Physics > Unit and Dimensions > 3.0 SI units
Unit and Dimensions
1.0 Introduction
2.0 Physical quantity
3.0 SI units
3.1 Definition of standard units
3.2 System of units
3.3 Rules for writing units
3.4 Characteristics of a standard unit
3.5 Advantages of SI
4.0 SI prefixes
5.0 Conversion of units
6.0 Important practical units
7.0 Dimensions
8.0 Dimensional formula
9.0 Dimensional equation
10.0 List of dimensional formula
11.0 Application of dimensional analysis
11.1 To check the dimensional consistency of equations
11.2 To deduce relation among the physical quantities
11.3 To convert one system of unit into another system of unit
12.0 Limitations of dimensional analysis
3.3 Rules for writing units
3.2 System of units
3.3 Rules for writing units
3.4 Characteristics of a standard unit
3.5 Advantages of SI
11.2 To deduce relation among the physical quantities
11.3 To convert one system of unit into another system of unit
Rule 1: Unit is never written with capital initial letter even if it is named after a scientist.
Example: SI unit of pressure is "pascal" (correct) and Pascal (wrong)
Rule 2: If a unit is named after a scientist, the symbol is a capital letter.
Example:
| Quantity | Symbol |
| Force | newton $(N)$ |
| Pressure | pascal $(Pa)$ |
| Temperature | kelvin $(K)$ |
| Frequency | hertz $(Hz)$ |
| Energy | joule $(J)$ |
Rule 3: If the unit is not named after a scientist, the symbol is not a capital letter.
Example:
| Quantity | Symbol |
| Length | metre $(m)$ |
| Mass | kilogram $(kg)$ |
| Time | second $(s)$ |
| Luminous intensity | candela $(cd)$ |
Rule 4: Symbol of a unit should not be in plural form
Example: A force of $10$ newtons should be written as $10\ N$ and not $10\ Ns$.
| Wrong representation | Correct representation |
| $Ns$ | $N$ |
| $Ks$ | $K$ |
| Mols | mol |
Rule 5: Symbols of the derived unit is written either by putting a dot or leaving a space between the symbol of two units.
Impulse: $N\ s$ or $N.s$
Work done: $N\ m$ or $N.m$
Strength of magnet: $A\ m$ or $A.m$
