In 1960, an international committee agreed on a set of definitions and standards to describe the physical quantities. The system that was established is called the system international (SI).
Due to simplicity and convenience this system of units is being used by the world’s scientific community and by most nations.
The SI units are build-up from three kinds of units:
- Base units
- Supplementary units
- Derived units
Base units
“The units defined arbitrarily for the measurement of seven base quantities in comparison with them are called base units.” The names of base units along with respective physical quantities and symbols are given below:
The
standard definitions of base units are given below:
1.
Meter: The unit of length is meter. Before
1960, it was defined as:
a.
Definition 1: “The distance between two lines marked
on the bar of an alloy of platinum (90%) and iridium (10%) kept under
controlled conditions at the international bureau of weights and measures in France”.
The 11th
general conference on weights and measures (1960) redefined the standard as
follows:
b.
Definition 2: “One meter is a length equal to
1,650,763.73 wavelength in vacuum of the orange red radiation emitted by the
krypton-86 atom”.
In 1983, the meter
was redefined as:
c.
Definition 3: “It is the distance travelled by light
in vacuum during a time of 1/299,792,458 second”.
A few sub-multiple and multiples of
meter are:
2.
Kilogram: The unit of mass is kilogram. It is
defined as: “the mass of a platinum (90%) and iridium (10%) alloy cylinder,
3.9cm in diameter and 3.9cm in height, kept at the international bureau of
weights and measures in France”.
This mass standard was established in 1901.
A few conversion relations of kg are:
3.
Second: The unit of time is termed as second.
a.
Definition 1: “One second is equal to 1/86400 part
of an average day of the year 1900 A.D”.
In 1967, an
international committee redefined second as
b.
Definition 2: “One second is equal to the duration
in which the outer most electron of the cesium-133 atom makes 9,192,631,770
vibrations”.
A few sub-multiples and multiples of
second are as given below:
4.
Kelvin: The unit of temperature is Kelvin.
Definition: “It is the fraction 1/273.16 of the thermodynamic
temperature of the triple point of water”.
The triple point of a
substance means the temperature at which solid, liquid and water vapour phases
are in equilibrium. The triple point of water is taken as 273.16K. This
standard was adopted in 1967.
5.
Ampere: The unit of electric current is
ampere.
Definition: “One ampere is that constant current which if maintained in
two straight parallel conductors of infinite length, of negligible circular
cross-section and placed a meter apart in vacuum, would produce between those conductors
a force equal to 2x10-7 Newton per meter of length”.
This unit was
established in 1971.
6.
Candela: The unit of light intensity (or
luminous intensity) is candela, which is defined as:
Definition: “One candela is the luminous intensity in the perpendicular
direction of a surface of 1/600,000 square meter of a black body radiator at
the solidification temperature of platinum under standard atmospheric pressure”.
This definition was
adopted in 1967.
7.
Mole: The unit of amount of substance is
mole.
Definition: “One mole is the amount of substance
of a system which contains as many elementary entities as there are atoms in
0.012 kg of carbon-12”. This standard was adopted in 1971. When the unit mole
is used, the element entities must be specified. There may be atoms, molecules,
ions, electrons, other particles or specified groups of such particles. One
mole of any substance contain 6.0225x1023 entities.
Supplementary units
Those SI
units which are not included either in base units or in derived units are
called supplementary units. These are two in number as given below:
Radian: The
radian is the plane angle between two radii of a circle which cut off on the
circumference on arc, equal in length to the radius of the circle (see Figure).
 |
| Total angle of the circumference = 2 π rad |
Steradian: The steradian is the solid angle
(three dimensional angle) subtended at the centre of a sphere by an area of its
surface equal to the square of radius of the sphere (see Figure)
Derived units
SI-units
for measuring all other physical quantities are derived from the base and
supplementary units. These are called derived units. Some of the derived units
are given as under:
Scientific notation
“When the
numbers are expressed in standard form, it is called scientific notation, which
employs power of ten”.
Rule: whiles writing the numbers in
scientific notation, there should be only one non-zero digit to the left of
decimal.
Examples:
Conventions for indicating units
Following
points should be kept in mind while using units:
- Full name of the units does not begin with a capital letter even if named after a scientist. e.g. newton.
- The symbol of unit named after a scientist has initial capital letter, such as ‘N’ for newton.
- The prefix should be written before the unit without any space, such as 1 x 10-3 m = 1 mm
- A combination of units is written each with one space apart. e.g. newton meter = N m
- Compound prefixes are not allowed. e.g. 1µµF may be written as 1pF
- A number such as 5.0 x 104 cm may be expressed in scientific notation as 5.0 x 102 m
- When a multiple of a base unit is raised to a power, the power applies to the whole multiple and not the base unit alone. e.g. 1 km2 = 1 (km)2 = 1 x 106 m2
- Measurement in practical work should be recorded immediately in the most convenient unit. e.g. reading of screw gauge in ‘mm’ mass in ‘grams’. But before calculation for the result, all measurements must be converted into SI-units.
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