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| Need of Measurement and System of Units |
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| Before all the branches of science were clubbed together under the nomenclature 'Natural philosophy', under which all observations of subjective nature were being carried out and spirit of enquiry was almost non-existent, we were satisfied with simple explanations. |
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| The subjective interpretation including measurement, obviously varies from person to person, since the interpretation is based on one's senses. Gradually, the generations began to ask 'how' things are happening. Thus, the observations became more objective. Thus, physics - one of the many sciences, became a subject of observation and measurement. |
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| When we quantify, what we are observing, then such an observation is a science or knowledge. However, without the quantification of an observation, thought is simply a process of gaining knowledge, but has yet to reach a stage, to be called a 'science'. |
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| Hence, to call a subject 'science', the thought process ultimately has to be quantified suitably, using appropriate units of measurement. |
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| There are 7 physical quantities viz., Length, mass, time, temperature, electric current, luminous intensity and amount of substance. |
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| Length, mass and time are called fundamental physical quantities. |
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| The units of fundamental physical quantities are called fundamental units. They are m, kg and sec. These units can neither be derived from one another nor can be resolved into other units. They are independent to each other. |
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| Units of physical quantities can be expressed in terms of fundamental units and such units are called derived units. |
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| Unit of area can be an example for derived unit. If L is the length of square then L x L = L2 is its area. Similarly, the volume of a cube is L x L x L = L3 cubic area. Units of any physical quantity can be derived from its defining equation. |
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| The characteristics of several chosen units are: |
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 they must not vary with place and time |
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 should be easily reproducible |
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 should be well-defined |
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 should be of proper size |
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 should not change easily with the changing physical conditions like temperature, pressure etc. |
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 should be accessible, i.e. not too small when compared to the quantities to be measured. |
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| System of units: |
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| Following are the common system of units to measure mass, length and time. |
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| CGS |
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| Fundamental units of length, mass and time are centimetre, gram and second. |
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| MKS |
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| Fundamental units of length, mass and time are metre, kilogram and second. It is a coherent system of units in mechanics. |
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| FPS |
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| Known as British system of units, it is not a metric system. It stands for foot, pound and second. Its use is declining in scientific work. |
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| SI |
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| It is a new system introduced by General Conference of Weights and Measures in 1960. It is called "Le Systeme International d' Unites". |
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| It has seven basic and three supplementary units |
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| Supplementary quantities and their units |
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| Prefixes are used for large and small quantities. The following table gives prefixes, their symbol and their values in powers of 10, as used in SI units. |
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| Among the systems of units mentioned above, the SI units i.e., Le systeme internationale d' unites, is a new, comprehensive and rationalised system of units, accepted by the 11th conference of weights and measures in 1960. It is used in science and technology all over the world, because of the following advantages over others. |
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 The units are more logical and coherent. |
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 The units are more easily reproducible. |
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 The units do not change with time as they are based on the properties of an atom. |
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 The units are more rationalised viz., heat energy, electric energy and mechanical energy are measured in joule (J). |
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 The units are more comprehensive, covering all disciplines of science and technology. |
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 The units are convertible into CGS system very easily, whenever needed. |
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 The units are a metric system in multiples and subsequently, can be expressed as the powers of 10. |
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| However, the SI units are governed by the following rules: |
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 Full names of the units do not begin with capital letter |
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| E.g., newton and not Newton. |
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 The symbols of units, named after a scientists, have initial capital letter. |
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| E.g., J for joule, K for Kelvin. |
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 Symbols do not have plural form. |
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| E.g., 5 kg (not 5 kgs), 25 m (not 25 metres). |
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 A unit symbol is represented by the first letter of the unit name and no full stop, comma or colon is put after the symbol, with a few exceptions like Hz, Pa, cd and mol. |
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 Multiplication of units is neither shown by leaving a space nor by using crosses or dots. |
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| E.g., Nm and not N m or N x m. |
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 Division of units is indicated by use of a stroke (/) sign (not more than one line) or -ve powers. |
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 A zero precedes a decimal. |
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| E.g., 0.7kg & not .7kg |
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 Compound prefixes should be avoided. |
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 A hyphen is not put in-between the number and the unit, when the number is used as an adjective. |
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| E.g., 16 mm film. For writing the thickness of a glass of 10 mm, it is written as 10 mm and not 10-mm. |
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 A space must be left between a number and a unit. |
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| E.g., 6 kg and not 6kg |
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| Length |
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| The shortest distance between the two ends of a body is its length. During 1120 A.D., the King of England had enforced the usage of 'Yard' in his country, as the unit of length, which was equal to the distance from the tip of his nose to the end of his out stretched arm. Similarly, the King of France had declared the length of the Royal foot of the King as a measure and called it one foot. But in 1799, the foot was replaced by 'metre', equal to one-tenmillionth the distance of the equator to the north pole along a longitudinal line that passes through Paris. |
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| In 1889, 1 metre was defined as the distance between 2 lines on a specific platinum-iridium bar stored under controlled conditions near Paris. Since the S and T demanded more precise standard, in 1960, 1 metre was redefined as 1, 650, 763.73 wavelengths of a particular orange-red light emitted by atoms of krypton-86 in a gas discharge tube. But of late, 1 metre is defined as the length of the path travelled by light in vacuum during a time interval of 1/299, 792, 458 of a second. This unit is believed to be absolutely constant. |
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| Planck length |
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| Distance travelled by light in a time interval of one planck time, the shortest distance possible, is 10-43 m. |
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| Fermi (F) |
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| One fermi = 10-15 m used to measure nuclear dimensions. |
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| Angstrom (A) |
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| One angstrom = 10-10 m used to measure atomic dimensions. |
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| Astronomical unit (AU) |
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| It is used in measuring long distances converted with solar studies in astronomy. |
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| 1 Au = 1.496 x 1011m, the mean distance between the sun and the Earth. |
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| Light year (1 yr) |
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| is the distance travelled by light in vacuum in one year. |
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| Distance = velocity x time |
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| One light year = 3 x 108 m/s x 1 |
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| = 3 x 108 x 365 x 24 x 60 x 60 x m/s x s |
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| = 9.46 x 1015 m |
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| This unit is used to measure intergalactic distances as well as stellar distances. |
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| Parallactic second |
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| The distance at which the mean radius of the Earth's orbit subtends an angle of one second of arc. |
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| Note: |
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 1 Au = 1.496 x 1011 m |
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 1 ly = 9.46 x 1015 m |
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 1 parsec = 3.0857 x 1016 m |
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 1 F = 10-15 m |
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 1 micron or 1mm = 10-6 m |
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