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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.
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