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Units and Measurement

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Physics Part I

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Class 10 Class 12
One mole of an ideal gas at standard temperature and pressure occupies 22.4 L (molar volume). What is the ratio of molar volume to the atomic volume of a mole of hydrogen ? (Take the size of hydrogen molecule to be about 1 Å). Why is this ratio so large?

Diameter space of space hydrogen space atom comma space straight D space equals space 0.5 space straight A with straight o on top space equals space 0.5 cross times 10 to the power of negative 10 space end exponent straight m

Volume comma space straight V space equals space 4 over 3 straight pi space open parentheses straight D over 2 close parentheses cubed space space

equals space 5.238 space cross times space 10 to the power of negative 31 end exponent space straight m cubed space

Ratio space of space molar space volume space to space volume space of space one space atom space is comma space

fraction numerator 22.4 over denominator 5.238 cross times 10 to the power of negative 28 end exponent end fraction space equals space 4.276 space cross times space 10 to the power of 28 space

Ratio space os space large space because semicolon space

straight i right parenthesis thin space No. space of space atoms space in space straight a space mlole space of space gas space is space large. space

ii right parenthesis thin space Distance space between space hydrogen space molecules space almost equal to space 10 space straight A with 0 on top

What is the need of measurement?

To get the complete knowledge of any physical quantity, measurement is needed. Measurement of any physical quantity involves comparison with a certain basic, arbitrarily chosen, internationally accepted reference standard called unit. Knowledge without measurement is incomplete and unsatisfactory. 

Define unit.

The quantity used as a standard of measurement is called unit. 

Example: Unit of time is second. 


Define physical quantity.


A measurable quantity in terms of which laws of Physics can be expressed is called physical quantity.

The result of a measurement of a physical quantity is expressed by a number accompanied by a unit.


Give four examples of physical quantities.

Examples of Physical quantitie are force, mass, density and power.

What is measurement?

Measurement is the process of attaching a numeric value to an aspect of a natural phenomenon. Measurement is making the quantitative knowledge of a physical quantity.