A physical quantity is any quantity or property which can be measured and a numerical value obtained. Physical quantities have both magnitude (size) and units. Both must be given when stating the value of a physical quantity.
Using different units to represent the same quantity may prove to be confusing, especially when one has to convert between these units.
For example: In day to day activities, the volume of a liquid may be measured in: pints, quarts, gallons, etc. There is no common relationship between these units and converting between them may lead to confusion.
Scientists have been able to avoid such confusion by agreeing on a single general system of units which proves to be quite useful, especially when converting between units.
This system is called Systeme International (S.I.) and is based on a defined unit for each of seven (7) fundamental (base) physical quantities. These are shown in Table 1.0.
Table 1.0: Fundamental Quantities and their Units.
|Fundamental/Base Quantity||Fundamental/Base Units|
|amount of substance||n||mole||mol|
*Won’t be examined/used at this level. Included for completeness.
* All other physical quantities (and units) are called derived quantities (and units).
* Derived quantities (and units) have been derived from the base quantities and their units by using equations.
The volume of a cube of sides (l) is given by:
V = l x l x l
Since volume (a derived quantity) is found by multiplying three lengths (fundamental quantities), the unit of volume is found by multiplying the base units of length.
Therefore, units of volume: m x m x m = m3
Other derived quantities include:
- Speed = mass/volume 🡪 S.I. units: kg/m3 (kg m-3)
- Density = distance/time 🡪 S.I. units: m/s (ms-1)