2. Learning Objectives
At the conclusion of this section, students should be
able to:
Identify the basic units of measurement
Define and use the SI derived units for force,
pressure, energy, work, temperature and power
Convert units to multiple and sub-multiple units
Transpose a given equation for any variable in the
equation
Perform basic calculations of electrical and
related mechanical quantities given any
combination of units, multiple units or sub-
multiple units.
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3. Resources
Hampson & Hanssen, “Electrical Trade Principles – A practical
approach”
Pgs 2 – 5, 15 – 25 & 421 including review questions
Chisholm Moodle E Learning
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13. Substitution
Take the electrical quantities of: Power (P),
Voltage (V), Current (I) and Resistance (R). There
are two equations that use these quantities, they
are:
P = V x I and V = I x R
Suppose we want to calculate power when only
current (I) and resistance (R) is known.
Substitution will enable power to be calculated.
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14. Substitution
V IR
Substituting IR for V in the power equation,
P I R I
2
I R
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15. Multiples and Submultiples Pg. 4
Prefix Symbol Exponential Multiplier
format
tera T 1012 1 000 000 000 000
giga G 109 1 000 000 000
mega M 106 1 000 000
kilo k 103 1 000
milli m 10-3 0.001
micro 10-6 0.000 001
nano n 10-9 0.000 000 001
pico p 10-12 0.000 000 000 001
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18. Standard Measurement Units
Previous measuring systems
Imperial System
English units of pound (mass), foot (length) and
To help understand,
not for examination
degree Fahrenheit (temperature)
CGS Units
Centimetre for length, gram for mass, second for
time
MKSA System
Metre, (length)
Kilogram, (mass)
Second (time) and
Ampere (electric current)
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19. SI (Systeme International)
The SI system is an expansion of the MKSA
and includes three new base units. These are
the kelvin (temperature), the mol (amount of
Matter} and the candela (luminous intensity).
This brought the total number of base units*
to seven.
*Base units are a set of mutually independent (fundamental)
units from which all other units can be derived.
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21. Derived Quantities
Velocity (distance traveled in a given time)
Acceleration (the rate of change in velocity)
Force (the physical action capable of moving a body)
Torque (twisting force eg produced by a motor)
Pressure (force per unit area)
Electrical charge (1 Amp flowing for 1 second)
Voltage (electrical pressure)
Resistance (opposition to current flow)
Energy (the capacity to do work)
Work (force acting through a distance)
Power (rate of doing work)
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23. Abbreviations and Conventions
(shortened names for things) (agreed standard ways to do or
write things)
1. There should be a space between the
numeric value and the unit symbol.
For example five milliamps is written as
5 mA and not 5mA
(A ‘hard’ space in a typed document will prevent this; 240
V i.e. the unit symbol appearing on the next line.)
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24. Abbreviations and Conventions
2. When writing numbers above 999, they
should be clustered into groups of three.
For example,
1 000 or 20 000 or 0.000 006 78
and not 1000 or 20000 or 0.00000678
(This reduces the chance of mis-reading a number’s
size by mis-counting zero’s)
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25. Abbreviations and Conventions
5. A leading zero should precede a decimal value.
For example
0.351 and not .351
(This makes it easier to recognise a missing decimal
point, for instance, on a well-used drawing 0 351
would be obvious but 351 could lead to a major
error!)
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