Physics: Units, Measurements, Instruments, and Scientists

Physics questions on units, measurements, and instruments appear in every PSC exam. Expect 3-6 questions. This note covers SI units, derived units, instruments, and key scientists.

SI Base Units (7 Fundamental Units)

Quantity	Unit	Symbol
Length	Metre	m
Mass	Kilogram	kg
Time	Second	s
Electric Current	Ampere	A
Temperature	Kelvin	K
Amount of Substance	Mole	mol
Luminous Intensity	Candela	cd

Mnemonic: “Lucky Men Travel Every Tuesday Afternoon Leisurely” (Length, Mass, Time, Electric current, Temperature, Amount, Luminous intensity)

Key facts:

• SI = Systeme International d’Unites (adopted 1960, 11th CGPM)
• Kilogram is the only base unit with a prefix
• 1 metre was redefined in 1983 using speed of light

Important Derived Units

Quantity	Unit	Symbol	Formula
Force	Newton	N	kg.m/s²
Energy/Work	Joule	J	N.m = kg.m²/s²
Power	Watt	W	J/s
Pressure	Pascal	Pa	N/m²
Frequency	Hertz	Hz	s⁻¹
Electric Charge	Coulomb	C	A.s
Voltage	Volt	V	W/A
Resistance	Ohm	Ω	V/A
Capacitance	Farad	F	C/V
Magnetic Flux	Weber	Wb	V.s
Magnetic Field	Tesla	T	Wb/m²
Inductance	Henry	H	Wb/A
Illuminance	Lux	lx	lm/m²
Radioactivity	Becquerel	Bq	disintegrations/s

CGS and Other Unit Systems

System	Length	Mass	Time
CGS	Centimetre	Gram	Second
FPS	Foot	Pound	Second
MKS	Metre	Kilogram	Second
SI	Metre	Kilogram	Second

Common Conversions (PSC Favourites)

Conversion	Value
1 light year	9.46 × 10¹² km
1 parsec	3.26 light years
1 Astronomical Unit (AU)	~1.496 × 10⁸ km (Earth-Sun distance)
1 angstrom (Å)	10⁻¹⁰ m
1 nanometre	10⁻⁹ m
1 micron	10⁻⁶ m
1 horse power	746 watts
1 calorie	4.186 joules
1 atmosphere	1.013 × 10⁵ Pa = 760 mmHg
1 electron volt (eV)	1.6 × 10⁻¹⁹ J
Absolute zero	0 K = -273.15°C

Measuring Instruments

Instrument	Measures	Key Detail
Ammeter	Electric current	Connected in series
Voltmeter	Potential difference	Connected in parallel
Galvanometer	Small currents	Detects current direction
Barometer	Atmospheric pressure	Invented by Torricelli (mercury)
Manometer	Gas pressure	U-tube type
Hygrometer	Humidity	Wet and dry bulb
Hydrometer	Specific gravity of liquids	Based on Archimedes’ principle
Lactometer	Purity of milk	Special hydrometer
Thermometer	Temperature	Mercury/alcohol/digital
Pyrometer	Very high temperatures	Used in furnaces; non-contact
Calorimeter	Heat energy	Water equivalent method
Seismograph	Earthquake intensity	Records seismic waves
Richter Scale	Earthquake magnitude	Logarithmic scale
Anemometer	Wind speed	Rotating cups
Rain gauge	Rainfall	Graduated cylinder
Sphygmomanometer	Blood pressure	Mercury or digital
Stethoscope	Heart/lung sounds	Invented by Laennec
ECG (Electrocardiograph)	Heart’s electrical activity	Invented by Einthoven
EEG (Electroencephalograph)	Brain’s electrical activity	Brain waves
Odometer	Distance travelled	In vehicles
Speedometer	Speed of vehicle	Instantaneous speed
Tachometer	Engine RPM	Rotational speed
Altimeter	Altitude	Based on atmospheric pressure
Fathometer	Ocean depth	Uses sonar/echo
Spectrometer	Wavelength of light	Prism/grating based
Geiger-Muller Counter	Radioactivity	Counts ionizing particles
Vernier Caliper	Small lengths	Least count: 0.01 cm
Screw Gauge (Micrometer)	Very small lengths	Least count: 0.001 cm

Scientists and Their Discoveries/Laws

Scientist	Discovery/Law
Newton	Laws of Motion, Universal Gravitation, Calculus, Reflecting telescope
Einstein	Theory of Relativity (E=mc²), Photoelectric effect (Nobel 1921)
Galileo	Telescope (astronomical use), Laws of falling bodies, Jupiter’s moons
Archimedes	Buoyancy principle, Lever principle
Faraday	Electromagnetic induction, Laws of electrolysis
Maxwell	Electromagnetic wave theory
Ohm	Ohm’s Law (V = IR)
Ampere	Electromagnetism, Ampere’s Law
Volta	Electric battery (voltaic pile)
Joule	Mechanical equivalent of heat
Watt	Improved steam engine
Pascal	Pressure transmission in fluids (Pascal’s Law); hydraulic press
Boyle	Boyle’s Law (P ∝ 1/V at constant T)
Charles	Charles’s Law (V ∝ T at constant P)
Bernoulli	Bernoulli’s Principle (fluid dynamics)
Doppler	Doppler Effect (change in frequency due to motion)
Kelvin	Absolute temperature scale
Celsius	Celsius temperature scale
Fahrenheit	Fahrenheit temperature scale
Hertz	Electromagnetic waves (experimental proof)
Rutherford	Nuclear model of atom; discovered proton
Chadwick	Discovered neutron (1932)
J.J. Thomson	Discovered electron (1897)
Bohr	Atomic model (electron orbits)
Curie (Marie)	Radioactivity; discovered Polonium, Radium (2 Nobel Prizes)
Roentgen	X-rays (1895; first Nobel in Physics, 1901)
Becquerel	Natural radioactivity (uranium)
Planck	Quantum theory (E = hf)
Heisenberg	Uncertainty Principle
Hubble	Expanding universe
Raman (C.V.)	Raman Effect (scattering of light); Nobel 1930; 28 Feb = National Science Day
S.N. Bose	Bose-Einstein statistics; Boson named after him
Homi Bhabha	Father of Indian nuclear program
Vikram Sarabhai	Father of Indian space program
A.P.J. Abdul Kalam	Missile Man of India

Laws of Physics — Quick Reference

Law	Statement (Simplified)
Newton’s 1st Law	Body at rest stays at rest (Inertia)
Newton’s 2nd Law	F = ma
Newton’s 3rd Law	Every action has equal and opposite reaction
Law of Gravitation	F = Gm₁m₂/r²
Ohm’s Law	V = IR
Boyle’s Law	PV = constant (at constant T)
Charles’s Law	V/T = constant (at constant P)
Snell’s Law	n₁ sin θ₁ = n₂ sin θ₂ (refraction)
Coulomb’s Law	F = kq₁q₂/r² (electrostatics)
Hooke’s Law	Stress ∝ Strain (within elastic limit)
Archimedes’ Principle	Buoyant force = weight of displaced fluid
Pascal’s Law	Pressure applied to confined fluid transmitted equally
Bernoulli’s Principle	Fast-moving fluid = low pressure

Electromagnetic Spectrum (Increasing Frequency)

Radio waves → Microwaves → Infrared → Visible light (VIBGYOR) → Ultraviolet → X-rays → Gamma rays

Type	Wavelength Range	Application
Radio waves	>1 mm	Broadcasting, communication
Microwaves	1 mm–1 m	Radar, cooking, mobile phones
Infrared	700 nm–1 mm	Night vision, remote controls, heating
Visible light	400–700 nm	Vision (Violet shortest, Red longest)
Ultraviolet	10–400 nm	Sterilization, vitamin D synthesis
X-rays	0.01–10 nm	Medical imaging, security
Gamma rays	Less than 0.01 nm	Cancer treatment, nuclear reactions

Quick-Fire Recall

• SI unit of force? Newton
• Speed of light? 3 × 10⁸ m/s
• Speed of sound in air? ~343 m/s (at 20°C)
• Who discovered X-rays? Roentgen
• Ammeter connected in? Series
• Blood pressure instrument? Sphygmomanometer
• National Science Day? 28 February (Raman Effect)
• 1 horse power = ? 746 watts
• Ocean depth measured by? Fathometer
• Father of Indian space program? Vikram Sarabhai