The lux is the SI derived unit of illuminance and luminous emittance, measuring luminous flux per unit area. It is equal to one lumen per square metre. In photometry, this is used as a measure of the intensity, as perceived by the human eye, of light that hits or passes through a surface, it is analogous to the radiometric unit watt per square metre, but with the power at each wavelength weighted according to the luminosity function, a standardized model of human visual brightness perception. In English, "lux" is used as plural form. Illuminance is a measure of. One can think of luminous flux as a measure of the total "amount" of visible light present, the illuminance as a measure of the intensity of illumination on a surface. A given amount of light will illuminate a surface more dimly if it is spread over a larger area, so illuminance is inversely proportional to area when the luminous flux is held constant. One lux is equal to one lumen per square metre: 1 lx = 1 lm/m2 = 1 cd·sr/m2. A flux of 1000 lumens, concentrated into an area of 1 square metre, lights up that square metre with an illuminance of 1000 lux.
However, the same 1000 lumens, spread out over 10 square metres, produces a dimmer illuminance of only 100 lux. Achieving an illuminance of 500 lux might be possible in a home kitchen with a single fluorescent light fixture with an output of 12000 lumens. To light a factory floor with dozens of times the area of the kitchen would require dozens of such fixtures. Thus, lighting a larger area to the same level of lux requires a greater number of lumens; as with other SI units, SI prefixes can be used, for example. Here are some examples of the illuminance provided under various conditions: The illuminance provided by a light source on a surface perpendicular to the direction to the source is a measure of the strength of that source as perceived from that location. For instance, a star of apparent magnitude 0 provides 2.08 microlux at the Earth's surface. A perceptible magnitude 6 star provides 8 nanolux; the unobscured Sun provides an illumination of up to 100 kilolux on the Earth's surface, the exact value depending on time of year and atmospheric conditions.
This direct normal illuminance is related to the solar illuminance constant Esc, equal to 128000 lux. The illuminance on a surface depends on. For example, a pocket flashlight aimed at a wall will produce a given level of illumination if aimed perpendicular to the wall, but if the flashlight is aimed at increasing angles to the perpendicular, the illuminated spot becomes larger and so is less illuminated; when a surface is tilted at an angle to a source, the illumination provided on the surface is reduced because the tilted surface subtends a smaller solid angle from the source, therefore it receives less light. For a point source, the illumination on the tilted surface is reduced by a factor equal to the cosine of the angle between a ray coming from the source and the normal to the surface. In practical lighting problems, given information on the way light is emitted from each source and the distance and geometry of the lighted area, a numerical calculation can be made of the illumination on a surface by adding the contributions of every point on every light source.
The lux is a measure of luminous emittance, the number of lumens given off by a surface per square metre, regardless of how that light is distributed in terms of the directions in which it is emitted. It differs from the luminance. A white surface with one lux falling on it will emit one lux. Like all photometric units, the lux has a corresponding "radiometric" unit; the difference between any photometric unit and its corresponding radiometric unit is that radiometric units are based on physical power, with all wavelengths being weighted while photometric units take into account the fact that the human eye's image-forming visual system is more sensitive to some wavelengths than others, accordingly every wavelength is given a different weight. The weighting factor is known as the luminosity function; the lux is one lumen per square metre, the corresponding radiometric unit, which measures irradiance, is the watt per square metre. There is no single conversion factor between lx and W/m2; the peak of the luminosity function is at 555 nm.
For monochromatic light of this wavelength, the amount of illuminance for a given amount of irradiance is maximum: 683.002 lux per 1 W/m2. Other wavelengths of visible light produce fewer lux per watt-per-meter-squared; the luminosity function falls to zero for wavelengths outside the visible spectrum. For a light source with mixed wavelengths, the number of lumens per watt can be calculated by means of the luminosity function. In order to appear reasonably "white", a light source cannot consist of the green light to which the eye's image-forming visual photoreceptors are most sensitive, but must include a generous mixture of red and blue wavelengths, to which they are much less sensitive; this means that white light sources produce far fewer lumens per watt than the theoretical maximum of 683.002 lm/W. The ratio between the actual number of lumen
John Napier of Merchiston. He was the 8th Laird of Merchiston, his Latinized name was Ioannes Neper. John Napier is best known as the discoverer of logarithms, he invented the so-called "Napier's bones" and made common the use of the decimal point in arithmetic and mathematics. Napier's birthplace, Merchiston Tower in Edinburgh, is now part of the facilities of Edinburgh Napier University. Napier died from the effects of gout at home at Merchiston Castle and his remains were buried in the kirkyard of St Giles. Following the loss of the kirkyard there to build Parliament House, he was memorialised at St Cuthbert's at the west side of Edinburgh. Napier's father was Sir Archibald Napier of Merchiston Castle, his mother was Janet Bothwell, daughter of the politician and judge Francis Bothwell, Lord of Session, a sister of Adam Bothwell who became the Bishop of Orkney. Archibald Napier was 16 years old; as was the common practice for members of the nobility at that time, he was tutored and did not have formal education until he was 13, when he was sent to St Salvator's College, St Andrews.
He did not stay in college long. It is believed that he dropped out of school in Scotland and travelled in mainland Europe to better continue his studies. Little is known about those years, when, or with whom he might have studied, although his uncle Adam Bothwell wrote a letter to John's father on 5 December 1560, saying "I pray you, sir, to send John to the schools either to France or Flanders, for he can learn no good at home", it is believed that this advice was followed. In 1571, aged 21, returned to Scotland, bought a castle at Gartness in 1574. On the death of his father in 1608, Napier and his family moved into Merchiston Castle in Edinburgh, where he resided the remainder of his life, he died at the age of 67. Many mathematicians at the time were acutely aware of the issues of computation and were dedicated to relieving practitioners of the calculation burden. In particular, the Scottish mathematician John Napier was famous for his devices to assist with computation, he invented a well-known mathematical artifact, the ingenious numbering rods more quaintly known as “Napier's bones,” that offered mechanical means for facilitating computation.
In addition, Napier recognized the potential of the recent developments in mathematics those of prosthaphaeresis, decimal fractions, symbolic index arithmetic, to tackle the issue of reducing computation. He appreciated that, for the most part, practitioners who had laborious computations did them in the context of trigonometry. Therefore, as well as developing the logarithmic relation, Napier set it in a trigonometric context so it would be more relevant, his work, Mirifici Logarithmorum Canonis Descriptio contained fifty-seven pages of explanatory matter and ninety pages of tables of numbers related to natural logarithms. The book has an excellent discussion of theorems in spherical trigonometry known as Napier's Rules of Circular Parts. See Pentagramma mirificum. Modern English translations of both Napier's books on logarithms and their description can be found on the web, as well as a discussion of Napier's Bones and Promptuary, his invention of logarithms was taken up at Gresham College, prominent English mathematician Henry Briggs visited Napier in 1615.
Among the matters they discussed were a re-scaling of Napier's logarithms, in which the presence of the mathematical constant now known as e was a practical difficulty. Neither Napier nor Briggs discovered the constant e. Napier delegated to Briggs the computation of a revised table; the computational advance available via logarithms, the converse of powered numbers or exponential notation, was such that it made calculations by hand much quicker. The way was opened to scientific advances, in astronomy and other areas of physics. Napier made further contributions, he improved Simon Stevin's decimal notation. Lattice multiplication, used by Fibonacci, was made more convenient by his introduction of Napier's bones, a multiplication tool using a set of numbered rods. Napier may have worked in isolation, but he had contact with Tycho Brahe who corresponded with his friend John Craig. Craig announced the discovery of logarithms to Brahe in the 1590s, it has been shown that Craig had notes on a method of Paul Wittich that used trigonometric identities to reduce a multiplication formula for the sine function to additions.
Napier had an interest in the Book of Revelation, from his student days at St Salvator's College, St Andrews. Under the influence of the sermons of Christopher Goodman, he developed a anti-papal reading, he further used the Book of Revelation for chronography, to predict the Apocalypse, in A Plaine Discovery of the Whole Revelation of St. John, which he regarded as his most important work. Napier believed that would occur in 1688 or 1700, he dated the seventh trumpet to 1541. In his dedication of the Plaine Discovery to James VI, dated 29 Jan 1594, Napier urged the king to see "that justice be done against the enemies of God's church,"
The decibel is a unit of measurement used to express the ratio of one value of a power or field quantity to another on a logarithmic scale, the logarithmic quantity being called the power level or field level, respectively. It can be used to express a change in an absolute value. In the latter case, it expresses the ratio of a value to a fixed reference value. For example, if the reference value is 1 volt the suffix is "V", if the reference value is one milliwatt the suffix is "m". Two different scales are used when expressing a ratio in decibels, depending on the nature of the quantities: power and field; when expressing a power ratio, the number of decibels is ten times its logarithm to base 10. That is, a change in power by a factor of 10 corresponds to a 10 dB change in level; when expressing field quantities, a change in amplitude by a factor of 10 corresponds to a 20 dB change in level. The decibel scales differ by a factor of two so that the related power and field levels change by the same number of decibels in, for example, resistive loads.
The definition of the decibel is based on the measurement of power in telephony of the early 20th century in the Bell System in the United States. One decibel is one tenth of one bel, named in honor of Alexander Graham Bell. Today, the decibel is used for a wide variety of measurements in science and engineering, most prominently in acoustics and control theory. In electronics, the gains of amplifiers, attenuation of signals, signal-to-noise ratios are expressed in decibels. In the International System of Quantities, the decibel is defined as a unit of measurement for quantities of type level or level difference, which are defined as the logarithm of the ratio of power- or field-type quantities; the decibel originates from methods used to quantify signal loss in telegraph and telephone circuits. The unit for loss was Miles of Standard Cable. 1 MSC corresponded to the loss of power over a 1 mile length of standard telephone cable at a frequency of 5000 radians per second, matched the smallest attenuation detectable to the average listener.
The standard telephone cable implied was "a cable having uniformly distributed resistance of 88 Ohms per loop-mile and uniformly distributed shunt capacitance of 0.054 microfarads per mile". In 1924, Bell Telephone Laboratories received favorable response to a new unit definition among members of the International Advisory Committee on Long Distance Telephony in Europe and replaced the MSC with the Transmission Unit. 1 TU was defined such that the number of TUs was ten times the base-10 logarithm of the ratio of measured power to a reference power. The definition was conveniently chosen such that 1 TU approximated 1 MSC. In 1928, the Bell system renamed the TU into the decibel, being one tenth of a newly defined unit for the base-10 logarithm of the power ratio, it was named the bel, in honor of the telecommunications pioneer Alexander Graham Bell. The bel is used, as the decibel was the proposed working unit; the naming and early definition of the decibel is described in the NBS Standard's Yearbook of 1931: Since the earliest days of the telephone, the need for a unit in which to measure the transmission efficiency of telephone facilities has been recognized.
The introduction of cable in 1896 afforded a stable basis for a convenient unit and the "mile of standard" cable came into general use shortly thereafter. This unit was employed up to 1923 when a new unit was adopted as being more suitable for modern telephone work; the new transmission unit is used among the foreign telephone organizations and it was termed the "decibel" at the suggestion of the International Advisory Committee on Long Distance Telephony. The decibel may be defined by the statement that two amounts of power differ by 1 decibel when they are in the ratio of 100.1 and any two amounts of power differ by N decibels when they are in the ratio of 10N. The number of transmission units expressing the ratio of any two powers is therefore ten times the common logarithm of that ratio; this method of designating the gain or loss of power in telephone circuits permits direct addition or subtraction of the units expressing the efficiency of different parts of the circuit... In 1954, J. W. Horton argued that the use of the decibel as a unit for quantities other than transmission loss led to confusion, suggested the name'logit' for "standard magnitudes which combine by addition".
In April 2003, the International Committee for Weights and Measures considered a recommendation for the inclusion of the decibel in the International System of Units, but decided against the proposal. However, the decibel is recognized by other international bodies such as the International Electrotechnical Commission and International Organization for Standardization; the IEC permits the use of the decibel with field quantities as well as power and this recommendation is followed by many national standards bodies, such as NIST, which justifies the use of the decibel for voltage ratios. The term field quantity is deprecated by ISO 80000-1. In spite of their widespread use, suffixes are not recognized by the IEC or ISO. ISO 80000-3 describes definitions for units of space and time; the decibel for use in acoustics is defined in ISO 80000-8. The major difference from the article below is that for acoustics the decibel has no
The kilogram or kilogramme is the base unit of mass in the International System of Units. Until 20 May 2019, it remains defined by a platinum alloy cylinder, the International Prototype Kilogram, manufactured in 1889, stored in Saint-Cloud, a suburb of Paris. After 20 May, it will be defined in terms of fundamental physical constants; the kilogram was defined as the mass of a litre of water. That was an inconvenient quantity to replicate, so in 1799 a platinum artefact was fashioned to define the kilogram; that artefact, the IPK, have been the standard of the unit of mass for the metric system since. In spite of best efforts to maintain it, the IPK has diverged from its replicas by 50 micrograms since their manufacture late in the 19th century; this led to efforts to develop measurement technology precise enough to allow replacing the kilogram artifact with a definition based directly on physical phenomena, now scheduled to take place in 2019. The new definition is based on invariant constants of nature, in particular the Planck constant, which will change to being defined rather than measured, thereby fixing the value of the kilogram in terms of the second and the metre, eliminating the need for the IPK.
The new definition was approved by the General Conference on Weights and Measures on 16 November 2018. The Planck constant relates a light particle's energy, hence mass, to its frequency; the new definition only became possible when instruments were devised to measure the Planck constant with sufficient accuracy based on the IPK definition of the kilogram. The gram, 1/1000 of a kilogram, was provisionally defined in 1795 as the mass of one cubic centimetre of water at the melting point of ice; the final kilogram, manufactured as a prototype in 1799 and from which the International Prototype Kilogram was derived in 1875, had a mass equal to the mass of 1 dm3 of water under atmospheric pressure and at the temperature of its maximum density, 4 °C. The kilogram is the only named SI unit with an SI prefix as part of its name; until the 2019 redefinition of SI base units, it was the last SI unit, still directly defined by an artefact rather than a fundamental physical property that could be independently reproduced in different laboratories.
Three other base units and 17 derived units in the SI system are defined in relation to the kilogram, thus its stability is important. The definitions of only eight other named SI units do not depend on the kilogram: those of temperature and frequency, angle; the IPK is used or handled. Copies of the IPK kept by national metrology laboratories around the world were compared with the IPK in 1889, 1948, 1989 to provide traceability of measurements of mass anywhere in the world back to the IPK; the International Prototype Kilogram was commissioned by the General Conference on Weights and Measures under the authority of the Metre Convention, in the custody of the International Bureau of Weights and Measures who hold it on behalf of the CGPM. After the International Prototype Kilogram had been found to vary in mass over time relative to its reproductions, the International Committee for Weights and Measures recommended in 2005 that the kilogram be redefined in terms of a fundamental constant of nature.
At its 2011 meeting, the CGPM agreed in principle that the kilogram should be redefined in terms of the Planck constant, h. The decision was deferred until 2014. CIPM has proposed revised definitions of the SI base units, for consideration at the 26th CGPM; the formal vote, which took place on 16 November 2018, approved the change, with the new definitions coming into force on 20 May 2019. The accepted redefinition defines the Planck constant as 6.62607015×10−34 kg⋅m2⋅s−1, thereby defining the kilogram in terms of the second and the metre. Since the second and metre are defined in terms of physical constants, the kilogram is defined in terms of physical constants only; the avoirdupois pound, used in both the imperial and US customary systems, is now defined in terms of the kilogram. Other traditional units of weight and mass around the world are now defined in terms of the kilogram, making the kilogram the primary standard for all units of mass on Earth; the word kilogramme or kilogram is derived from the French kilogramme, which itself was a learned coinage, prefixing the Greek stem of χίλιοι khilioi "a thousand" to gramma, a Late Latin term for "a small weight", itself from Greek γράμμα.
The word kilogramme was written into French law in 1795, in the Decree of 18 Germinal, which revised the older system of units introduced by the French National Convention in 1793, where the gravet had been defined as weight of a cubic centimetre of water, equal to 1/1000 of a grave. In the decree of 1795, the term gramme thus replaced gravet, kilogramme replaced grave; the French spelling was adopted in Great Britain when the word was used for the first time in English in 1795, with the spelling kilogram being adopted in the United States. In the United Kingdom both spellings are used, with "kilogram" having become by far the more common. UK law regulating the units to be used when trading by weight or measure does not prevent the use of either spelling. In the 19th century the French word kilo, a shortening of kilogramme, was imported into the English language where it has been used to mean both kilogram and kilometre. While kilo is acceptable in many generalist texts
In mathematics, the logarithm is the inverse function to exponentiation. That means the logarithm of a given number x is the exponent to which another fixed number, the base b, must be raised, to produce that number x. In the simplest case, the logarithm counts repeated multiplication of the same factor; the logarithm of x to base b is denoted as logb . More exponentiation allows any positive real number to be raised to any real power, always producing a positive result, so the logarithm for any two positive real numbers b and x where b is not equal to 1, is always a unique real number y. More explicitly, the defining relation between exponentiation and logarithm is: log b = y if b y = x. For example, log2 64 = 6, as 26 = 64; the logarithm to base 10 is called the common logarithm and has many applications in science and engineering. The natural logarithm has the number e as its base; the binary logarithm uses base 2 and is used in computer science. Logarithms were introduced by John Napier in the early 17th century as a means to simplify calculations.
They were adopted by navigators, scientists and others to perform computations more using slide rules and logarithm tables. Tedious multi-digit multiplication steps can be replaced by table look-ups and simpler addition because of the fact—important in its own right—that the logarithm of a product is the sum of the logarithms of the factors: log b = log b x + log b y, provided that b, x and y are all positive and b ≠ 1; the present-day notion of logarithms comes from Leonhard Euler, who connected them to the exponential function in the 18th century. Logarithmic scales reduce wide-ranging quantities to tiny scopes. For example, the decibel is a unit used to express ratio as logarithms for signal power and amplitude. In chemistry, pH is a logarithmic measure for the acidity of an aqueous solution. Logarithms are commonplace in scientific formulae, in measurements of the complexity of algorithms and of geometric objects called fractals, they help describing frequency ratios of musical intervals, appear in formulas counting prime numbers or approximating factorials, inform some models in psychophysics, can aid in forensic accounting.
In the same way as the logarithm reverses exponentiation, the complex logarithm is the inverse function of the exponential function applied to complex numbers. The discrete logarithm is another variant. Addition and exponentiation are three fundamental arithmetic operations. Addition, the simplest of these, can be undone by subtraction: adding, say, 2 to 3 gives 5; the process of adding 2 can be undone by subtracting 2: 5 − 2 = 3. Multiplication, the next-simplest operation, can be undone by division: doubling a number x, i.e. multiplying x by 2, the result is 2x. To get back x, it is necessary to divide by 2. For example 2 ⋅ 3 = 6 and the process of multiplying by 2 is undone by dividing by 2: 6 / 2 = 3; the idea and purpose of logarithms is to undo a fundamental arithmetic operation, namely raising a number to a certain power, an operation known as exponentiation. For example, raising 2 to the third power yields 8, because 8 is the product of three factors of 2: 2 3 = 2 × 2 × 2 = 8 The logarithm of 8 is 3, reflecting the fact that 2 was raised to the third power to get 8.
This subsection contains a short overview of the exponentiation operation, fundamental to understanding logarithms. Raising b to the n-th power, where n is a natural number, is done by multiplying n factors equal to b; the n-th power of b is written bn, so that b n = b × b × ⋯ × b ⏟ n factors Exponentiation may be extended to by, where b is a positive number and the exponent y is any real number. For example, b − 1 is the reciprocal of b. Raising b to the power 1/2 is the square root of b. More raising b to a rational power p/q, where p and q are integers, is given by b p / q = b p q, the q-th root of bp. Any irrational number y can be approximated to arbitrary precision by rational numbers; this can be used to compute the y-th power of b: for example 2 ≈ 1.414... and
Joule heating known as Ohmic heating and resistive heating, is the process by which the passage of an electric current through a conductor produces heat. Joule's first law known as the Joule–Lenz law, states that the power of heating generated by an electrical conductor is proportional to the product of its resistance and the square of the current: P ∝ I 2 R Joule heating affects the whole electric conductor, unlike the Peltier effect which transfers heat from one electrical junction to another. James Prescott Joule first published in December 1840, an abstract in the Proceedings of the Royal Society, suggesting that heat could be generated by an electrical current. Joule immersed a length of wire in a fixed mass of water and measured the temperature rise due to a known current flowing through the wire for a 30 minute period. By varying the current and the length of the wire he deduced that the heat produced was proportional to the square of the current multiplied by the electrical resistance of the immersed wire.
In 1841 and 1842, subsequent experiments showed that the amount of heat generated was proportional to the chemical energy used in the voltaic pile that generated the current. This led Joule to reject the caloric theory in favor of the mechanical theory of heat. Resistive heating was independently studied by Heinrich Lenz in 1842; the SI unit of energy was subsequently named the joule and given the symbol J. The known unit of power, the watt, is equivalent to one joule per second. Joule heating is caused by the body of the conductor. A voltage difference between two points of a conductor creates an electric field that accelerates charge carriers in the direction of the electric field, giving them kinetic energy; when the charged particles collide with ions in the conductor, the particles are scattered. Thus, energy from the electrical field is converted into thermal energy. Joule heating is referred to as ohmic heating or resistive heating because of its relationship to Ohm's Law, it forms the basis for the large number of practical applications involving electric heating.
However, in applications where heating is an unwanted by-product of current use the diversion of energy is referred to as resistive loss. The use of high voltages in electric power transmission systems is designed to reduce such losses in cabling by operating with commensurately lower currents; the ring circuits, or ring mains, used in UK homes are another example, where power is delivered to outlets at lower currents, thus reducing Joule heating in the wires. Joule heating does not occur in superconducting materials, as these materials have zero electrical resistance in the superconducting state. Resistors create electrical noise, called Johnson–Nyquist noise. There is an intimate relationship between Johnson–Nyquist noise and Joule heating, explained by the fluctuation-dissipation theorem; the most fundamental formula for Joule heating is the generalized power equation: P = I where P is the power converted from electrical energy to thermal energy, I is the current travelling through the resistor or other element, V A − V B is the voltage drop across the element.
The explanation of this formula is: = × Assuming the element behaves as a perfect resistor and that the power is converted into heat, the formula can be re-written by substituting Ohm's law, V = I ∗ R, into the generalized power equation: P = I V = I 2 R = V 2 / R where R is the resistance. When current varies, as it does in AC circuits, P = U I where t is time and P is the instantaneous power being converted from electrical energy to heat. Far more the average power is of more interest than the instantaneous power: P a v g = U rms I rms = I rms 2 R = U rms 2 / R where "avg" denotes average over one or more cycles, "rms" denotes root mean square; these formulas are valid for an ideal resistor, with zero reactance. If the reactance is nonzero, the formulas are modified: P a v g = U rms I rms cos ϕ = I rms 2 Re = U rms 2 Re
The pascal is the SI derived unit of pressure used to quantify internal pressure, Young's modulus and ultimate tensile strength. It is defined as one newton per square metre, it is named after the French polymath Blaise Pascal. Common multiple units of the pascal are the hectopascal, equal to one millibar, the kilopascal, equal to one centibar; the unit of measurement called. Meteorological reports in the United States state atmospheric pressure in millibars. In Canada these reports are given in kilopascals; the unit is named after Blaise Pascal, noted for his contributions to hydrodynamics and hydrostatics, experiments with a barometer. The name pascal was adopted for the SI unit newton per square metre by the 14th General Conference on Weights and Measures in 1971; the pascal can be expressed using SI derived units, or alternatively SI base units, as: 1 P a = 1 N m 2 = 1 k g m ⋅ s 2 = 1 J m 3 where N is the newton, m is the metre, kg is the kilogram, s is the second, J is the joule. One pascal is the pressure exerted by a force of magnitude one newton perpendicularly upon an area of one square metre.
The unit of measurement called a standard atmosphere is 101325 Pa.. This value is used as a reference pressure and specified as such in some national and international standards, such as the International Organization for Standardization's ISO 2787, ISO 2533 and ISO 5024. In contrast, International Union of Pure and Applied Chemistry recommends the use of 100 kPa as a standard pressure when reporting the properties of substances. Unicode has dedicated code-points U+33A9 ㎩ SQUARE PA and U+33AA ㎪ SQUARE KPA in the CJK Compatibility block, but these exist only for backward-compatibility with some older ideographic character-sets and are therefore deprecated; the pascal or kilopascal as a unit of pressure measurement is used throughout the world and has replaced the pounds per square inch unit, except in some countries that still use the imperial measurement system or the US customary system, including the United States. Geophysicists use the gigapascal in measuring or calculating tectonic stresses and pressures within the Earth.
Medical elastography measures tissue stiffness non-invasively with ultrasound or magnetic resonance imaging, displays the Young's modulus or shear modulus of tissue in kilopascals. In materials science and engineering, the pascal measures the stiffness, tensile strength and compressive strength of materials. In engineering use, because the pascal represents a small quantity, the megapascal is the preferred unit for these uses; the pascal is equivalent to the SI unit of energy density, J/m3. This applies not only to the thermodynamics of pressurised gases, but to the energy density of electric and gravitational fields. In measurements of sound pressure or loudness of sound, one pascal is equal to 94 decibels SPL; the quietest sound a human can hear, known as the threshold of hearing, is 20 µPa. The airtightness of buildings is measured at 50 Pa; the units of atmospheric pressure used in meteorology were the bar, close to the average air pressure on Earth, the millibar. Since the introduction of SI units, meteorologists measure pressures in hectopascals unit, equal to 100 pascals or 1 millibar.
Exceptions include Canada. In many other fields of science, the SI is preferred. Many countries use the millibars. In all other fields, the kilopascal is used instead. Atmospheric pressure which gives the usage of the hbar end the mbar Centimetre of water Meteorology Metric prefix Orders of magnitude Pascal's law Pressure measurement