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Physical quantities and units

Learn about fundamental and derived physical quantities, SI units, supplementary units, and how units relate to the measurement of physical quantities.

Why this shows up in the exam

NEET tests your understanding of the classification of quantities and correct use of units in scientific problems.

How NEET tests this

Statement analysis · 3 QsDirect recall · 3 QsNumerical · 2 Qs

Learn the idea

A physical quantity is the product of a numerical value and its unit; when the unit changes, the numerical value changes inversely. The key insight is that smaller units give larger numbers because the product must stay constant.

🧠 Memory hook: Big unit, small number; small unit, big number – like buying apples in kilograms versus grams.

Get this one clearly and it pays off every single time it shows up in the paper. 🎯

Formulas & facts to keep ready

  • Seven fundamental quantities: length, mass, time, electric current, thermodynamic temperature, amount of substance, luminous intensity
  • SI base unit for each fundamental quantity (metre, kilogram, second, ampere, kelvin, mole, candela)
  • Supplementary units: radian and steradian are dimensionless derived units
  • Derived unit of absolute permittivity ε₀ is farad per metre (F m⁻¹)
  • Derived unit of self‑inductance is henry (H) = weber per ampere = volt‑second per ampere
  • Numerical value × unit = constant for a given physical quantity

How to approach it

  1. 1Read the statement and identify whether it concerns a change of unit or a specific unit asked
  2. 2Recall the SI unit (or derived unit) of the quantity from the NCERT table
  3. 3If a conversion is needed, use the relation: numerical value₁ × unit₁ = numerical value₂ × unit₂
  4. 4Check the direction: a smaller unit → larger numerical value, and vice‑versa

Worked example — watch it click

Assertion A: When we change the unit of a physical quantity, its numerical value changes. Reason R: Smaller the unit of measurement, larger is its numerical value.

  • ✅If both assertion and reason are true and reason is the correct explanation of assertion.
  • B)If both assertion and reason are true but reason is not the correct explanation of assertion.
  • C)If assertion is true but reason is false.
  • D)If both assertion and reason are false.

The concept behind this problem

The worked example checks that you understand the inverse relationship between unit size and numerical value and that the reason given correctly explains the assertion.

Step by step

  1. 1Assertion A is true: When unit changes, numerical value changes because physical quantity = numerical value × unit (constant product).
  2. 2Reason R is true: If unit is smaller, more units are needed to measure the same quantity, so numerical value is larger (e.g., 1 m = 100 cm).
  3. 3Reason R correctly explains why numerical value changes when unit changes - they are inversely related.
  4. 4Both true and R explains A.

Watch out

Students often accept the assertion but forget that the reason must *explain* why the numerical value changes, leading them to choose the wrong option.

Common slip-ups that cost marks

  • •Do not treat radian and steradian as having separate dimensions – they are dimensionless
  • •Remember that ε₀ can be written as F m⁻¹ (or equivalently C² N⁻¹ m⁻²); both are correct
  • •Self‑inductance is henry, not simply weber or tesla – H = Wb A⁻¹

🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.

Practise it

These are real questions from past NEET papers that test this exact idea.

Question 1 of 7

Assertion A: When we change the unit of a physical quantity, its numerical value changes. Reason R: Smaller the unit of measurement, larger is its numerical value.

Push further

More challenging

10 harder questions built from the past papers above — a step up in difficulty, with distractors designed so you can't get there by elimination. Written and checked by our reviewers, not from a real paper.

Question 1 of 10

Consider a physical quantity Q that is expressed as N1U1 = N2U2, where N represents the numerical value and U represents the unit. If U1 is a larger unit than U2, which of the following relationships must be true?

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