General Physics - 1
Physical Quantities & Units
Physical Quantities
1) Define a physical quantity.
2) Describe physical quantities.
3) Categorise physical quantities.
Physical Quantities
A physical quantity is any measurable quantity.
NOTE THAT:
A Physical quantity consists of a numerical MAGNITUDE and
a UNIT of measure.
* MAGNITUDE means size (How big/Small?)
* UNIT refers to the standard of measurement.
For example:
One may say “the MASS of a box is “50kg”
MAGNITUDE
50
kg
UNIT OF MEASURE
Classification of Physical Quantities
Physical Quantities
Base Quantities
Definition: These are fundamental
quantities that are not defined in
terms of other physical quantities.
Derived Quantities
Definition: These are physical
quantities obtained through
multiplication or division of the
fundamental quantities.
Activity 1: Given the list of Physical Quantities below, classify them as base
quantities or derived quantities.
Density, Mass, Area, Volume, Time, Speed, Length,
Temperature, Energy, Electric current, Luminous intensity,
Work, Amount of Substance, Power and Acceleration.
Classification of Physical Quantities
Activity 1: Given the list of Physical Quantities below, classify them as base quantities
or derived quantities.
Density, Mass, Area, Volume, Time, Speed, Length, Temperature, Energy, Electric current, Luminous
intensity, Work, Amount of Substance, Power and Acceleration.
BASE QUANTITIES
DERIVED QUANTITIES
1. Mass
1)Density
2. Time
2) Area
3. Length
3) Volume
4. Temperature
4) Speed
5. Electric Current
5)Energy
6. Luminous intensity
6) Work
7. Amount of Substance
7) Power, 8) Acceleration, etc.
The International System of Units
1) State the S.I base units and S.I derived units of given
physical quantities.
2) Explain the advantages of the S.I system of Units.
The International System of Units
The International System of Units, known by the
international abbreviation S.I is used to define
units of measurement in Physics.
The S.I system consists of two types of units,
namely:
1) S.I base units
2) S.I derived units
S.I Base Units
Base Units are the seven fundamental units of measurement
that are not defined in terms of other units.
Table 1.1 S.I Base Quantities and their Units
Physical Quantity
Symbol of Quantity Name of SI Unit Symbol of
Unit
Mass
m
Kilogram
Kg
Time
t
Second
s
l
Length
Metre
m
Temperature
T
Kelvin
K
I
Electric Current
Ampere
A
Amount of Substance n
Mole
mol
lv
Luminous Intensity
Candela
cd
S.I Derived Units
Derived Units are units of measurement derived from base units of the component base
quantities by multiplication or division or both.
Table 1.2
S.I Derived Quantities and their Units
Physical
Quantity
Symbol of Derivation
Quantity
Volume
Density
Speed
V
ρ
v
l*b*h
Mass/Volume
Distance/time
Name of SI Unit
Symbol of
Unit
Cubic metres
Kilograms per cubic meter
meters per second
m3
Kg/m3
m/s
Advantages of SI Units
The International System of Units serves the following reasons:
1) It is a rational system, in which only one unit is used for one
physical quantity therefore it creates uniformity.
2) It is a coherent system, which means all the derived units
can be easily obtained form basic and supplementary units.
3) It is a metric system, which means that multiples and
submultiples can be expressed as powers of 10 for quick
conversions.
Conversions of Units
1) Express multiples and submultiples of a given prefix to
the power of 10.
2) Recall and use the conversion table to convert units from
a multiple to a unit.
Conversions of Units
One of the advantages of the SI system of units is
that it is a metric system, which means that
multiples and submultiples can be expressed as
powers of 10 for quick conversions.
*The Conversion table is used to convert units*
Table 1.3 SI The Conversion Table
Prefix
TeraGigaMegaKiloDeciCentiMilliMicroNano-
Symbol
T
G
M
k
d
c
m
µ
n
Factor
1012
109
106
103
10-1
10-2
10-3
10-6
10-9
-12
WHEN USING THE CONVERSION TABLE
***NOTE THAT:
1. From a bigger prefix to a smaller one, multiply by the power.
2. From a smaller prefix to a bigger one, divide by the power.
3. Multiplication by a negative power is division.
Metric Conversions
Example 1 - Multiple to Unit Conversions
Convert the following:
(a)3km to Metres
(b)45mA to Amperes
(c)13µV to Volts
SOLUTION TO EXAMPLE 1a
SOLUTION TO EXAMPLE 1b
SOLUTION TO EXAMPLE 1c -TEST YOURSELF
convert 13µV to V
Multiple to Multiple Conversions
2) Recall and use the conversion table to convert units from
a Multiple to another Multiple.
Example 2 – Multiple to Multiple Conversions
Convert the following:
(a)3km to dm
(b)45mA to cA
(c)63µV to kV
Note that:
When converting from multiple to multiple. First
convert the given multiple to non multiple, then
convert the non multiple to the required multiple.
SOLUTION TO EXAMPLE 2a
Convert the following 3km to dm.
Follow the 2 steps process:
STEP 1
STEP 2
3km
m
dm
Step 1: 3km to m
3km = 3 x kilo x m (big to small, multiply)
= 3 x 1000 x m = 3000m
Step 2: 3000m to dm (big to small, multiply)
3000m = 3000m x deci
= 3000m x 101
= 30 000dm
SOLUTION TO EXAMPLE 2b
Convert the following 45mA to cA
Follow the 2 steps process:
STEP 1
STEP 2
45mA
A
cA
Step 1: 45mA to A
45mA = 45/milli x A (small to big, divide)
= 45 x 10-3 x A = 0.0045A
Step 2: 0.0045A to cA
0.0045A = 0.0045A x centi (big to small, multiply
= 0.0045A x 102
= 0.45cA
SOLUTION TO EXAMPLE 2c -TEST YOURSELF
convert 63µV to kV
END OF TOPIC
SEE THE TOPICAL QUESTION BANK:
RETENTION TEST