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| International System of Units |
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| In 1960, the Eleventh General Conference
of Weights and Measures recommended an International System of Units
(abbreviated as SI) based on the metric system of measurement. When the recommendation was accepted in 1968, it marked the first time in world history that a single system of units had been established internationally. This helped scientists working in different parts of the world to compare their data (measurements) easily. |
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| Scientists and engineers were leaders in
proposing SI units, but their adoption caused changes in many areas of life
all over the world. Road signs began to be marked in kilometers rather than
miles and articles were weighed in kilograms instead of pounds. An agreement
that benefited science has also been important in developing international
trade and communications. In this system the units are based on specific
quantities, which do not vary. For example, in this system: |
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Unit of Length is defined as the length of the path
traveled by light in vacuum during a time interval of
1/(2.99792458 X 108) seconds. |
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| Unit of Mass is defined as "the mass of a particular solid cylinder made of platinum-iridium alloy kept in Paris, known as the International Prototype Kilogram". |
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| Unit of time, second, is equal to the
duration of 9192631770 periods of the radiation corresponding to the
transition between two hyperfine levels of the ground state of the caesium
–133 atoms. |
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| In all the three definitions given above you will notice that the quantities do not vary and are independent of different environmental factors. |
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| The following table shows the basic units
in the SI system together with their symbols: |
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| Basic Units in the SI System |
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| Usually all small measurements are expressed by using the prefixes - deci, centi, milli, etc. with the units. |
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| For large measurements, we use deca, hecto, kilo etc. as prefixes with the units. The symbol and meaning of each prefix is given below |
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| It should be noted that while writing the units of a physical quantity certain guide lines have to be followed: |
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 Use
upper case to represent the symbol for a unit named after a scientist. |
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Use
lowercase to represent a unit not named after a scientist. |
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Do not
capitalize the first letter of the expansion of a unit named after a
scientist. |
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Never
use plural to represent a unit. |
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Insert
a period (.) or leave space to represent a compound unit formed by
multiplication of two or more units. |
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Use
negative power or solidus (/) for representing compound units formed by
dividing one unit by the other. |
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A
symbol for unit is represented by the first letter of the unit, except for
few units like hertz, Pascal, candela, mole, radian and steradian. |
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Never
use full stop or comma or colon after the symbol representing the unit. |
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Zero
should be placed before a decimal number. |
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Leave
space between a number and a unit. |
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A
hyphen should be placed in between the number and the unit when the number
is used as an adjective. |
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| Certain units of time which are not very commonly used are listed below: |
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| Year |
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| One year is the time which the earth takes for one revolution around the sun which is, equal to 365 days. |
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| Leap Year |
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| A leap year is the year in which the month of February is of 29 days and that year will have 366 days. |
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| Decade |
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| A decade is a period of 10 years. |
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| Century |
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| A century is a period of 100 years. |
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| Millennium |
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| A millennium is a period of 1000 years. |
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| Lunar month |
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| One lunar month is the time in which the moon completes one revolution around the earth. |
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| Units of all other physical quantities
can be derived from the basic units and hence are called "derived units".
The following table shows the list of various physical quantities, derived
formula and corresponding SI Units: |
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| Derived Units |
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