Physics

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SI units, conversions and measuring instruments

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⏱ 8 min read🧩 5 question types🎯 18 practice Q

The idea in one minute

The 7 SI base quantities

Quantity	SI unit	Symbol
Length	metre	m
Mass	kilogram	kg
Time	second	s
Electric current	ampere	A
Thermodynamic temperature	kelvin	K
Amount of substance	mole	mol
Luminous intensity	candela	cd

Derived units asked again and again

Quantity	Unit	Quantity	Unit
Force	newton (N)	Electric charge	coulomb (C)
Work / energy / heat	joule (J)	Potential difference	volt (V)
Power	watt (W)	Resistance	ohm (Ω)
Pressure	pascal (Pa)	Capacitance	farad (F)
Frequency	hertz (Hz)	Inductance	henry (H)
Magnetic field	tesla (T)	Magnetic flux	weber (Wb)
Power of a lens	dioptre (D)	Radioactivity	becquerel (Bq)
Luminous flux	lumen	Illuminance	lux
Sound level	decibel (dB)	Speed of ships	knot (nautical mile per hour)

Special units and conversions

Unit	Measures	Value
Light year	Distance (not time)	about 9.46 × 10¹⁵ m
Parsec	Distance	about 3.26 light years
Astronomical unit	Distance	Mean Earth–Sun distance, about 1.5 × 10¹¹ m
Ångström	Length (wavelength)	10⁻¹⁰ m
Fermi	Nuclear size	10⁻¹⁵ m
Nautical mile	Distance at sea	1,852 m
Horsepower	Power	about 746 W
Kilowatt-hour ('unit')	Electrical energy	3.6 × 10⁶ J
Calorie	Heat	about 4.18 J
Atmosphere	Pressure	about 1.013 × 10⁵ Pa

Instruments

Instrument	Measures / use
Barometer	Atmospheric pressure (Torricelli)
Hygrometer	Relative humidity
Hydrometer	Relative density of liquids
Lactometer	Purity (density) of milk
Anemometer	Wind speed
Altimeter	Altitude (aircraft)
Ammeter / Voltmeter	Current (in series) / potential difference (in parallel)
Galvanometer	Detects small currents
Seismograph	Earthquake waves
Sphygmomanometer	Blood pressure
Pyrometer	Very high temperatures (furnaces)
Fathometer	Ocean depth (echo sounding)
Odometer / Speedometer	Distance covered / speed of a vehicle
Tachometer	Rotation speed (rpm)
Manometer	Pressure of gases
Audiometer	Hearing

Why units matter

Every measurement has two parts: a number and a unit. "The rod is 5" means nothing; "the rod is 5 metres" is a measurement. To let scientists everywhere compare results, the world uses one agreed system — the SI system (Système International, adopted in 1960). Competitive exams ask more questions on units and instruments than on any other physics area, because they are pure recall.

The seven base quantities

Base (fundamental) quantities do not depend on any other quantity. SI has exactly seven:

Quantity	SI 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

Note: the unit of temperature is kelvin, not "degree kelvin", and not degree Celsius.

Derived units named after scientists

Every other quantity is derived by multiplying or dividing base quantities. Many derived units carry a scientist's name:

Force — newton (N) = kg·m/s². Work / energy / heat — joule (J). Power — watt (W) = J/s.
Pressure — pascal (Pa) = N/m². Frequency — hertz (Hz) = 1/s.
Charge — coulomb (C). Potential difference — volt (V). Resistance — ohm (Ω). Capacitance — farad (F).
Magnetic flux — weber (Wb). Magnetic field — tesla (T). Inductance — henry (H).
Radioactivity — becquerel (Bq). Illuminance — lux. Power of a lens — dioptre (D) = 1/metre. Some derived quantities have no special name: momentum is kg·m/s, velocity is m/s, acceleration is m/s², density is kg/m³.

Scalars and vectors

A scalar has only size (magnitude): mass, speed, distance, time, work, energy, power, temperature. A vector has size and direction: displacement, velocity, acceleration, force, momentum, weight. Easy test: if "towards the north" makes sense with it, it is a vector.

Quick conversions

km/h to m/s: multiply by 5/18. So 36 km/h = 10 m/s. m/s to km/h: multiply by 18/5.
Energy bill: units (kWh) = watts × hours ÷ 1000. A 100 W bulb for 10 hours uses 1 kWh.

Instruments — what they measure

Instrument	Measures
Ammeter / Voltmeter	Current / potential difference
Galvanometer	Detects small currents
Barometer	Atmospheric pressure
Manometer	Pressure of gases
Hygrometer	Relative humidity
Hydrometer	Density of liquids
Lactometer	Purity of milk (its density)
Anemometer	Wind speed
Altimeter	Altitude (aircraft)
Seismograph	Earthquake waves
Sphygmomanometer	Blood pressure
Pyrometer	Very high temperature
Tachometer	Rotational speed (rpm)
Odometer / Speedometer	Distance run / speed of a vehicle
Fathometer / SONAR	Depth of sea
Crescograph	Growth of plants

Question types you will see

Each type: how to recognise it, the method step by step, and one question to try.

Type 1very common3 practice Q

SI unit of a physical quantity

How to spot it:

'The SI unit of pressure / charge / magnetic flux / power is …', or 'which pair of quantity and unit is wrongly matched'.

Method

Decide first whether the quantity is a base quantity (7 units) or a derived one.
For derived quantities, recall the scientist-named unit: N, J, W, Pa, Hz, C, V, Ω, F, Wb, T, H.
If there is no named unit (momentum, velocity), build it: momentum = kg·m/s.

Why it works:

every unit is fixed by an international agreement, so recall is the only method — and elimination by dimensions helps.

Try this

The SI unit of magnetic field (magnetic flux density) is —

Show solution

Tesla (T). Weber is the unit of magnetic flux; tesla = weber per square metre.

Type 2very common4 practice Q

Instrument → what it measures

How to spot it:

An instrument name ending in '-meter' or '-graph' is given and its use is asked, or the reverse.

Method

Break the name into its root: hygro (moisture), hydro (water/liquid), anemo (wind), baro (weight of air = pressure), seismo (shaking), sphygmo (pulse).
Pick the option that fits the root.
Watch the pairs hygrometer vs hydrometer, manometer vs sphygmomanometer.

Why it works:

most names are Greek roots that describe the job.

Try this

An altimeter is used to measure —

Show solution

Altitude (height above sea level), mainly in aircraft. It works like a barometer: pressure falls as height rises.

Type 3common3 practice Q

Practical and astronomical units

How to spot it:

Units like light year, parsec, AU, knot, nautical mile, fathom, horsepower, carat or kWh — 'is a unit of …' or 'equals …'.

Method

Classify the unit by quantity: light year/parsec/AU/fathom/nautical mile = distance; knot = speed; kWh/calorie/eV = energy; horsepower = power.
Recall the headline value (nautical mile 1852 m, 1 hp ≈ 746 W, 1 kWh = 3.6 × 10⁶ J).
Beware the word 'year' and 'hour' in a unit name — they do not make it a unit of time.

Try this

A parsec is a unit of —

Show solution

Distance (astronomical). 1 parsec ≈ 3.26 light years.

Type 4common2 practice Q

Base vs derived units; scalar vs vector

How to spot it:

'Which is NOT a fundamental unit', 'which is a vector quantity', 'which pair are both scalars'.

Method

Base units: metre, kilogram, second, ampere, kelvin, mole, candela — everything else is derived.
Vector check: does direction change the meaning? Velocity, force, momentum, displacement, acceleration = vectors.
Speed, distance, mass, work, energy, power, time = scalars.

Try this

Which of the following is a scalar quantity? (a) Force (b) Momentum (c) Work (d) Displacement

Show solution

(c) Work. Work has only magnitude; force, momentum and displacement all need a direction.

Type 5common2 practice Q

Unit-conversion and energy-bill numericals

How to spot it:

A speed in km/h to convert, or appliance watts × hours to turn into kWh or ₹.

1\ \text{km/h}=\tfrac{5}{18}\ \text{m/s},\quad \text{units (kWh)}=\frac{\text{W}\times\text{h}}{1000}

Method

For speed: multiply km/h by 5/18 to get m/s (divide by 3.6).
For bills: watts × hours per day × days ÷ 1000 = units; multiply by the rate.
Check the size: a 1 kW appliance for 1 hour = exactly 1 unit.

Why it works:

1 km = 1000 m and 1 h = 3600 s, so 1000/3600 = 5/18.

Try this

Convert 54 km/h to m/s.

Show solution

54 × 5/18 = 15 m/s.

Formula sheet

Kilowatt-hour

1\ \text{kWh} = 3.6 \times 10^{6}\ \text{J}

1 'unit' of electricity

Horsepower

1\ \text{hp} \approx 746\ \text{W}

Light year

1\ \text{ly} \approx 9.46 \times 10^{15}\ \text{m}

a unit of distance

Shortcuts that save time

⚡ Base-unit roll call: 'Mighty Kings Seldom Answer Kind Monks Correctly'

Metre, Kilogram, Second, Ampere, Kelvin, Mole, Candela — the 7 SI base units. Anything else (newton, joule, volt, watt…) is derived.

Example

Which of these is NOT an SI base unit: kelvin, candela, newton, mole?

Show solution

Newton — it is derived (kg m s⁻²).

⚡ 'Meter' by what it measures

Hygro = humidity (hygiene/water), Hydro = liquid density, Lacto = milk, Anemo = wind (Greek anemos), Sphygmo = pulse/BP, Pyro = fire/high temperature, Seismo = earthquake, Alti = altitude.

Mistakes to avoid

Where most students lose marks on this subtopic.

Mistake 01

Treating the light year as a unit of time — it is a distance.

Mistake 02

Using kWh as a unit of power — it is a unit of energy.

Mistake 03

Confusing hygrometer (humidity) with hydrometer (liquid density).

Quick revision

Read this the night before the exam.

7 base units: m, kg, s, A, K, mol, cd.
Light year, parsec, AU = distance. kWh = energy (3.6 × 10⁶ J). Horsepower ≈ 746 W.
Nautical mile = 1852 m; knot = nautical mile per hour.
km/h × 5/18 = m/s.
Hygro = humidity, hydro = liquid density, lacto = milk, anemo = wind, sphygmo = blood pressure.

Practice: 18 questions

Sets of 10, mixed across the question types above. Every answer has a step-by-step explanation.

Topic test · 10 questions

Suggested time 3 min · wrong answers go to your mistake notebook automatically.

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