Seven SI Base Units of Measurement-Explanations, MCQs, FAQs & worksheets, CBSE Class 11

Introduction

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The Seven Base Units of Measurement

1. Length – Metre (m)

• Defined as the length of the path traveled by light in a vacuum in exactly 1/299,792,458 seconds.
• Based on the fundamental speed of light, c = 299,792,458 m/s.

2. Time – Second (s)

• Defined by 9,192,631,770 periods of oscillations of light emitted by a caesium-133 atom.
• Determined using highly precise atomic clocks.

3. Mass – Kilogram (kg)

• Defined by a prototype platinum-iridium cylinder kept at the International Bureau of Weights and Measures in Paris.
• The mass of a cylinder made of platinum-iridium alloy kept at International Bureau of Weights and Measures is defined as 1 kg.
• On atomic scale, 1 kilogram is equivalent to the mass of 5.0188 × 10²⁵ atoms of ₆C¹² (an isotope of carbon).
• Standardized worldwide for accurate weight measurement.

4. Electric Current – Ampere (A)

• Defined as the constant current that produces a force of 2 × 10⁻⁷ newton per metre between two parallel conductors 1 metre apart in a vacuum.
• Chosen because measuring current is more practical than measuring charge.

5. Temperature – Kelvin (K)

• Defined as 1/273.16 of the thermodynamic temperature of the triple point of water.
• The triple point of water is where solid, liquid, and gas states coexist.

6. Amount of Substance – Mole (mol)

• Defined as the amount of substance that contains as many entities as 0.012 kg of carbon-12.
• A mole contains Avogadro’s number of particles.

7. Luminous Intensity – Candela (cd)

• Defined as the luminous intensity of a source emitting radiation at 540 × 10¹² Hz.
• With a radiant intensity of 1/683 watt per steradian in a given direction.

FAQs on SI Base Units

Q1: Why is the speed of light used to define a metre? A1: The speed of light is a universal constant, making it a highly precise standard for defining length.

Q2: What is the significance of the second being based on caesium-133? A2: The oscillations of caesium-133 provide an extremely stable and accurate measure of time.

Q3: Why is electric current measured in amperes instead of charge? A3: Measuring current is easier and more practical than directly measuring charge.

MCQs on SI Base Units

Q1: What is the base unit of luminous intensity?

• (A) Watt
• (B) Joule
• (C) Candela
• (D) Ampere
  Answer: (C) Candela
  Explanation: Candela (cd) is the SI unit of luminous intensity.

Q2: The Kelvin scale is based on which fundamental physical concept?

• (A) Boiling point of water
• (B) Triple point of water
• (C) Freezing point of mercury
• (D) Average kinetic energy of particles
  Answer: (B) Triple point of water
  Explanation: The Kelvin scale is defined using the triple point of water.

Test Your Knowledge

Worksheet on SI Base Units

1. Define the following SI base units:
   • (a) Metre
   • (b) Second
   • (c) Kilogram
2. Convert the following measurements:
   • (a) 5 kilograms to grams
   • (b) 2 moles to number of entities (use Avogadro’s number)
   • (c) 300 Kelvin to Celsius
3. Fill in the blanks:
   • (a) The SI unit of electric current is ___.
   • (b) 1 mole contains ___ atoms/molecules.
   • (c) The SI unit of time is ___.

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