(General Science) PHYSICS - Fundamental SI Units, SI Derived Units, Names and Symbols
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GENERAL SCIENCE: PHYSICS
BASIC PHYSICAL PARAMETERS AND UNITS
Standards and Units: Laws of physics are expressed in terms of physical quantities such as time, force, temperature, density and numerous other parameters. Physical quantities are often divided into fundamental and derived quantities. Derived quantities arc those whose definitions are based on other physical quantities, e.g., speed, area, density, etc. Fundamental quantities are not defined of other physical quantities, e.g., length, mass and time.
Physical quantities may, in general, be divided in two classes:
- Scalar quantities
- Vector quantities
A scalar quantities is one which has only magnitude. A vector quantity has both magnitude and direction. Thus, when we say that the height of a tree is 20 metres or there is 5 litres of water in a bucket, we are dealing with scalar quantities. On the other hand, when we say that a force of 2 Newton (newton is a unit of force) is acting on a body, the information is incomplete unless we state the direction of force, for instance 2 Newton vertically upwards. Force is therefore a vector quantity. Mass, length, time, volume, speed, energy, work are examples of scalar quantities. Velocity, momentum, force, acceleration are examples of vector quantities .
The measurement of physical quantities involves two steps: (i) the choice of a standard (unit) and (ii) the comparison of the standard to the quantity to be measured. Thus a number and a unit determine the measure of a quantity. For example, when we say that the mass of a person is 75 kilogram, it means that his mass is 75 times the unit of mass, kilogram. Thus all measurements in physics require standard units. Earlier, workers in various countries used different systems of units. In 1960, the General Conference of Weights and Measures recommended that a metric system of measurements called the International System of Units, abbreviated as SI units, be used.
The seven fundamental SI units are given in the following table:
Fundamental SI Units
Base quantity |
Name |
Symbol |
---|---|---|
Length |
Meter |
M |
Mass |
kilogram |
kg |
Time |
Second |
S |
Electric current |
Ampere |
A |
Thermodynamic Temperature |
Kelvin |
K |
Amount of substance |
Mole |
mol |
Luminous intensity |
Candela |
cd |
Derived SI Units
Other quantities, called derived quantities, are defined in terms of the seven base quantities via a system of quantity equations. The SI derived units for these derived quantities are obtained from these equations and the seven SI base units. Examples of such SI derived units are given in the following table, where it should be noted that the symbol 1 for quantities of dimension 1 such as mass fraction is generally omitted.
Examples of SI Derived Units
SI derived unit |
||
---|---|---|
Derived quantity |
Name |
Symbol |
Area |
Square meter |
m^{2} |
Volume |
Cubic meter |
m^{3} |
Speed, velocity |
Meter per second |
m/s |
Acceleration |
Meter per second squared |
m/s^{2} |
Wave number |
Reciprocal meter |
m^{-1} |
Mass density |
Kilogram per cubic meter |
Kg/m^{3} |
Specific volume |
Cubic meter per kilogram |
m^{3}/kg |
Current density |
Ampere per square meter |
A/m^{2} |
Magnetic field strength |
Ampere per meter |
A/m |
Amount-of-substance concentration |
Mole per cubic meter |
mol/m^{3} |
Luminance |
Candela per square meter |
cd/m^{2} |
Mass fraction |
Kilogram per kilogram, which may be represented by the number 1 |
kg/kg=1 |
For ease of understanding and convenience, 22 SI derived units have been given special names and symbols, as shown in the following table.
SI derived units with special names and symbols
Derived quantity |
Name |
Symbol |
Expression in terms of other SI units |
---|---|---|---|
Plane angle |
radian |
rad |
- |
Solid angle |
steradian |
sr |
- |
Frequency |
hertz |
Hz |
- |
Force |
newton |
N |
- |
Pressure, stress |
pascal |
Pa |
N/m^{2} |
Energy, work, quantity of heat |
joule |
J |
N.M |
Power, radiant flux |
watt |
W |
J/s |
Electric charge, quantity of electricity |
coulomb |
C |
- |
Electric potential difference, electromotive force |
Volt |
V |
W/A |
Capacitance |
farad |
F |
C/V |
Electric resistance |
ohm |
Ω |
V/A |
Electric conductance |
siemens |
S |
A/V |
Magnetic flux | weber | Wb | V.s |
Magnetic flux density |
tesla |
T |
Wb/m^{2} |
Inductance |
henry |
H |
Wb/a^{2} |
Celsius temperature |
degree Celsius |
^{o}C |
- |
Luminous flux |
lumen |
lm |
cd.sr |
Illuminance |
lux |
lx |
lm/m^{2} |
Activity (of a radionuclide) |
becquerel |
Bq |
- |
Absorbed dose, specific energy (imparted), kerma |
gray |
Gy |
J/kh |
Dose equivalent |
sievert |
Sv |
J/kg |
Catalytic |
katal |
kat |
Some commonly used units other than SI units
- Light years: the light year is a unit of length and is equal to the distance travelled by light in one year. It is used to express large astronomical distance like the distance between the sun and earth etc. 1 light year = 9.46 x 10^{15}m
- An Astronomical Unit (A.U) is the mean distance from the centre of the earth to centre of the sun. 1 A. U = 1.495 x 10^{11} m.
- F. P. S system is used in Britain, where length is measured in Foots, mass in pounds and time in Seconds.
- In C.G.S system, length is measured in Centimeter, mass in Grams and time in Seconds.
- Barrel is the internationally used unit for measuring the volume of crude oil. 1 Barrel = 159 Litres.
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