(General Science) PHYSICS - Fundamental SI Units, SI Derived Units, Names and Symbols

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:

  1. Scalar quantities
  2. 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

m2

Volume

Cubic meter

m3

Speed, velocity

Meter per second

m/s

Acceleration

Meter per second squared

m/s2

Wave number

Reciprocal meter

m-1

Mass density

Kilogram per cubic meter

Kg/m3

Specific volume

Cubic meter per kilogram

m3/kg

Current density

Ampere per square meter

A/m2

Magnetic field strength

Ampere per meter

A/m

Amount-of-substance concentration

Mole per cubic meter

mol/m3

Luminance

Candela per square meter

cd/m2

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/m2

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

V/A

Electric conductance

siemens

S

A/V

Magnetic flux weber Wb V.s

Magnetic flux density

tesla

T

Wb/m2

Inductance

henry

H

Wb/a2

Celsius temperature

degree Celsius

oC

-

Luminous flux

lumen

lm

cd.sr

Illuminance

lux

lx

lm/m2

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 1015m
  • 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 1011 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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