Class 11 Physics Units and Measurement – SI Units

SI Units

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SI unit is an abbreviation of International System of Units, which is at present internationally accepted system of unit for measurement. There are seven fundamental quantities and two supplementary quantities are in this system of units.

Seven Fundamental or base quantities, their SI units and Definitions are given in Table.

Base QuantitySI Units
Lengthmetrem1 m is equal to 1650763.73 wavelengths in vacuum, of radiation corresponding to orange – red light of krypton – 86.
Masskilogramkg1 kg is equal to mass of 1 litre volume of water at 4 oC
Timeseconds1 s is equal to 9192631770 time periods of a particular radiation from Cesium – 133 atom.
Electric CurrentampereA1 ampere is the current which when flows through two infinitely long straight conductors of negligible cross-section placed at a distance of one metre in vacuum produces a force of 2 X 10-7 N / m between them.
Thermo dynamic TemperaturekelvinkThe Kelvin, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.
Amount of SubstancemolemolThe mole is the amount of substance of a system, which contains as many elementary particles (atoms, molecules, ions etc.) as there are atoms in 0.012 kilogram of carbon-12.
Luminous IntensitycandelacdIt is luminous intensity in a perpendicular direction of a surface of (1/600000) m2 of a black body at the temperature of freezing platinum under a pressure of 1.013 X 105 N / m2.
Plane AngleradianradIt is the plane angle between two radii of a circle which cut-off on the circumference, an arc equal in length to the radius.
Solid AnglesteradiansrThe steradian is the solid angle which having its vertex at the centre of the sphere, cut-off an area of the surface of sphere equal to that of a square with sides of length equal to the radius of the sphere.

Rules and conventions for writing SI units and their symbols

  1. The units named after scientists are not written with a capital initial letter. Eg. newton
  2. The symbols of the units named after scientist should be written by a capital letter. Eg. N for newton
  3. Small letters are used as symbols for units not derived from a proper name. Eg. m for metre, kg for kilogram
  4. No full stop or other punctuation marks should be used within or at the end of symbols. Eg. 10 m and not as 10 m.
  5. The symbols of the units do not take plural form. Eg. 10 m and not 10 ms
  6. When temperature is expressed in kelvin, the degree sign is omitted. Eg. 278 K not as 278 0K (If measured in Celsius scale, degree sign is to be included. Eg. 50 0C and not 50 C
  7. Use of solidus is recommended only for indicating a division of one letter unit symbol by another unit symbol. Not more than one solidus is used. Eg. m s-1 or m / s is correct. J / K mol or J K-1 mol-1 is correct and J / K / mol is not correct.
  8. Some space is always to be left between the number and the symbol of the unit and also between the symbols for compound units such as force, momentum, etc. Eg. 10m is not correct and 10 m is correct. Kgms-1 is not correct but kg m s-1 is correct.
  9. Only accepted symbols should be used. Eg. ampere is represented as A and not as amp. Or am. Second is represented as s and not as sec.
  10. Numerical value of any physical quantity should be expressed in scientific notation. Eg. density of mercury is 1.36 X 104 kg m-3 and not as 13600 kg m-3.

SI Prefixes

Some prefixes with its power of 10 and symbol are given in the table below:

Power of 10PrefixSymbol

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