# Physical Quantities,Standards and Units

What are Physical quantities?

Physics laws can be explained in terms of Physical quantities such as time, force,temperature,density etc..

• How Physical quantities are divided?

They are mainly divided into two i.e

1.Fundamental quantity
2.Derived quantity

Derived quantities definitions can be explained using other physical quantities where as fundamental physical quantities can be defined by itself as they are the basic

Some of the examples of derived quantities: Speed,area,density etc..

The Physical quantities can be measured in two step process:

1.A standard unit

2.Comparison of the standard unit to the quantity to be measured

Ex: This Dumbbell weighs 20 kgs. it means that the mass of the dumbbell is 20 times the units of mass which is a kilogram. Hence all measurements in physics can have a standard unit.In the ancient days different countries used different units but in 1960 the General Conference of Weights and Measures suggested to use a specific metric system of measurements called international system of units abbreviated as SI units to be used.

• Units for  Length:

The SI units for length is the metre denoted by ‘m’. Firstly it is defined as the distance between two lines on a platinum-iridium rod at 0°c placed at  International office of weights and measures at sevres near Paris.

However they re-defined it in 1983 as the length of the path travelled by light in vacuum during a time interval of 1/299792458 of a second is known as metre.

Different metric units for measuring lengths in terms of metre are either the multiples or sub multiples 10.

1 kilometre – 1000 or 10^3 m

2. centimetre -1/100 or 10^-2 m

3.millimeter – 1/1000 or 10^-3 m

The very small distances can be measured in micrometers or microns(µm),angstroms(A°),nanometres(nm) and femtometres(fm)

1m = 10^6 µm
= 10^9 nm
= 10^10 A°
= 10^15 fm

and at last the large distances can be measured  in kilometres

• Wavelengths of light in angstroms
• Nuclear sizes can be measured in femtometres
• The extra large distances like the distance between the sun and earth and other stars can be expressed in light years.The light year is a unit of length which is equal to the distance travelled by light in one year

1 Light year =  9.46*10^15 m

• Units for Mass:
The SI units for mass is kg .This standard kilogram is the mass of a particular solid cylinder of platinum-iridium alloy kept at sevres.

The following are the multiples and sub-multiples of kilograms generally used:
• 1 tonnet(t) = 10^3 kg
• 1 gram = 10^-3 kg
• 1 milligram (mg) = 10^-6 kg
•  Units for Time:
Firstly the SI units for time was decided as second(s).The second is a 1/86400th part of a mean solar day.The mean solar day is the average period between successive transits of the sun across the meridian, taken over 12 months ,at any part of the earth’s surface but however the speed of the earth’s rotation is slowing down with time, so it was neglected as its not accurate.
Again second was redefined in 1967 using an atomic clock which works on energy changes in gaseous caesium atoms. The caesium atoms in the atomic clock act like a pendulum in a pendulum clock. 