PHYSICAL WORLD
PHYSICAL QUANTITIES:
All quantities in terms of which laws of physics can be expressed and which can be measured directly or indirectly are called Physical quantities.
e.g. Length, mass, temperature, speed, force etc.
UNITS FOR MEASUREMENT:
Measurement of a physical quantity involves its comparison with the standard of the same kind.
The chosen standard of the same kind taken as reference to measure a physical quantity is called the unit of that quantity.
When making measurements of a physical quantity, the result is expressed as a number followed by the unit.
The number expresses the ratio of the measured quantity to some fixed standard and the unit is the name or symbol for the standard.
Over the years, a large number of standards have been defined for the physical measurement and many systems of units have evolved. Recently there has been an attempt to simplify the language of science by the adoption of a system of units, the system international, SI (In French, its English equivalent being International system), which is intended to be used universally. This system of units and measures in 1948, instructed an international committee to study the establishment of a complete set of rules for units of measurement.
Measurement of a physical quantity involves its comparison with the standard of the same kind.
The chosen standard of the same kind taken as reference to measure a physical quantity is called the unit of that quantity.
When making measurements of a physical quantity, the result is expressed as a number followed by the unit.
The number expresses the ratio of the measured quantity to some fixed standard and the unit is the name or symbol for the standard.
Over the years, a large number of standards have been defined for the physical measurement and many systems of units have evolved. Recently there has been an attempt to simplify the language of science by the adoption of a system of units, the system international, SI (In French, its English equivalent being International system), which is intended to be used universally. This system of units and measures in 1948, instructed an international committee to study the establishment of a complete set of rules for units of measurement.
TYPES OF SYSTEMS OF UNITS:
The following systems of units have been used- The French system or C.G.S. system and international system of units or S.I. system.
C.G.S. system. In this system the units of length, mass and time are in centimeter, gram and second respectively.
The centimeter is one- hundredth part of a metre and is defined as the distance between two lines engraved on the polished surface of certain platinum iridium bar, which is kept at the international Bureau of weights and Measures (BIPM) at, serves near Paris. It is also defined in terms of the standard wavelength of light and is equal to 1650763.73 wavelengths in vaccum or radiation corresponding to transition between the energy levels2p10 and 5d5 of krypton 86 atoms.
The Gram is one-thousandth part of a kilogram. Kilogram is the mass of a certain piece of platinum iridium alloy preserved in a vault at Serves France by the IBPM. The gram is also defined as equal to the mass of 1cc of pure water at 3.98o C under the atmospheric pressure.
The second or the mean solar second is defined as 1/ 86400th part of a mean solar day i.e. the time between two consecutive noon’s or the average time which elapses during a year between two successive passages of the sun across any one straight line drawn from pole to pole on the earths surface.
The mean sideral second is 1.86400th part of the sideral day of the true period of revolution of the earth on its axis or the interval, which elapses between two consecutive passages of a fixed star across the meridian.
For physical measurements a second since 1964, has been defined as 9192631770 time periods of radiation corresponding to the uninterrupted transition between the hyperfine levels of the ground state of Cesium (C133) atom
Additional Units for length
One light year = distance travelled in one year
= 1016 m
One parsec = 3.08 x 1016 m = 3.26 light year
The following systems of units have been used- The French system or C.G.S. system and international system of units or S.I. system.
C.G.S. system. In this system the units of length, mass and time are in centimeter, gram and second respectively.
The centimeter is one- hundredth part of a metre and is defined as the distance between two lines engraved on the polished surface of certain platinum iridium bar, which is kept at the international Bureau of weights and Measures (BIPM) at, serves near Paris. It is also defined in terms of the standard wavelength of light and is equal to 1650763.73 wavelengths in vaccum or radiation corresponding to transition between the energy levels2p10 and 5d5 of krypton 86 atoms.
The Gram is one-thousandth part of a kilogram. Kilogram is the mass of a certain piece of platinum iridium alloy preserved in a vault at Serves France by the IBPM. The gram is also defined as equal to the mass of 1cc of pure water at 3.98o C under the atmospheric pressure.
The second or the mean solar second is defined as 1/ 86400th part of a mean solar day i.e. the time between two consecutive noon’s or the average time which elapses during a year between two successive passages of the sun across any one straight line drawn from pole to pole on the earths surface.
The mean sideral second is 1.86400th part of the sideral day of the true period of revolution of the earth on its axis or the interval, which elapses between two consecutive passages of a fixed star across the meridian.
For physical measurements a second since 1964, has been defined as 9192631770 time periods of radiation corresponding to the uninterrupted transition between the hyperfine levels of the ground state of Cesium (C133) atom
Additional Units for length
One light year = distance travelled in one year
= 1016 m
One parsec = 3.08 x 1016 m = 3.26 light year
F.P.S. system: In this system the unit of length is foot, the unit of mass is pound and the unit of time is second.
The foot is one third of the distance between two lines of platinum iridium bar at a temperature of 62o F kept at the standard office of the Board of Trade London.
Second - same as defined above.
M.K.S. System: The unit of mass in this system is kilogram, the unit of length is meter and unit of time is second.
UNITS:
A unit of quantity may be defined as fixed amount of that quantity to which comparison is the basis of all measurements. In order to form an exact idea of the physical quantity it is necessary to express the standard or unit in which the quantity is measured and also the number of times the quantity contains unit.
FUNDAMENTALS AND DERIVED QUANTITIES:
Fundamental quantities are length, mass and time. They are also called indefinable of mechanics. All the other physical quantities can be expressed in terms of one or more of these quantities and are known as derived quantities
E.g. Area, volume, speed etc
M.K.S system
UNIT OF LENGTH:
Meter is defined as the distance between the centers of two transverse lines engraved on polished surface of platinum iridium bar, at the international bureau of weights and measurements.
UNIT OF TIME:
Seconds-1/86400 of the mean solar day is taken as the unit of time called as second.
SI UNITS:
M.K.S units are confined to the field of mechanics only. This system could not meet the requirements of various fields like electrical engineering, thermodynamics chemical engineering etc. so a comprehensive & internationally recognized system of unit was recommended in 1960 consisting of 7 basic units and two supplementary units
| Quantity | Unit | Abbreviation |
| Mass | Kilogram | Kg |
| Length | Meter | m |
| Time | Second | s |
| Electric current | Ampere | A |
| Temperature | Kelvin | K |
| Luminous intensity | Candela | Cd |
| Quantity of matter | Mole | mol |
MULTIPLES AND SUBMULTIPLES:
One of the virtues of the SI unit of system is the standardization of the use of prefixes to indicate multiples & submultiples. The prefixes in the following table with their symbols indicated in the brackets are to be avoided.
One of the virtues of the SI unit of system is the standardization of the use of prefixes to indicate multiples & submultiples. The prefixes in the following table with their symbols indicated in the brackets are to be avoided.
| Prefix | Symbol | Multipliers |
| Tera | T | 1012 |
| Giga | G | 109 |
| Mega | M | 106 |
| Kilo | K | 103 |
| Hecto | (h) | 102 |
| Deca | (da) | 101 |
| Deci | (d) | 10-1 |
| Centi | (C) | 10-2 |
| Milli | m | 10-3 |
| Micro | m | 10-6 |
| Nano | n | 10-9 |
| Pico | P | 10-12 |
| Femto | f | 10-15 |
| Atto | a | 10-18 |
DIMENSION OF VOLT
V = w/q
Q = It
W = ML2T-2 IT
= ML2T-3I-1 DIMENSION OF AMPERE
I = V I
= ML2T-3I-2
THE METHOD OF WRITING THE UNITS:
The use of small and capital letters for various symbols should be correctly given otherwise they will create confusion.
V = w/q
Q = It
W = ML2T-2 IT
= ML2T-3I-1 DIMENSION OF AMPERE
I = V I
= ML2T-3I-2
THE METHOD OF WRITING THE UNITS:
The use of small and capital letters for various symbols should be correctly given otherwise they will create confusion.
| Quantity | Correct | Incorrect |
| Mega | M | m |
| Second | s | sec |
| Metre | m | M or mtr |
| Gram | g | gm or gr or grm |
| The use of small and capital letters for various symbols should be correctly given otherwise they will create confusion. | ||
| Mega | M | m |
| Metre | m | M |
| Hour | h | T or Hr |
| Ton | t | T |
| Year | a | yr |
| The proper space (single space) is to be left between numerical quantity and symbol or between two symbols. | ||
| Newton Metre | 5 N m | 5Nm |
| Metre Kelvin | 10 m K | 10mK |
| No space is to be left between prefix and symbol of the units | ||
| Millimetre | mm | m m |
| Kilogram | kg | k g |
| Symbols are not to be pluralized. Do not add “s” for plural. | ||
| 8 kilograms | 8 kg | 8 kgs |
| 220 volts | 220 V | 20 Vs |
If a set dimensions is in the same unit the unit symbol is to be given in the last dimensions only.
25 x 10 x 5 mm instead of 25 mm x 10 mm x 5mm
4 x 25 x 2 mm instead of 4 m x 25 m x 2 m
The symbols of units synonymous to scientist’s names should be always in capital. The first letter in units synonymous to scientist’s names should not be capitalized when the unit is written in full.
25 x 10 x 5 mm instead of 25 mm x 10 mm x 5mm
4 x 25 x 2 mm instead of 4 m x 25 m x 2 m
The symbols of units synonymous to scientist’s names should be always in capital. The first letter in units synonymous to scientist’s names should not be capitalized when the unit is written in full.
| Quantity | Correct | Incorrect |
| Force | N (newton) | n (Newton) |
| Power | W (watt) | w (Watt) |
| Frequency | hertz | Hertz |
| Force | newton | Newton |
TEMPERATURE:The S.I unit for temperature is Kelvin with symbol K. for practical purposes Celsius temperature scale can also be used.
When expressing temperature in Celsius, the word ‘ degree’ or symbol ‘o’ is to be used
| Correct | Incorrect |
| 5 degree Celsius | 5 Celsius |
| 5o C | 5 C |
It is incorrect to use ‘degree centigrade’ for measurement of temperature only degree Celsius is to be used for measuring temperature.
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