Standard Units (SI Units)

The International System of Units (abbreviated SI) is the metric system used in science, industry, and medicine.

Brief history of the SI

The creation of the decimal Metric System at the time of the French Revolution and the subsequent deposition of two platinum standards representing the meter and the kilogram, on 22 June 1799, in the Archives de la République in Paris can be seen as the first step in the development of the present International System of Units.

In 1832, Gauss strongly promoted the application of this Metric System, together with the second defined in astronomy, as a coherent system of units for the physical sciences. Gauss was the first to make absolute measurements of the earth’s magnetic force in terms of a decimal system based on the three mechanical units millimeter, gram and second for, respectively, the quantities length, mass and time. In later years, Gauss and Weber extended these measurements to include electrical phenomena

These applications in the field of electricity and magnetism were further developed in the 1860s under the active leadership of Maxwell and Thomson through the British Association for the Advancement of Science (BAAS). They formulated the requirement for a coherent system of units with base units and derived units. In 1874 the BAAS introduced the CGS system, a three-dimensional coherent unit system based on the three mechanical units centimeter, gram and second, using prefixes ranging from micro to mega to express decimal submultiples and multiples. The following development of physics as an experimental science was largely based on this system.

The sizes of the coherent CGS units in the fields of electricity and magnetism, proved to be inconvenient so, in the 1880s, the BAAS and the International Electrical Congress, predecessor of the International Electrotechnical Commission (IEC), approved a mutually coherent set of practical units. Among them were the ohm for electrical resistance, the volt for electromotive force, and the ampere for electric current.

After the establishment of the Meter Convention on May, 20 1875 the CIPM concentrated on the construction of new prototypes taking the meter and kilogram as the base units of length and mass. In 1889 the 1st CGPM sanctioned the international prototypes for the meter and the kilogram. Together with the astronomical second as unit of time, these units constituted a three-dimensional mechanical unit system similar to the CGS system, but with the base units meter, kilogram and second. 
  

• No matter what field of science you enter, you will need to take measurements, understand them, communicate them to others, and be able to repeat them. In other words, we all have to speak the same basic language.
• The ability to obtain accurate measurements and communicate those measurements is a key requirement for progress.
• These are the seven basic units in the SI system: the kilogram (kg) (mass), the second (s) (time), the Kelvin (K) (temperature),  the ampere (A) (electric current), the mole (mol) (amount of a substance), the candela (cd) (luminous intensity), and the meter (m), (distance).

The Seven SI Units

This figure displays the fundamental SI units and the combinations leading to more complex measurement units.

1. 

   SI Units

   SI = Système International d'unités

   1. Quantity, Name, Symbol
   2. SI Prefixes
   3. SI Derived Units
   4. cgs Units

   Quantity, Name, Symbol

   length

   metre (meter): m

   (the correct English spelling of the unit is "metre", but the variant "meter" is frequently used in the United States)

    

   mass

   kilogram: kg

    

   time

   second: s

    

   electric current

   ampere: A

    

   thermodynamic temperature

   kelvin: K

    

   amount of substance

   mole: mol

    

   luminous intensity

   candela: cd

   
   

   SI Derived Units

   (Please note: all units to the right of the slash are actually in the denominator!)

   Frequency

   hertz: Hz = 1/s

    

   Force

   newton: N = m kg/s²

    

   Pressure, stress

   pascal: Pa = N/m² = kg/m s²

    

   Energy, work, quantity of heat

   joule: J = N m = m² kg/s²

    

   Power, radiant flux

   watt: W = J/s = m² kg/s³

    

   Quantity of electricity, electric charge

   coulomb: C = s A

    

   Electric potential

   volt: V = W/A = m² kg/s³ A

    

   Capacitance

   farad: F = C/V = s⁴ A²/m² kg

    

   Electric resistance

   ohm: Omega = V/A = m² kg/s³ A²

    

   Conductance

   siemens: S = A/V = s³ A²/m² kg

    

   Magnetic flux

   weber: Wb = V s = m² kg/s² A

    

   Magnetic flux density, magnetic induction

   tesla: T = Wb/m² = kg/s² A

    

   Inductance

   henry: H = Wb/A = m² kg/s² A²

    

   Luminous flux

   lumen: lm = cd sr

    

   Illuminance

   lux: lx = lm/m² = cd sr/m²

    

   Activity (ionizing radiations)

   becquerel: Bq = 1/s

    

   Absorbed dose

   gray: Gy = J/kg = m²/s²

    

   Dynamic viscosity

   pascal second: Pa s = kg/m s

    

   Moment of force

   metre newton: N m = m² kg/s²

    

   Surface tension

   newton per metre: N/m = kg/s²

    

   Heat flux density, irradiance

   watt per square metre: W/m² = kg/s³

    

   Heat capacity, entropy

   joule per kelvin: J/K = m² kg/s² K

    

   Specific heat capacity, specific entropy

   joule per kilogram kelvin: J/kg K = m²/s² K

    

   Specific energy

   joule per kilogram: J/kg = m²/s²

    

   Thermal conductivity

   watt per metre kelvin: W/m K = m kg/s³ K

    

   Energy density

   joule per cubic metre: J/m³ = kg/m s²

    

   Electric field strength

   volt per metre: V/m = m kg/s³ A

    

   Electric charge density

   coulomb per cubic metre: C/m³ = s A/m³

    

   Electric displacement, electric flux density

   coulomb per square metre: C/m² = s A/m²

    

   Permittivity

   farad per metre: F/m = s⁴ A²/m³ kg

    

   Permeability

   henry per metre: H/m = m kg/s² A²

    

   Molar energy

   joule per mole: J/mol = m² kg/s² mol

    

   Molar entropy, molar heat capacity

   joule per mole kelvin: J/mol K = m² kg/s² K mol

    

   Exposure (ionizing radiations)

   coulomb per kilogram: C/kg = s A/kg

    

   Absorbed dose rate

   gray per second: Gy/s = m²/s³

    The Importance of Standard Units in Everyday Life

   The system of measurements is very important in everyday life . In the past , various systems of measurements were used. For example , length was measured in units like foot , yard , chain and mile . Weight was measured in units like pound , ounce , kati , tahil . 
    Today , many countries in the world use the same units of measurements . We say that they use standard units . The standard units used are the S.I. units .
    Under this system of units , mass is measured in kilograms (kg) and length is measured in metres (m).
    The use of standard units makes it easier for people from different countries to communicate with each other . Furthermore , the use of a standard units means a measurement in that unit has the same value anywhere in the world .

   
   

   Shailesh kr shukla

   directoratace@gmail.com