Physics · Chapter 01

Physical Quantities & SI Units

Every measurement in physics needs a standard reference, and the SI system gives the whole world one common set of units. The sections below start from what a unit is, build up the seven base units, then collect the most-asked derived units, their symbols and the decimal prefixes.

📏 7 topics🎯 94+ points📝 self-test

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Topic 01

Definition & Basics

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Physical Quantities & SI Units — downloadable PDF

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A physical quantity is anything that can be measured, and a unit is the agreed standard we compare it against. The SI system is the modern, internationally accepted form of the metric system.

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Key Point

A physical unit is a standard quantity used as a reference for measuring any physical quantity.

S.I. stands for Système International, the internationally accepted standard system of units.
The SI system has exactly 7 base (fundamental) units, and all other units are derived from them.
Exam tip: always memorise the symbols of units, since most questions test the quantity↔unit↔symbol match.

📝 Quick self-test 2 MCQs · 2 fill-ups

What does 'S.I.' stand for in the SI system of units?

Standard International
Système International
Scientific Instrument
System of India

✔ B. Système International — S.I. stands for Système International, the internationally accepted standard system of units.

How many base (fundamental) units does the SI system have?

✔ C. 7 — The SI system has exactly 7 base units, from which all others are derived.

A physical unit is a standard quantity used as a for measuring any physical quantity.

✔ reference

The SI system has exactly base (fundamental) units.

✔ 7

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Topic 02

The Seven SI Base Units

These are the foundation of the whole system; every other unit is built by combining them. Note that radian and steradian (in the source list) are not base units — they are supplementary/derived units, so the true base units are only seven.

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Key Point

Length is measured in metre (m) — the distance light travels in a tiny fraction of a second.

The seven SI base units

Physical Quantity	SI Unit	Symbol
Length (distance)	metre	m
Mass	kilogram	kg
Time	second	s
Electric current	ampere	A
Temperature	kelvin	K
Amount of substance	mole	mol
Luminous intensity	candela	cd

Mass is measured in kilogram (kg), the only base unit that already carries a prefix (kilo).
Time is measured in second (s).
Electric current is measured in ampere (A).
Temperature is measured in kelvin (K) — note: not "degree kelvin", just kelvin.
Amount of substance is measured in mole (mol) — this is the 7th base unit that the source list omits.
Luminous intensity is measured in candela (cd).

📝 Quick self-test 2 MCQs · 2 fill-ups

Which of these is the SI base unit for the amount of substance?

candela
mole
kelvin
ampere

✔ B. mole — Amount of substance is measured in mole (mol), the 7th base unit.

Which base unit already carries a prefix in its name?

metre
second
kilogram
ampere

✔ C. kilogram — Kilogram (kg) is the only base unit that already carries a prefix (kilo).

Luminous intensity is measured in the SI base unit (cd).

✔ candela

Electric current is measured in the SI base unit (A).

✔ ampere

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Topic 03

Supplementary Units

Beyond the seven base units, two angular units are classed separately.

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Key Point

Angle (plane angle) is measured in radian (rad) — a supplementary unit, not a base unit.

Solid angle is measured in steradian (sr) — also a supplementary unit.

📝 Quick self-test 2 MCQs · 2 fill-ups

The radian is used to measure which quantity?

Solid angle
Plane angle
Frequency
Illuminance

✔ B. Plane angle — Plane angle is measured in radian (rad), a supplementary unit.

The steradian (sr) is the unit of which quantity?

Plane angle
Luminous flux
Solid angle
Magnetic flux

✔ C. Solid angle — Solid angle is measured in steradian (sr), a supplementary unit.

Plane angle is measured in radian, which is classed as a unit, not a base unit.

✔ supplementary

Solid angle is measured in the supplementary unit (sr).

✔ steradian

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Topic 04

Common Derived Units (Mechanics)

Derived units are formed by multiplying or dividing the base units. The mechanical quantities below are among the most frequently asked.

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Key Point

Force is measured in newton (N); 1 N = 1 kg m s⁻².

Important derived units — mechanics

Physical Quantity	SI Unit	Symbol
Force	newton	N
Energy / Work	joule	J
Power	watt	W
Pressure	pascal	Pa
Frequency	hertz	Hz
Speed / Velocity	metre per second	m s⁻¹
Angular velocity	radian per second	rad s⁻¹
Momentum	kilogram metre per second	kg m s⁻¹
Angular momentum	kilogram metre² per second	kg m² s⁻¹
Moment of inertia	kilogram metre²	kg m²
Impulse	newton second	N s
Surface tension	newton per metre	N m⁻¹
Viscosity	newton second per metre²	N s m⁻²

Energy and work share the same unit, joule (J).
Power is measured in watt (W); 1 W = 1 J s⁻¹.
Pressure is measured in pascal (Pa); 1 Pa = 1 N m⁻².
Frequency is measured in hertz (Hz), equal to one cycle per second.
Speed and velocity are measured in metre per second (m s⁻¹).
Angular velocity is measured in radian per second (rad s⁻¹).
Momentum is measured in kilogram metre per second (kg m s⁻¹).
Angular momentum is measured in kilogram metre² per second (kg m² s⁻¹).
Moment of inertia is measured in kilogram metre² (kg m²).
Impulse has the unit newton second (N s), the same dimension as momentum.
Surface tension is measured in newton per metre (N m⁻¹).
Viscosity (coefficient of viscosity) is measured in newton second per metre² (N s m⁻²).

📝 Quick self-test 2 MCQs · 2 fill-ups

Which SI derived unit is used for pressure?

newton
pascal
joule
watt

✔ B. pascal — Pressure is measured in pascal (Pa), where 1 Pa = 1 N m⁻².

Impulse has the same dimension as which quantity?

Force
Energy
Momentum
Power

✔ C. Momentum — Impulse has the unit newton second (N s), the same dimension as momentum.

Energy and work share the same SI unit, the (J).

✔ joule

Surface tension is measured in per metre (N m⁻¹).

✔ newton

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Topic 05

Common Derived Units (Heat)

A couple of heat-related quantities are commonly asked in exams.

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Key Point

Thermal conductivity is measured in watt per metre per kelvin (W m⁻¹ K⁻¹) — exam-correct uses kelvin, not degree celsius.

Important derived units — heat

Physical Quantity	SI Unit	Symbol
Thermal conductivity	watt per metre per kelvin	W m⁻¹ K⁻¹
Specific heat	joule per kilogram per kelvin	J kg⁻¹ K⁻¹

Specific heat is measured in joule per kilogram per kelvin (J kg⁻¹ K⁻¹).

📝 Quick self-test 2 MCQs · 2 fill-ups

The SI unit of thermal conductivity is:

W m⁻¹ K⁻¹
J kg⁻¹ K⁻¹
W m⁻²
J K⁻¹

✔ A. W m⁻¹ K⁻¹ — Thermal conductivity is measured in watt per metre per kelvin (W m⁻¹ K⁻¹).

Specific heat is measured in which SI unit?

W m⁻¹ K⁻¹
J kg⁻¹ K⁻¹
J mol⁻¹ K⁻¹
cal g⁻¹

✔ B. J kg⁻¹ K⁻¹ — Specific heat is measured in joule per kilogram per kelvin (J kg⁻¹ K⁻¹).

The exam-correct unit of thermal conductivity uses kelvin, giving per metre per kelvin.

✔ watt

Specific heat is measured in joule per kilogram per .

✔ kelvin

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Topic 06

Common Derived Units (Electricity, Magnetism & Light)

Electrical, magnetic and optical quantities each carry their own named units, several honouring famous scientists.

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Key Point

Electric charge is measured in coulomb (C).

Important derived units — electricity, magnetism & light

Physical Quantity	SI Unit	Symbol
Electric charge	coulomb	C
Electric potential	volt	V
Capacitance	farad	F
Resistance	ohm	Ω
Inductance	henry	H
Magnetic flux	weber	Wb
Luminous flux	lumen	lm
Illuminance	lux	lx
Optical wavelength	angstrom	Å

Electric potential (voltage) is measured in volt (V).
Capacitance is measured in farad (F).
Resistance is measured in ohm (Ω).
Inductance is measured in henry (H).
Magnetic flux is measured in weber (Wb).
Luminous flux is measured in lumen (lm).
Illuminance is measured in lux (lx).
Optical wavelength is often expressed in angstrom (Å), where 1 Å = 10⁻¹⁰ m.

📝 Quick self-test 2 MCQs · 2 fill-ups

The SI unit of magnetic flux is:

tesla
weber
henry
farad

✔ B. weber — Magnetic flux is measured in weber (Wb).

Which unit is used for capacitance?

farad
henry
ohm
coulomb

✔ A. farad — Capacitance is measured in farad (F).

Inductance is measured in the unit (H).

✔ henry

Optical wavelength is often expressed in angstrom, where 1 Å = m.

✔ 10⁻¹⁰

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Topic 07

SI Prefixes (Multiples & Submultiples)

Prefixes attach to a unit to express very large or very small values neatly — handy for converting between scales.

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Key Point

kilo (k) means 10³, so 1 km = 1000 m.

Common SI prefixes

Prefix	Symbol	Factor
giga	G	10⁹
mega	M	10⁶
kilo	k	10³
deci	d	10⁻¹
centi	c	10⁻²
milli	m	10⁻³
micro	µ	10⁻⁶
nano	n	10⁻⁹

milli (m) means 10⁻³, so 1 mm = 0.001 m.
micro (µ) means 10⁻⁶ and nano (n) means 10⁻⁹.

📝 Quick self-test 2 MCQs · 2 fill-ups

The SI prefix 'kilo (k)' represents which factor?

10⁶
10³
10⁻³
10⁹

✔ B. 10³ — Kilo (k) means 10³, so 1 km = 1000 m.

Which prefix represents the factor 10⁻⁶?

nano
milli
micro
centi

✔ C. micro — Micro (µ) means 10⁻⁶.

The prefix milli (m) means 10 raised to the power .

✔ −3

The prefix nano (n) represents the factor .

✔ 10⁻⁹

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Recap

Quick Revision

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Key Point

The SI system has 7 base units: metre, kilogram, second, ampere, kelvin, mole, candela.

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).
Radian and steradian are supplementary units, not base units.
Force → newton (N), energy/work → joule (J), power → watt (W).
Pressure → pascal (Pa) and frequency → hertz (Hz).
Resistance → ohm (Ω), potential → volt (V), charge → coulomb (C).
Magnetic flux → weber (Wb) and inductance → henry (H).
kilo = 10³, milli = 10⁻³, micro = 10⁻⁶, the most-asked prefixes.

Test Yourself

Take 5 questions at a time — tap an option to check. After each round, revise the notes above and take the retest for 5 fresh questions, until you've mastered the whole chapter.