Chapter 14: Frequently Asked Facts for SSC/RRB Exams

This final chapter compiles the kind of short, direct, fact-based questions that repeatedly appear in SSC (CGL, CHSL, MTS, GD Constable) and Railway (RRB NTPC, Group D, ALP) general science sections on the topic of units and measurement. Reviewing this list a day or two before the exam is an efficient way to lock in the high-frequency facts.

14.1 Unit of a Given Quantity — Rapid-Fire Table

14.2 Important Conversion Facts

• 1 newton = 10⁵ dyne
• 1 joule = 10⁷ erg
• 1 kilowatt-hour (1 unit of electricity) = 3.6 × 10⁶ joule
• 1 horsepower (hp) = 746 watt (approximately)
• 1 atmosphere (atm) = 1.013 × 10⁵ pascal ≈ 760 mm of mercury
• 1 calorie = 4.186 joule (mechanical equivalent of heat, J = 4.18 cal approx.)
• 1 electron-volt (eV) = 1.602 × 10⁻¹⁹ joule
• 1 angstrom (Å) = 10⁻¹⁰ metre
• 1 light-year ≈ 9.46 × 10¹⁵ metre (distance light travels in one year)
• 1 astronomical unit (AU) ≈ 1.496 × 10¹¹ metre (average Earth–Sun distance)
• 0°C = 273.15 K = 32°F; 100°C = 373.15 K = 212°F
• Absolute zero = 0 K = −273.15°C (lowest theoretically possible temperature)

14.3 Frequently Tested Facts and Trivia

• The SI system was formally adopted by the 11th General Conference on Weights and Measures (CGPM) in 1960.
• The Metre Convention (Treaty of the Metre) was signed in Paris in 1875, establishing the International Bureau of Weights and Measures (BIPM).
• The kilogram is the only SI base unit whose name itself contains a prefix ("kilo").
• Before 2019, the kilogram was the last SI base unit defined by a physical object — the International Prototype of the Kilogram (a platinum-iridium cylinder kept near Paris); since the 2019 redefinition, all SI base units are defined via fundamental physical constants.
• The 2019 redefinition of the SI base units took effect on 20 May 2019, which is now celebrated annually as World Metrology Day.
• Kelvin is written without a degree symbol ("°"); it is simply "K", not "°K".
• The mole is defined using the Avogadro constant, NA = 6.022 × 10²³ per mole, named after the Italian scientist Amedeo Avogadro.
• The candela is the SI base unit related to human visual perception (luminous intensity), based on the response of the human eye to green light of frequency 540 × 10¹² Hz.
• Radian (plane angle) and steradian (solid angle) are dimensionless SI derived units used for angular measurement; they were formerly called "supplementary units".
• Absolute zero (0 K) is the temperature at which molecular motion is theoretically minimum; it can be approached but never actually reached.
• Sound cannot travel through vacuum because it is a mechanical wave requiring a medium, whereas light (an electromagnetic wave) can travel through vacuum.
• The speed of sound is generally greatest in solids, less in liquids, and least in gases, because sound travels faster through media with closely packed molecules.
• Vernier callipers and screw gauges are precision instruments used to measure small lengths, and their precision is expressed in terms of "least count" — the smallest measurement that can be read directly from the instrument.
• The CGS unit of force is the dyne, and the CGS unit of energy is the erg — both are frequently confused with their SI counterparts, newton and joule, in exam questions.
• India's National Physical Laboratory (NPL), located in New Delhi, is the country's official custodian of national measurement standards and is responsible for realising and maintaining the SI units in India.
• Torque and work have the same dimensional formula ([ML²T⁻²]) but are measured in different named units in practice — work in joule, torque in newton-metre — to avoid confusing a scalar quantity (work) with a vector-like quantity (torque).
• Plane angle (radian) and solid angle (steradian) are dimensionless because they are defined as ratios of two lengths (arc/radius) or two areas (surface area/radius²) respectively.
• The concept of "significant figures" tells us about the precision of a measurement, while "dimensional analysis" tells us about the nature/consistency of a physical relationship — the two serve very different purposes and should not be confused.
• Bilingual convention in India: SI units are the legal units of measurement in India as per the Standards of Weights and Measures Act, and the metric/SI system has been compulsory for trade and commerce in India since the 1950s–60s reforms.