Measurement is a process of comparing a physical quantity with a standard quantity. The standard quantity used to compare a physical quantity for its measurement is called unit.
The internationally accepted modern system of units used in science is known as SI units.
If a physical quantity P is "n" times the standard quantity (unit) u, then P = nu
In SI units there are 7 basic units. These are:
The units obtained in terms of basic units are called derived units.
In international system of units known as SI the basic units are defined as under:
Kilogram: One kilogram is the mass of a particular cylinder made of Platinum Iridium alloy kept at the International Bureau of Weight and Measures in France.
Metre: One metre is the distance travelled by light in vacuum in a time interval of 1/299792458 second.
Second: One second is the time required for cesium-133 atom to undergo 9192631770 vibrations.
Kelvin: One kelvin is equal to 1/273.15 of the thermodynamic temperature of triple point of water.
Ampere: One ampere is the current that when flowing through two long parallel wires, each of length equal to one metre, separated by one metre in free space, results in a force of 2 × 107 N between the two wires.
Candela: One candela is the luminous intensity, in a given direction of a source that emits monochromatic radiation of frequency 540 × 1012 Hz and that has a radiant intensity of 1/683 watt per steradian in that direction.
Mole: One mole is the amount of any substance, which contains, as may elementary units, as there are atoms in exactly 0.012 kg of C-12 isotopes of carbon.
No space is required between the prefix and the symbol of the unit.
The prefixes are used only with the units and not alone.
Use only one prefix at a time.
SI prefix is not used with the unit °C.
While writing the value of physical quantity, the number and the unit are separated by a space.
No space is given between number and °C, degree, minute and second of plane angle.
The symbols of the unit are not changed while writing them in plural.
The symbols of the units are not followed by a full stop except at the end of a sentence.
In writing the SI unit obtained as a combination of unit a space is given between the symbols.
When using powers with a unit name the modifier is used after the unit name.