Physical Quantities | Physics | CSEC CXC lessons online – Study Guide

Physical Quantities | Physics | CSEC CXC lessons online – Study Guide

Physical Quantities

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
Name Symbol Name Symbol
length l metre m
mass m kilogram kg
time t second s
temperature T, θ kelvin K
electric current I ampere A
amount of substance n mole mol
luminous intensity* candela cd

*Won’t be examined/used at this level. Included for completeness.

N.B.

*              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.

 

For example:

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:

  1. Speed = mass/volume 🡪                  S.I. units: kg/m3 (kg m-3)
  2. Density = distance/time 🡪              S.I. units: m/s (ms-1)