SI Units

SI Units, Nomenclature & Conventions

  • SI: Le Système International d’Unités

  • Unit Types:

    • Base Units:

      • 7 well-defined units that are dimensionally independent.

      • All other units can be expressed in terms of these base units.

    • Derived Units:

      • Formed as products of powers of the base units.

      • Many combinations have special names.

Base Units

  • Traceability: All base units are traceable to an international standard.

  • Base Quantities:

    • Length: metre (m)

    • Mass: kilogram (kg)

    • Time: second (s)

    • Electric Current: ampere (A)

    • Thermodynamic Temperature: kelvin (K)

    • Amount of Substance: mole (mol)

    • Luminous Intensity: candela (cd)

Some Accepted Non-SI Units

  • Unit Names and Definitions:

    • Electronvolt (eV): 1 eV = 160 x 10^-21 J

    • Bar: 1 bar = 100 kPa

    • Minute (min): 1 min = 60 s

    • Hour (h): 1 h = 60 min = 3600 s

    • Day (d): 1 d = 24 h = 86,400 s

    • Degree (°): 1° = (π/180) rad

    • Minute (of arc, ¢): 1¢ = (1/60)°

    • Second (of arc, ²): 1² = (1/60)¢

    • Litre (l, L): 1 L = 10^-3 m³

Some Derived Units of Relevance to EEE

  • Key Derived Quantities:

    • Angular Velocity: radian per second (rad/s)

    • Area: square metre (m²)

    • Volume: cubic metre (m³)

    • Speed, Velocity: metre per second (m/s)

    • Acceleration: metre per second squared (m/s²)

    • Density: kilogram per cubic metre (kg/m³)

    • Concentration: mole per cubic metre (mol/m³)

    • Celsius Temperature: degree Celsius (°C)

    • Frequency: hertz (Hz)

    • Force: newton (N)

    • Pressure, Stress: pascal (Pa)

    • Energy, Work, Heat: joule (J)

    • Power: watt (W)

Additional Derived Units

  • Electric Charge: coulomb (C)

  • Electric Potential Difference, emf: volt (V)

  • Electric Field Strength: volt per metre (V/m)

  • Electric Charge Density: coulomb per cubic metre (C/m³)

  • Current Density: ampere per square metre (A/m²)

  • Magnetic Field Strength: ampere per metre (A/m)

  • Magnetic Flux: weber (Wb)

  • Electric Resistance: ohm (Ω)

  • Capacitance: farad (F)

  • Inductance: henry (H)

Standards and Conventions

  • Unit Symbols and Formatting:

    • Unit symbols are written in upright roman type.

    • No space between prefix and unit symbols (e.g., kW, µm).

    • Always a space between the number and unit (e.g., 5 mA).

  • SI Prefixes:

    • Names and Multipliers:

      • Yocto: 10^-24 (y)

      • Zepto: 10^-21 (z)

      • Atto: 10^-18 (a)

      • Femto: 10^-15 (f)

      • Pico: 10^-12 (p)

      • Nano: 10^-9 (n)

      • Micro: 10^-6 (µ)

      • Milli: 10^-3 (m)

      • Kilo: 10^3 (k)

      • Mega: 10^6 (M)

      • Giga: 10^9 (G)

      • Tera: 10^{12} (T)

      • Peta: 10^{15} (P)

      • Exa: 10^{18} (E)

      • Zetta: 10^{21} (Z)

      • Yotta: 10^{24} (Y)

  • Notes:

    • Compound prefixes are never used (e.g., 1 nm, not 1 mmm).