Chapter 1 — Units and Measurement

Chapter 1 of the Class 11 Physics NCERT textbook, "Units and Measurement", introduces the SI system of units—seven fundamental base quantities (length, mass, time, electric current, temperature, amount of substance, and luminous intensity) and their standard units—along with significant figures to express measurement precision and dimensional analysis to verify physical equations.

• Measurement and the SI system — Physics is quantitative, so every quantity is measured by comparing it against an agreed standard called a unit. The SI system builds all measurements from seven base units, with every other quantity derived as a combination of these bases.
• Expressing precision reliably — A recorded value must communicate how precisely it was measured, which is the job of significant figures. Scientific notation removes ambiguity about which digits count, keeping calculations honest about the accuracy of the underlying data.
• Dimensions as a checking tool — Dimensions describe the nature of a quantity independent of its magnitude. Dimensional analysis lets you test whether an equation is consistent and deduce relationships, though consistency alone cannot guarantee an equation is fully correct.

1. 01SI system consists of seven base units (metre, kilogram, second, ampere, kelvin, mole, candela) for fundamental quantities; all other units are derived combinations of these.
2. 02Significant figures represent measurement precision; rules govern addition/subtraction (match decimal places) and multiplication/division (match significant figure count of least precise value).
3. 03Dimensions use square brackets [L], [M], [T], etc., to describe the nature of physical quantities independent of magnitude.
4. 04Dimensional formulae express how base quantities combine to form a derived quantity; dimensional equations check if formulas are consistent.
5. 05Dimensional analysis deduces relations among physical quantities and verifies equations, but cannot determine dimensionless constants like 2π in T = 2π√(l/g).