Units and Measurement
Class 11 Physics · Physics Part I · Chapter 1
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Summary
Physics is a quantitative science in which every physical quantity is expressed as a number accompanied by a unit, a number obtained by comparing the quantity with an internationally accepted reference standard. Units of the chosen base quantities are called base units, and units of all other (derived) quantities are combinations of these. The Systeme International (SI), revised in 2018, uses seven base units — metre, kilogram, second, ampere, kelvin, mole and candela — now defined through fixed numerical values of fundamental constants such as the speed of light \(c\), the Planck constant \(h\) and the caesium frequency. Because every measurement carries error, results are reported using significant figures, with rules for non-zero digits, embedded zeros and trailing zeros, and arithmetic operations preserve the precision of the least precise input. Scientific notation \(a\times10^{b}\) avoids ambiguity and gives the order of magnitude. The dimensions of a quantity are the powers to which the base quantities \([M],[L],[T]\) etc. are raised; force, for example, has dimensions \([M\,L\,T^{-2}]\). Dimensional analysis, based on the principle of homogeneity, is used to check the consistency of equations, convert units and deduce relations among quantities, though it cannot find dimensionless constants and a dimensionally correct equation need not be physically correct.
The international system of unitsSignificant figuresDimensions of physical quantitiesDimensional formulae and equationsDimensional analysis and its applicationsOrder of magnitude and scientific notation
Key terms
Unit
An internationally accepted reference standard used to measure a physical quantity, against which the quantity is compared.
Base unit
The unit of a fundamental quantity; SI has seven — metre, kilogram, second, ampere, kelvin, mole and candela.
Significant figures
The reliably known digits of a measurement plus the first uncertain digit, indicating the precision of the result.
Order of magnitude
The power of ten nearest to a quantity, found by writing it as \(a\times10^{b}\) and rounding \(a\).
Dimensions
The powers to which the base quantities are raised to represent a physical quantity, written in square brackets.
Principle of homogeneity
The rule that every term in a physically meaningful equation must have the same dimensions.
Extra questions & answers
Try one from this chapter
How many base units does the SI system have?