SI in the context of Electrical resistivity

The hectare (/ˈhɛktɛər, -tɑːr/; SI symbol: ha) is a non-SI metric unit of area equal to a square with 100-metre sides (1 hm), that is, 10,000 square metres (10,000 m), and is primarily used in the measurement of land. There are 100 hectares in one square kilometre. An acre is about 0.405 hectares and thus one hectare is about 2.47 acres.

In 1795, when the metric system was introduced, the are was defined as 100 square metres, or one square decametre, and the hectare ("hecto-" + "are") was thus 100 ares or ⁠1/100⁠ km (10000 square metres). When the metric system was further rationalised in 1960, resulting in the International System of Units (SI), the are was not included as a recognised unit. The hectare, however, remains as a non-SI unit accepted for use with the SI and whose use is "expected to continue indefinitely". Though the dekare/decare daa (1000 m) and are (100 m) are not officially "accepted for use", they are still used in some contexts.

In the field of solid mechanics, torsion is the twisting of an object due to an applied torque. Torsion could be defined as strain or angular deformation, and is measured by the angle a chosen section is rotated from its equilibrium position. The resulting stress (torsional shear stress) is expressed in either the pascal (Pa), an SI unit for newtons per square metre, or in pounds per square inch (psi) while torque is expressed in newton metres (N·m) or foot-pound force (ft·lbf). In sections perpendicular to the torque axis, the resultant shear stress in this section is perpendicular to the radius.

In non-circular cross-sections, twisting is accompanied by a distortion called warping, in which transverse sections do not remain plane. For shafts of uniform cross-section unrestrained against warping, the torsion-related physical properties are expressed as:

The micrometre (Commonwealth English) or micrometer (American English) (SI symbol: μm) is a unit of length in the International System of Units (SI) equalling 10 metre (SI standard prefix "micro-" = 10); that is, one millionth of a metre (or one thousandth of a millimetre, 0.001 mm, or about 0.00004 inch).

The nearest smaller common SI unit is the nanometre, equivalent to one thousandth of a micrometre, one millionth of a millimetre or one billionth of a metre (10 or 0.000000001 m).

The nanometre (international spelling as used by the International Bureau of Weights and Measures; SI symbol: nm), or nanometer (American spelling), is a unit of length in the International System of Units (SI), equal to one billionth (short scale) or one thousand million (long scale) of a metre (0.000000001 m) and to 1000 picometres. One nanometre can be expressed in scientific notation as 1 × 10 m and as ⁠1/1000000000⁠ m.

In physics and many other areas of science and engineering the intensity or flux of radiant energy is the power transferred per unit area, where the area is measured on the plane perpendicular to the direction of propagation of the energy. In the SI system, it has units watts per square metre (W/m), or kg⋅s in base units. Intensity is used most frequently with waves such as acoustic waves (sound), matter waves such as electrons in electron microscopes, and electromagnetic waves such as light or radio waves, in which case the average power transfer over one period of the wave is used. Intensity can be applied to other circumstances where energy is transferred. For example, one could calculate the intensity of the kinetic energy carried by drops of water from a garden sprinkler.

The word "intensity" as used here is not synonymous with "strength", "amplitude", "magnitude", or "level", as it sometimes is in colloquial speech.

Electrical resistivity (also called volume resistivity or specific electrical resistance) is a fundamental specific property of a material that measures its electrical resistance or how strongly it resists electric current. A low resistivity indicates a material that readily allows electric current. Resistivity is commonly represented by the Greek letter ρ (rho). The SI unit of electrical resistivity is the ohm-metre (Ω⋅m). For example, if a 1 m solid cube of material has sheet contacts on two opposite faces, and the resistance between these contacts is 1 Ω, then the resistivity of the material is 1 Ω⋅m.

Electrical conductivity (or specific conductance) is the reciprocal of electrical resistivity. It represents a material's ability to conduct electric current. It is commonly signified by the Greek letter σ (sigma), but κ (kappa) (especially in electrical engineering) and γ (gamma) are sometimes used. The SI unit of electrical conductivity is siemens per metre (S/m). Resistivity and conductivity are intensive properties of materials, giving the opposition of a standard cube of material to current. Electrical resistance and conductance are corresponding extensive properties that give the opposition of a specific object to electric current.

Luminosity is an absolute measure of radiated electromagnetic energy per unit time, and is synonymous with the radiant power emitted by a light-emitting object. In astronomy, luminosity is the total amount of electromagnetic energy emitted per unit of time by a star, galaxy, or other astronomical objects.

In SI units, luminosity is measured in joules per second, or watts. In astronomy, values for luminosity are often given in the terms of the luminosity of the Sun, L_⊙. Luminosity can also be given in terms of the astronomical magnitude system: the absolute bolometric magnitude (M_bol) of an object is a logarithmic measure of its total energy emission rate, while absolute magnitude is a logarithmic measure of the luminosity within some specific wavelength range or filter band.

Relative density, also called specific gravity, is a dimensionless quantity defined as the ratio of the density (mass divided by volume) of a substance to the density of a given reference material. Specific gravity for solids and liquids is nearly always measured with respect to water at its densest (at 4 °C or 39.2 °F); for gases, the reference is air at room temperature (20 °C or 68 °F). The term "relative density" (abbreviated r.d. or RD) is preferred in SI, whereas the term "specific gravity" is gradually being abandoned.

If a substance's relative density is less than 1 then it is less dense than the reference; if greater than 1 then it is denser than the reference. If the relative density is exactly 1 then the densities are equal; that is, equal volumes of the two substances have the same mass. If the reference material is water, then a substance with a relative density (or specific gravity) less than 1 will float in water. For example, an ice cube, with a relative density of about 0.91, will float. A substance with a relative density greater than 1 will sink.

The kilometre (SI symbol: km; /ˈkɪləmiːtər/ or /kɪˈlɒmətər/), spelt kilometer in American and Philippine English, is a unit of length in the International System of Units (SI), equal to one thousand metres (kilo- being the SI prefix for 1000). It is the preferred measurement unit to express distances between geographical places on land in most of the world; notable exceptions are the United States and the United Kingdom where the statute mile is used.

In photometry, luminous intensity is a measure of the wavelength-weighted power emitted by a light source in a particular direction per unit solid angle, based on the luminosity function, a standardized model of the sensitivity of the human eye. The SI unit of luminous intensity is the candela (cd), an SI base unit.

Electric power is the rate of transfer of electrical energy within a circuit. Its SI unit is the watt, the general unit of power, defined as one joule per second. Standard prefixes apply to watts as with other SI units: thousands, millions and billions of watts are called kilowatts, megawatts and gigawatts respectively.

In common parlance, electric power is the production and delivery of electrical energy, an essential public utility in much of the world. Electric power is usually produced by electric generators, but can also be supplied by sources such as electric batteries. It is usually supplied to businesses and homes (as domestic mains electricity) by the electric power industry through an electrical grid.

A decametre (International spelling as used by the International Bureau of Weights and Measures and by most English speaking countries, United States spelling dekameter or decameter), symbol dam ("da" for the SI prefix deca-, "m" for the SI unit metre), is a unit of length in the International System of Units (SI) equal to ten metres.

While any combination of SI prefix and unit can be used, many are rarely used in practice; the decametre is used less frequently than other units of length. One practical use is for altitude of geopotential heights in meteorology. The volumetric form cubic decametre is convenient for describing large volumes of water such as in rivers and lakes; a volume of one cubic decametre (dam) is equivalent to a capacity of one megalitre (ML).

An hour (symbol: h; also abbreviated hr) is a unit of time historically reckoned as 1⁄24 of a day and defined contemporarily as exactly 3,600 seconds (SI). There are 60 minutes in an hour, and 24 hours in a day.

The hour was initially established in the ancient Near East as a variable measure of 1⁄12 of the night or daytime. Such seasonal hours, also known as temporal hours or unequal hours, varied by season and latitude.

The digit or finger is an ancient and obsolete non-SI unit of measurement of length. It was originally based on the breadth of a human finger. It was a fundamental unit of length in the Ancient Egyptian, Mesopotamian, Hebrew, Ancient Greek and Roman systems of measurement.

In astronomy a digit is one twelfth of the diameter of the sun or the moon.

The gram (originally gramme; SI unit symbol g) is a unit of mass in the International System of Units (SI) equal to one thousandth of a kilogram.

Originally defined in 1795 as "the absolute weight of a volume of pure water equal to the cube of the hundredth part of a metre [1 cm], and at the temperature of melting ice", the defining temperature (0 °C) was later changed to the temperature of maximum density of water (approximately 4 °C). Subsequent redefinitions agree with this original definition to within 30 parts per million (0.003%), with the maximum density of water remaining very close to 1 g/cm, as shown by modern measurements.

The light-second is a unit of length useful in astronomy, telecommunications and relativistic physics. It is defined as the distance that light travels in free space in one second, and is equal to exactly 299792458 m (approximately 983571055 ft or 186282 miles).

Just as the second forms the basis for other units of time, the light-second can form the basis for other units of length, ranging from the light-nanosecond (299.8 mm or just under one international foot) to the light-minute, light-hour and light-day, which are sometimes used in popular science publications. The more commonly used light-year is also currently defined to be equal to precisely 31557600 light-seconds, since the definition of a year is based on a Julian year (not the Gregorian year) of exactly 365.25 d, each of exactly 86400 SI seconds.