SI Units
Physics · SI इकाइयां
📋Quick Overview
SI stands for 'Système International d'Unités' (International System of Units). It was adopted in 1960 at the 11th General Conference on Weights and Measures (CGPM) in Paris. SI has 7 base units from which ALL other units are derived. Think of it like 7 building blocks — every other unit is built from these 7.
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EXAM TRAP: Light year, Parsec, AU are units of DISTANCE — not time! This is asked repeatedly in exams.
📖7 Base SI Units
| Quantity | SI Unit | Symbol | Easy Way to Remember |
|---|---|---|---|
| Length | Meter | m | Ruler measures in meters |
| Mass | Kilogram | kg | Your weight in kg |
| Time | Second | s | Clock ticks in seconds |
| Electric Current | Ampere | A | Named after André-Marie Ampère |
| Temperature | Kelvin | K | Named after Lord Kelvin |
| Amount of Substance | Mole | mol | Avogadro's number = 6.022 × 10²³ |
| Luminous Intensity | Candela | cd | Candela = Candle (light brightness) |
📖CGS vs SI (MKS) System
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Key conversions: 1 Newton = 10⁵ Dyne | 1 Joule = 10⁷ Erg | 1 kg = 1000 g | 1 m = 100 cm
📖Important Derived Units
| Quantity | SI Unit | Formula | Named After |
|---|---|---|---|
| Force | Newton (N) | kg·m/s² | Isaac Newton |
| Energy / Work | Joule (J) | kg·m²/s² = N·m | James Prescott Joule |
| Power | Watt (W) | J/s = kg·m²/s³ | James Watt |
| Pressure | Pascal (Pa) | N/m² | Blaise Pascal |
| Frequency | Hertz (Hz) | 1/s (per second) | Heinrich Hertz |
| Electric Charge | Coulomb (C) | A·s | Charles Coulomb |
| Voltage | Volt (V) | W/A = J/C | Alessandro Volta |
| Resistance | Ohm (Ω) | V/A | Georg Ohm |
| Area | m² | m × m | — |
| Volume | m³ | m × m × m | — |
| Speed / Velocity | m/s | m ÷ s | — |
| Acceleration | m/s² | m/s ÷ s | — |
📝Important Unit Conversions
- •1 km = 1000 m | 1 mile = 1.6 km | 1 nautical mile = 1.852 km
- •1 Horse Power (HP) = 746 Watts
- •1 calorie = 4.186 Joules
- •1 atmosphere (atm) = 1.013 × 10⁵ Pascal = 760 mmHg
- •1 electron volt (eV) = 1.6 × 10⁻¹⁹ Joule
- •1 Angstrom (Å) = 10⁻¹⁰ m (used for atomic sizes)
- •1 Bar = 10⁵ Pascal (unit of pressure)
- •1 kWh (unit) = 3.6 × 10⁶ Joule (electricity bill unit)