Physical Quantities, Units and Measurements

Physical Quantities Definition

A physical quantity is a quantity that can be measured. It is defined by two components: a numerical magnitude and a unit. For example, in the measurement $4.5\,m$, the numerical magnitude is $4.5$ and the unit is $m$.

The International System of Units (SI Units)

To ensure a universal standard and avoid confusion caused by varying individual measurements, the International System of Units (SI, from the French: Système International d'Unités) was adopted.

Base Quantities and SI Units

There are seven basic physical quantities. The base quantities and their corresponding SI units are:

• Length: metre ($m$)
• Mass: kilogram ($kg$)
• Time: second ($s$)
• Electric current: ampere ($A$)
• Thermodynamic temperature: kelvin ($K$)
• Amount of substance: mole ($mol$)

SI Unit Prefixes

Prefixes and their symbols are used to indicate decimal multiples and sub-multiples of SI units:

• Sub-multiples: nano ($n$), micro ($u$), milli ($m$), centi ($c$), deci ($d$)
• Multiples: kilo ($k$), mega ($M$), giga ($G$), tera ($T$)

Amount of Substance and Avogadro's Constant

In Chemistry, the mole ($mol$) measures the amount of substance. One mole contains $6.02 \times 10^{23}$ particles (such as atoms, molecules, ions, or electrons). This value, $6.02 \times 10^{23}$, is known as Avogadro's constant.

Matters of Detail

• Disciplinary Idea: Matter and energy compose the Universe; physical measurements require a common system like SI units (e.g., kilograms for mass and joules for energy).
• Safety Illustration: An overhead bridge sign marked $4.5\,m$ warns vehicles exceeding that height not to pass underneath.