SI unit

(noun)

The modern form of the metric system used extensively in the sciences (abbreviated SI from French: Système International d'Unités).

Related Terms

Examples of SI unit in the following topics:

  • SI Unit Prefixes

    • The basic SI units can be expressed as fractions and multiples of basic units by using a set of simple prefixes.
    • Now that we know about the SI system and what it provides to the scientist and engineer, we can examine some aspects of actual measurement.
    • The SI system utilizes a standard system of prefixes to the basic units that allow them to be more relevant to and descriptive of relative magnitude.
    • Briefly review the basic SI units before you study the prefixes.
    • A prefix may be used to identify multiples of the original unit or fractions of the original unit.

  • SI Units of Pressure

    • The SI unit of pressure is the pascal (Pa), which is equal to one Newton per meter squared (N/m2).
    • The International System of Units (abbreviated SI from the French Système International d'Unités) is the basis of the metric system.
    • The SI was established in 1960 and is based on the metre-kilogram-second system rather than the centimetre-gram-second system.
    • Derived units are unlimited in number and are formed by multiplying and dividing the seven base units and other derived units; for example, the SI derived unit of speed is meters per second, m/s.
    • The unit of pressure in the SI system is the pascal (Pa), defined as the force of one newton per square meter:

  • Standard Units (SI Units)

    • The International System of Units (abbreviated SI) is the metric system used in science, industry, and medicine.
    • The SI units of measurement have an interesting history.
    • This figure displays the fundamental SI units and the combinations that lead to more complex units of measurement.
    • Chemistry - Scientific Units the SI and Metric Units - YouTube
    • Causey teaches scientific units of the SI system, the metric system, and the CGS system.

  • Converting from One Unit to Another

    • The most widely used system of units is the International System of Units, or SI.
    • There are seven SI base units, and all other SI units can be derived from these base units.
    • The seven base SI units are: [Physical Quantity: unit symbol (unit name)]
    • The base units of SI are actually not the smallest set possible; smaller sets have been defined.
    • Derived units are based on units from the SI system of units.

  • Molality

    • The SI unit for molality is mol/kg, or moles solute per kg of solvent.
    • However, following the SI system of units, the National Institute of Standards and Technology, which is the United States' authority on measurement, considers the term "molal" and the unit symbol "m" to be obsolete, and suggests using mol/kg or another related SI unit instead.
    • We multiply the moles by the reciprocal of the given molality (3.0 moles/kg) so that our units appropriately cancel.

  • Bond Energy

    • Bond energies are commonly given in units of kcal/mol or kJ/mol, and are generally called bond dissociation energies when given for specific bonds, or average bond energies when summarized for a given type of bond over many kinds of compounds.
    • Such average values are often referred to as standard bond energies, and are given here in units of kcal/mole.
    • The SI unit of energy is the joule, symbol J.
    • A useful site for unit conversions may be reached by clicking the following link (http://www.chemie.fu-berlin.de/chemistry/general/units_en.html).

  • Measuring Radiation Exposure

    • Non-SI units are still prevalent as well, where dose is often reported in rads, and dose equivalent in rems.
    • It should not be confused with the unit of radioactive activity, the becquerel (Bq).

  • Volume and Density

    • The unit for volume is a unit derived from the SI unit of length and is not a fundamental SI measurement.
    • Density is another measurement derived from SI basic units.
    • The density of a material is defined as its mass per unit volume.
    • Density is calculated by the dividing the mass by the volume, so that density is measured as units of mass/volume, often g/mL.
    • The measuring cup expresses liquid volume in the standard SI units of liters and milliliters.

  • Silicate Units, Silicate Chains, Silicate Sheets

    • The basic unit of silicate, [SiO4]4- tetrahedron, can form single and double chains and sheets.
    • The basic building block of all silicate minerals is the [SiO4]4− tetrahedron.
    • There are four covalent Si−O bonds.
    • If two [SiO4]4− tetrahedrons share an oxygen atom at one common vertex, an [Si2O7]6− ion is formed.
    • SiO4 tetrahedrons can be arranged to form sheets.

  • Enantioselective Aldol Reactions

    • Note that the last example shows the addition of an acetate unit, so enantioselectivity is not accompanied by diastereoselectivity.
    • In this example the re-face of the enolate bonds to the si-face of the aldehyde.
    • Steric hindrance of the Lewis acid with the Z-methyl group changes the facial selectivity of the aldehyde from re to si.
    • As a result, the bulky t-butyl group serves to direct bonding from the re-face of the enolate to the si-face of the carbonyl group in the lithium enolate, but changes the facial selectivity of both reactants in the enol borinate.
    • The α-siloxy substituent again chelates with the magnesium favoring a transition state in which the si-face of the enolate bonds to the si-face of the carbonyl group (top equation).