as :K = 0.4835979 GHz/µV.

Explanation

The electrical potential difference can be thought of as the ability to move electrical charge through a resistance. In essence, the volt measures how much kinetic energy each electron carries. The number of electrons is measured by the charge, in coulombs. Thus the volt is multiplied by the current flow, in amperes which are one coulomb per second, to yield the total electrical power in the current, in Watts. At a time in physics when the word force was used loosely, the potential difference was named the electromotive force or emf - a term which is still used in certain contexts.

Electrical potential difference ("voltage")

Between two points in an electric field, such as exists in an electrical circuit, the potential difference is equal to the difference in their electrical potentials. This difference is proportional to the electrostatic force that tends to push electrons or other charge-carriers from one point to the other. Potential difference, electrical potential and electromotive force are measured in volts, leading to the commonly used term voltage and the symbol V (sometimes $\mathcal$ is used for voltage). Voltage is additive in the following sense: the voltage between A and C is the sum of the voltage between A and B and the voltage between B and C. Two points in an electric circuit which are connected by an ideal conductor, without resistance and without the presence of a changing magnetic field, have a potential difference of zero. But other pairs of points may also have a potential difference of zero. If two such points are connected with a conductor, no current will flow through the connection. The various voltages in a circuit can be computed using Kirchhoff's circuit laws. Voltage is a property of an electric field, not individual electrons. An electron moving across a voltage difference experiences a net change in energy, often measured in electron-volts. This effect is analogous to a mass falling through a given height difference in a gravitational field.

Hydraulic analogy

If one thinks of an electrical circuit in analogy to water circulating in a network of pipes, driven by pumps in the absence of gravity, then the potential difference corresponds to the fluid pressure difference between two points. If there is a pressure difference between two points, then water flowing from the first point to the second will be able to do work, such as driving a turbine. This hydraulic analogy (see separate article for full details) is a useful method of teaching a range of electrical concepts. In a hydraulic system, the work done to move water is equal to the pressure multiplied by the volume of water moved. Similarly, in an electrical circuit, the work done to move electrons or other charge-carriers is equal to 'electrical pressure' (an old term for voltage) multiplied by the quantity of electrical charge moved. Voltage is a convenient way of quantifying the ability to do work.

Technical definition

The electrical potential difference is defined as the amount of work per charge needed to move electric charge from the second point to the first, or equivalently, the amount of work that unit charge flowing from the first point to the second can perform. The potential difference between two points a and b is the line integral of the electric field E: :

V_a - V_b = \int _a ^b \mathbf\cdot d\mathbf = \int _a ^b E \cos \phi dl.

Examples

Voltage sources

line integral Common sources of emf include:
- the battery
- the dynamo (a potential difference is generated between the ends of an electrical conductor that moves perpendicularly to a magnetic field)
- electrostatic induction (as when two different electrically insulating materials are rubbed together to produce an electrostatic discharge)
- the capacitor (really a storage device for energy produced by an emf from another source)

Common voltages

capacitor Nominal voltages of familiar sources:
- Nerve cell action potential: 40 millivolts
- Single-cell, non-rechargeable battery (e.g. AAA, AA, C and D cells): 1.5 volts
- Automobile electrical system: 12 volts
- Household mains electricity: 120 volts North America, 230 volts Europe
- Rapid transit third rail: 600 to 700 volts
- High voltage electric power transmission lines: 110 kilovolts and up (1150 kV is the record as of 2005)
- Lightning: 100 megavolts

Measuring instruments

Lightning Instruments for measuring potential differences include the voltmeter, the potentiometer (measurement device), and the oscilloscope. The voltmeter works by measuring the current through a fixed resistor, which, according to Ohm's Law, is proportional to the potential difference across it. The potentiometer works by balancing the unknown voltage against a known voltage in a bridge circuit. The cathode-ray oscilloscope works by amplifying the potential difference and using it to deflect an electron beam from a straight path, so that the deflection of the beam is proportional to the potential difference.

History of the volt

In 1800, as the result of a professional disagreement over the galvanic response advocated by Luigi Galvani, Alessandro Volta developed the so-called Voltaic pile, a forerunner of the battery, which produced a steady electric current. Volta had determined that the most effective pair of dissimilar metals to produce electricity was zinc and silver. In the 1880s, the International Electrical Congress, now the International Electrotechnical Commission (IEC), approved the volt for electromotive force. The volt was defined as the potential difference across a conductor when a current of one ampere dissipates one watt of power. Prior to the development of the Josephson junction voltage standard, the volt was maintained in national laboratories using specially constructed batteries called standard cells. The United States used a design called the Weston cell from 1905 to 1972.

Watt

:For other uses, see: Watt (disambig) The watt (symbol: W) is the SI derived unit of power.
Definition
One watt is one joule of energy per second. : 1 W = 1 J/s = 1 newton meter per second
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.
SI multiples

Conversions

- 1 watt ≈ 3.41214163 BTU/h
- 1 horsepower ≈ 745.700 W
- 1 horsepower (electrical British) = 746 W
- 1 horsepower (electrical European) = 736 W
- 1 horsepower ("metric") = 735.498 75 W
Derived and qualified units for power distribution
A watt is a unit of power or the amount of energy per unit time.
Kilowatt-hour, MWd
When paired with a unit of time the term watt is used for expressing energy consumption. For example, a kilowatt hour, is the amount of energy expended by a one kilowatt device over the course of one hour; it equals 3.6 megajoules (1 hour = 3600 seconds). A megawatt day (MWd or MW·d) is equal to 86.4 GJ (1 day = 86400 seconds). These units are often used in the context of power plants and home energy bills. For the use of watts as a measurement of transmitter power in radio, see effective radiated power and nominal power.
MWe, MWt
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). This is a term that refers to thermal power produced. SI prefixes can be used, for example megawatt thermal (MWt) and gigawatt thermal (GWt). For example, a nuclear power plant might use a fission reactor to generate heat (thermal output) which creates steam to drive a turbine to generate electricity. See nuclear proliferation for discussion of a reactor that generates 200 MWt (50 MWe), and another reactor that generates 800 MWt (200 MWe).
See also

- SI
- Kilowatt hour (kW·h)
- Watt balance
- Conversion of units
- Orders of magnitude (power)
- James Watt
- RMS
- Back to the Future
External links

- Nelson, Robert A., "[http://www.aticourses.com/international_system_units.htm The International System of Units] Its History and Use in Science and Industry". Via Satellite, February 2000. Category:SI derived units Category:Units of power ko:와트 ja:ワット simple:Watt

Kilowatt hour

The watt hour (symbol W·h) is a unit of energy. The watt hour is not an SI unit because it contains the non-SI unit (hour). The SI unit of energy is the joule (J).

Definition

1 W·h = 3 600 J

Multiples

Conversions

Explanation

The watt hour is derived from the multiplication of the SI unit of power (watt) and the non-SI unit of time (hour). The kilowatt-hour is commonly used for electrical energy and natural gas energy. This may be because domestic appliances often quote power in kilowatts. Many electric utility companies use the kilowatt hour for billing. Megawatt-hours are used for metering of larger amounts of electrical energy. For example, a power plant's daily output is likely to be meas