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System of Units

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Topics

  • Fundamental Quantities
  • The Seven Fundamental Quantities
  • Historical Development of Standards
  • Evolution of Unit Systems
Maharashtra State Board: Class 7, 11

Fundamental Quantities

Think of fundamental quantities like the primary colors - red, blue, and yellow. Just as you can mix these to create any other color, you can combine fundamental quantities to measure everything else in physics. 

Fundamental quantities are the basic physical quantities that: 

  • Cannot be defined using other quantities
  • They are independent of each other
  • Form the foundation for all other measurements
Maharashtra State Board: Class 7, 11

The Seven Fundamental Quantities

Quantity SI Unit Symbol What It Measures Everyday Example
Length metre m Distance, height, width Your height, room length
Mass kilogram kg Amount of matter Your weight, book mass
Time second s Duration of events Class period, heartbeat
Temperature kelvin K Hotness or coldness Body temperature, weather
Electric Current ampere A Flow of electricity Current in your phone charger
Luminous Intensity candela cd Brightness of light LED bulb brightness
Amount of Substance mole mol Number of particles Atoms in a sample
Maharashtra State Board: Class 11

Historical Development of Standards

How Standards Evolved

Early Days (Before 1800s): 

  • People used body parts: foot, hand span, cubit
  • Kings' measurements became local standards
  • Led to confusion in trade and science 

Modern Era (1800s-Present): 

  • Mass Standard: A special platinum-iridium cylinder kept in Paris serves as the world's kilogram reference[Original content]
  • Length Standard: Originally based on marks on a platinum-iridium bar, now defined using the speed of light[Original content]
  • Time Standard: Based on Earth's rotation, refined to atomic clock precision[Original content] 

Real-world Impact: These standards enable: 

  • International trade and manufacturing
  • Scientific research collaboration
  • Space missions and GPS accuracy
Maharashtra State Board: Class 7, 11

Evolution of Unit Systems

System Full Name Units When Used Example
CGS Centimetre-Gram-Second cm, g, s Historical/some science Lab measurements
MKS Metre-Kilogram-Second m, kg, s Before SI Engineering
FPS Foot-Pound-Second ft, lb, s British system Construction
SI Système International Seven base units Current worldwide Modern science

Why SI Wins: Uses the decimal system, making conversions super easy! Converting meters to centimeters? Just multiply by 100[Original content].

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