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SI Units and Prefixes Overview

Sep 3, 2025

Overview

This lecture explains the SI (International System of Units) base units, how derived units are formed, and the use of SI prefixes for representing large or small quantities.

SI Base Units

  • SI units are international standard units for measuring physical quantities.
  • There are seven SI base quantities: length, mass, time, temperature, amount of substance, electric current, and luminous intensity.
  • The SI unit and symbol for length is meter (m).
  • The SI unit and symbol for mass is kilogram (kg).
  • The SI unit and symbol for time is second (s).
  • The SI unit and symbol for temperature is kelvin (K).
  • The SI unit and symbol for amount of substance is mole (mol).
  • The SI unit and symbol for electric current is ampere (A).
  • The SI unit and symbol for luminous intensity is candela (cd).

Derived SI Units and Examples

  • Derived units are built from the seven base units.
  • The SI unit for area is square meter (m²), calculated as length × length.
  • The SI unit for volume is cubic meter (m³), calculated as length × length × length.
  • The SI unit for speed or velocity is meter per second (m/s), calculated as distance ÷ time.
  • The SI unit for acceleration is meter per second squared (m/s²), calculated as speed ÷ time.
  • The SI unit for force is kilogram meter per second squared (kg·m/s²), derived from mass × acceleration.
  • Units in a denominator can be written as negative exponents (e.g., m·s⁻² for m/s²).

SI Prefixes and Symbols

  • SI prefixes are used to express very large or very small quantities.
  • Giga (G): 10⁹
  • Mega (M): 10⁶
  • Kilo (k): 10³
  • Centi (c): 10⁻²
  • Milli (m): 10⁻³
  • Micro (μ): 10⁻⁶
  • Nano (n): 10⁻⁹
  • Pico (p): 10⁻¹²
  • Uppercase and lowercase letters in symbols are important (e.g., M for Mega, m for milli).
  • Example: 1.2 × 10⁻³ grams can be written as 1.2 mg (milligrams).
  • Example: 5.22 × 10⁻⁹ meters can be written as 5.22 nm (nanometers).
  • Example: 8.9 × 10⁹ Hz can be written as 8.9 GHz (gigahertz).

Key Terms & Definitions

  • SI (International System of Units) — Standard system for measurement used worldwide in science.
  • Base unit — A fundamental unit from which others are derived (e.g., meter, kilogram).
  • Derived unit — A unit defined by combinations of base units (e.g., m², m/s).
  • SI prefix — Symbol added to a unit to indicate a multiple or fraction (e.g., kilo-, milli-).

Action Items / Next Steps

  • Review the SI prefixes and their symbols.
  • Practice converting large and small numbers using SI prefixes.

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