The International System of Units: Class 11 Physics Notes, Definition, Working Principle, Formula & Real-Life Applications

SI Base Units

1. Length – Metre ( $m$): Defined by the distance light travels in a vacuum in $\frac{1}{299792458}$ seconds.
2. Mass – Kilogram ( $\mathrm{kg}$): Defined by Planck’s constant $h=6.62607015\times {10}^{-34}\text{\hspace{0.17em}J·s}$.
3. Time – Second ( $s$): Defined by $9192631770$ periods of the cesium-133 atom’s hyperfine transition.
4. Electric Current – Ampere ( $A$): Defined by the flow of $\frac{1}{1.602176634\times {10}^{-19}}$ elementary charges per second.
5. Thermodynamic Temperature – Kelvin ( $K$): Defined by the Boltzmann constant $k=1.380649\times {10}^{-23}\text{\hspace{0.17em}J·K⁻¹}$.
6. Amount of Substance – Mole ( $\mathrm{mol}$): Defined by Avogadro’s number $N_A=6.02214076\times {10}^{23}$ elementary entities.
7. Luminous Intensity – Candela ( $\mathrm{cd}$): Defined by the luminous efficacy of monochromatic radiation of frequency $540\times {10}^{12}\text{\hspace{0.17em}Hz}$ at $683\text{\hspace{0.17em}lm·W⁻¹}$.

These units form the foundation for all measurements, and JEE Main problems often test their definitions or conversions.

 Basic SI units

Table 2.2 Some units retained for general use (Though outside SI)

• Numerical Problems: Calculating quantities like work $J$ or power $W$.
• Unit Conversions: Converting between SI and non-SI units (e.g., $J\to \mathrm{dJ}$).
• Dimensional Analysis: Verifying equations using base-unit dimensions.
• Formula Application: Using correct units in formulas like $F=ma$ or $W=Fd$.

Understanding SI units prevents errors in calculations and enhances problem-solving efficiency.