SI units

(noun)

International System of Units (abbreviated SI from French: Le Système international d'unités). It is the modern form of the metric system.

Related Terms

Examples of SI units in the following topics:

  • Prefixes and Other Systems of Units

    • SI prefixes precede a basic unit of measure to indicate a multiple or fraction of the unit.
    • A metric prefix, or SI prefix, is a unit prefix that precedes a basic unit of measure to indicate a multiple or fraction of the unit.
    • Some units that are widely used are not a part of the International System of Units and are considered Non-SI Units.
    • These units, though not officially part of SI Units, are generally accepted for use in conjunction with SI units.
    • Apply prefixes to units and distinguish between SI and customary units

  • Time

    • Time is the fundamental physical quantity of duration and is measured by the SI Unit known as the second.
    • Time is one of the seven fundamental physical quantities in the International System (SI) of Units.
    • Periodic events and motion have long served as standards for units of time.
    • Today, the SI Unit of the second is defined in terms of radiation emitted by cesium atoms.

  • Mass

    • The SI unit of mass is the kilogram (kg).
    • It is also the only SI unit that is directly defined by an artifact, rather than a fundamental physical property that can be reproduced in different laboratories.
    • Four of the seven base units in the SI system are defined relative to the kilogram, so the stability of this measurement is crucial for accurate and consistent measurements.
    • This value, though given in kilograms, is actually the non-SI unit of measure known as the kilogram-force.

  • Length

    • Length is a physical measurement of distance that is fundamentally measured in the SI unit of a meter.
    • Many different units of length are used around the world.
    • In the United States, the U.S. customary units operationally describe length in terms of the basic unit of an inch.
    • As such, a standard unit of measurement that is internationally accepted is needed.
    • The basic unit of length as identified by the International System of Units (SI) is the meter.

  • Application of Bernoulli's Equation: Pressure and Speed

    • The kinetic energy of the fluid is stored in static pressure, $p_s$, and dynamic pressure, $\frac{1}{2}\rho V^2$, where \rho is the fluid density in (SI unit: kg/m3) and V is the fluid velocity (SI unit: m/s).
    • The SI unit of static pressure and dynamic pressure is the pascal.
    • Static pressure is simply the pressure at a given point in the fluid, dynamic pressure is the kinetic energy per unit volume of a fluid particle.

  • Period and Frequency

    • The period is the duration of one cycle in a repeating event, while the frequency is the number of cycles per unit time.
    • The frequency is defined as the number of cycles per unit time.
    • In SI units, the unit of frequency is the hertz (Hz), named after the German physicist Heinrich Hertz: 1 Hz indicates that an event repeats once per second.
    • The SI unit for period is the second.
    • Angular frequency is often represented in units of radians per second (recall there are 2π radians in a circle).

  • Electric Flux

    • If the electric field is uniform, the electric flux passing through a surface of vector area S is $\Phi_E = \mathbf{E} \cdot \mathbf{S} = ES \cos \theta$ where E is the magnitude of the electric field (having units of V/m), S is the area of the surface, and θ is the angle between the electric field lines and the normal (perpendicular) to S.
    • Electric flux has SI units of volt metres (V m), or, equivalently, newton metres squared per coulomb (N m2 C−1).
    • Thus, the SI base units of electric flux are kg·m3·s−3·A−1.

  • Mass

    • Mass is a physical property of matter that depends on size and shape of matter, and is expressed as kilograms by the SI system.
    • This relation is called a unit.
    • The International System of Units (SI) measures mass in kilograms, or kg.
    • There are other units of mass, including the following (only the first two are accepted by the SI system):

  • Absolute Temperature

    • Absolute temperature is the most commoly used thermodyanmic temperature unit and is the standard unit of temperature.
    • The kelvin (or "absolute temperature") is the standard thermodyanmic temperature unit.
    • It is one of the seven base units in the International System of Units (SI) and is assigned the unit symbol K.
    • By international agreement, the unit kelvin and its scale are defined by two points: absolute zero and the triple point of Vienna Standard Mean Ocean Water (water with a specified blend of hydrogen and oxygen isotopes).

  • Length

    • In most systems of measurement, the unit of length is a fundamental unit, from which other units are defined.
    • There are several units that are used to measure length.
    • Units of length may be based on lengths of human body parts, the distance traveled in a number of paces, the distance between landmarks or places on the Earth, or arbitrarily on the length of some fixed object.In the International System of Units (SI), the basic unit of length is the meter and is now defined in terms of the speed of light.
    • In U.S. customary units, English or Imperial system of units, commonly used units of length are the inch, the foot, the yard, and the mile.
    • Units used to denote distances in the vastness of space, as in astronomy, are much longer than those typically used on Earth and include the astronomical unit, the light-year, and the parsec.