Units and Measurement

Units and Measurement

Introduction : Experiments and measurement forms the basic of the Physics.We can get knowledge of the surrounding by sense of vision, hearing, Touching,etc.To study and measure phenomenon in physics we have to perform experiments.In physics we come across Various experiments that needs to be measured for further analysis.For measurements of same ,we need to have good knowledge on units and measurements.

Physics : Physics is a branch of science which deals with the study of secret of matter.

Unit

• Any  physical Quantity can be measured and expressed in term of number and units.
• Hence for the measurement of a physical quantity, we need a certain reference standard.
• The reference standard used for the measurement of  a physical Quantity is called a unit

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  Properties of a Unit

  A good unit should have following properties

  • It should be easily available

  • It should be easily invariable( should not change with space and time)

  • It should be universally accepted

  • It should be reproducible and not perishable.

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Systems of Units

Earlier used system

CGS : Centimeter , Gram,  second

MKS : Metre,  Kilogram, Second

FPS : Foot,   Pound,   Second

In 1971, International General Conference on weights and Measures recommended the use of International System Of Units called as SI Units : System International.

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Physical Quantity

Any physical property  that can be quantified and measured is called Physical Quantity

Physical Quantity is classified into two category viz.

1. Fundamental Quantity
2. Derived Quantity

Fundamental Quantity: The physical quantities which do not depend on any other physical quantities for their measurements are known as fundamental quantities. Eg: Mass, Length, etc.

Fundamental units: The units used to measure fundamental quantities are called fundamental units. 

The Fundamental Quantities are given In table Below:

Derived Quantity: The physical quantities which depend on any other physical quantitiesfor their measurements are known as derived quantities. Eg: Force, Work,Energy,etc.

Derived units: The units used to measure derived quantities are called derived units.

Some of the Derived Quantities are given in Table below:

Supplementary Units

Supplementary units : Besides, the seven fundamental or basic units, there are two more units called supplementary units: (i) Plane angle dθ and (ii) Solid angle dΩ

Plane angle (dθ) : This is the ratio of the length of an arc of a circle to the radius of the circle .Thus dθ = ds/r is the angle subtended by the arc at the centre of the circle. It is measured in radian (rad). An angle θ in radian is denoted as θc .

Solid angle (dΩ) : This is the 3-dimensional analogue of dθ and is defined as the area of a portion of surface of a sphere to the square of radius of the sphere. Thus dΩ = dA/r2 is the solid angle subtended by area ds. It is measured in steradians (sr). A sphere of radius r has surface area 4πr2. Thus, the solid angle subtended by the entire sphere at its centre is Ω = 4πr2 /r2 = 4π sr. 

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Convention for SI Units

Unit of every physical quantity should be represented by its symbol.

1. Full name of a unit always starts with smaller letter even if the name is after a person, e.g., 1 newton, 1 joule, etc. But symbol for unit named after a person should be in capital letter, e.g., N after scientist Newton, J after scientist Joule, etc.

2. Symbols for units do not take plural form for example, force of 20 N and not 20 newtons or not 20 Ns.

3. Symbols for units do not contain any full stops at the end of recommended letter, e.g., 25 kg and not 25 kg..

4. The units of physical quantities in numerator and denominator should be written as one ratio for example the SI unit of acceleration is m/s2 or m s-2 but not m/s/s.

5. Use of combination of units and symbols for units is avoided when physical quantity is expressed by combination of two. e.g., The unit J/kg K is correct while joule/kg K is not correct.

6. Space or hyphen must be introduced while indicating multiplication of two units e.g., m/s should be written as m s-1 or m-s-1 and Not as ms-1 (because ms will be read as millisecond).

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