मराठी
CISCEICSE ICSE Class 6
पाठ्यपुस्तक उत्तरे७१९४
संकल्पना स्पष्टीकरण आणि व्हिडिओ३४२
अभ्यासक्रम

Measurement of Mass

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Topics

  • Introduction
  • The Old Kilogram Standard
  • The New Kilogram Standard
  • Measuring Atoms & Molecules: Atomic Mass Unit (amu)
Maharashtra State Board: Class 11

Introduction

Mass tells us how much matter something contains. The SI unit of mass is the kilogram (kg).

Maharashtra State Board: Class 11

The Old Kilogram Standard

  • For 130 years, the kilogram was defined by a metal cylinder (platinum-iridium) kept in France.
  • This “international prototype” could gain dirt or change slightly over time. 
Maharashtra State Board: Class 11

The New Kilogram Standard

  • Since 2019, the kilogram is defined by a physical constant (the Planck constant) and measured using electricity.
  • Imagine an electromagnet pulling down one side of a very sensitive balance, with a known current balancing 1 kg on the other side.
Year How Kilogram Was Defined
1889 Platinum-iridium cylinder (IPK)
2019 Planck constant and electromagnet
Maharashtra State Board: Class 11

Measuring Atoms & Molecules: Atomic Mass Unit (amu)

  • Kilograms are too large for atoms and molecules.
  • Instead, scientists use the atomic mass unit (amu).
  • 1 amu = one-twelfth the mass of a single carbon-12 atom (C¹²).
  • 1 amu = 1.6605402 × 10⁻²⁷ kg. 

Why Use Carbon-12? 

  • It’s very stable and the most common form of carbon on Earth.
  • Other elements have more isotopes, making them less accurate for standards. 

Test Yourself

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