2.
Pick the odd one out:
a. Speed of light in vacuum
b. Charge of the electron
c. Diameter of the Earth
d. Gravitational constant
Odd one out: c. Diameter of the Earth
The options a, b, and d represent fundamental physical constants with fixed, universal values, while c measures a macroscopic planetary property that varies slightly.
Option Analysis
Speed of light in vacuum (a): Exactly 299,792,458 m/s, a defined universal constant in relativity and electromagnetism.
Charge of the electron (b): Fixed at -1.602 × 10^{-19} C, a fundamental quantized property unchanged across conditions.
Diameter of the Earth (c): Approximately 12,742 km equatorial average, but varies due to oblateness (equatorial 12,756 km, polar 12,714 km) and geoid fluctuations up to 110 m from tides, earthquakes, or ice melt.
Gravitational constant (d): 6.67430 × 10^{-11} N·m²/kg², a universal constant governing gravity despite minor measurement uncertainties.
Introduction to Physics Constants Puzzle
In physics MCQs like “pick the odd one out: speed of light in vacuum, charge of electron, diameter of Earth, gravitational constant,” the key lies in distinguishing universal constants from variable measurements. Speed of light, electron charge, and gravitational constant remain invariant, while Earth diameter varies.
Why Diameter of Earth Stands Out
Diameter of Earth (~12,742 km) reflects planetary structure, changing slowly from rotation-induced flattening or mass shifts. Unlike speed of light (exact, defined), electron charge (quantized), or gravitational constant (universal), it lacks fundamental invariance.
Fundamental Constants Explained
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Speed of light in vacuum: Basis for SI units, unchanging at 3×10^8 m/s.
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Charge of electron: Elementary value in atomic theory.
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Gravitational constant: Fixed in Newton’s law.
This distinction tests conceptual clarity in competitive exams.
