What is the SI System?
The International System of Units (SI) is the modern metric system used worldwide for scientific, technical, and everyday measurements. It replaced older systems like CGS, FPS, and MKS. SI units are divided into base units (fundamental) and derived units (combinations of base units).
SI Base Units
| Base Quantity | Unit Name | Symbol | Definition |
|---|---|---|---|
| Length | metre | m | Defined using the speed of light \( c = 299792458 \, \text{m/s} \). |
| Mass | kilogram | kg | Based on the Planck constant \( h = 6.62607015 \times 10^{-34} \, \text{J·s} \). |
| Time | second | s | Defined using the caesium-133 atom’s frequency \( \Delta \nu_{Cs} = 9192631770 \, \text{Hz} \). |
| Electric Current | ampere | A | Based on the elementary charge \( e = 1.602176634 \times 10^{-19} \, \text{C} \). |
| Temperature | kelvin | K | Defined using the Boltzmann constant \( k = 1.380649 \times 10^{-23} \, \text{J/K} \). |
| Amount of Substance | mole | mol | One mole contains \( 6.02214076 \times 10^{23} \) entities (Avogadro’s number). |
| Luminous Intensity | candela | cd | Defined using the luminous efficacy of light at \( 540 \times 10^{12} \, \text{Hz} \). |
Supplementary Units
- Plane Angle (radian): \( d\theta = \frac{\text{arc length } ds}{\text{radius } r} \).
- Solid Angle (steradian): \( d\Omega = \frac{\text{area } dA}{r^2} \).
Derived Units & Retained Units
Derived units are combinations of base units (e.g., speed = m/s). Some units outside SI are still used:
| Unit | Symbol | Value in SI |
|---|---|---|
| minute | min | 60 s |
| litre | L | \( 1 \, \text{dm}^3 = 10^{-3} \, \text{m}^3 \) |
| day | d | 86400 s |
| tonne | t | \( 10^3 \, \text{kg} \) |
Significant Figures
These indicate the precision of a measurement. Rules include:
- All non-zero digits are significant (e.g., 287.5 cm has 4 significant figures).
- Leading zeros are not significant (e.g., 0.02308 m has 4 significant figures).
- Changing units doesn’t change significant figures (e.g., 2.308 cm = 23.08 mm).
Example
A pendulum’s period is 1.62 s. Here, 1 and 6 are certain; 2 is uncertain → 3 significant figures.
Key NEET Topics
- 💡 The seven SI base units and their definitions (e.g., kilogram via Planck constant).
- 💡 Derived units and conversions (e.g., litre to cubic metre).
- 💡 Rules for counting significant figures in measurements.
- 💡 Units like mole (Avogadro’s number) and ampere (elementary charge).
- 💡 Commonly used non-SI units (e.g., minute, day, tonne).
Keep practicing unit conversions and sig fig problems—they’re NEET favorites! 🌟
