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. Base Units
• The Systeme Internationale’ (SI) is the
International Metric System
There are 6 Base Units in the SI system
Unit Symbol Quantity Symbol
Length l Metre m
Time s Second s
Mass m Kilogram kg
Current I Ampere A
Temperature T Kelvin K
Light I candela cd
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16. SI Derived Units
The six basic units are not sufficient to
act for all situations that arise in
measurement.
Derived units are used for all non-basic
situations.
Most derived units use the three basic
units of length, mass and time in
various combinations.
.
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17. SI Derived Units
The units used can be subdivided
into three groups:
mechanical, electrical and magnetic
although it must be realised there
are many more examples than those
listed
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18. 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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19. Derived Mechanical Units
Unit Symbol Quantity Symbol
Force F Newton N
Pressure P Pascal Pa
Energy & Work W Joule J
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20. Derived Electrical Units
Unit Symbol Quantity Symbol
Power P watt W
Frequency F hertz Hz
Potential V volt V
Charge Q coulomb C
Capacitance C farad F
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21. Multiples And Submultiples
In practical cases some SI values are
inconveniently large or small, In order to
choose values that are convenient to handle,
multiples or submultiples are used.
For example, if the resistance of an electrical
installation is measured at 15 000 000 ohms,
it is more convenient to refer to this value as
15 megohms.
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22. Multiples and Submultiples
Tera 1012 T
giga 109 G
mega 106 M
kilo 103 k
milli 10-3 m
micro 10-6
nano 10-9 n
pico 10-12 p
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24. Scientific Notation
• Another method of overcoming cumbersome
rows of figures is to notate numbers to a
value between 1 and 10 multiplied by 10 to
some power.
• For example, 6 800 000 can be expressed as
•
6.8 x 106 and
• 1250 as 1.25 x 103
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25. Examples:
• Given: 1.015 x 10 -8
– Answer: 0.00000001015 (8 places to left)
– Negative exponent move decimal to the left
– Given: 5.024 x 10 -3
– Answer: 5,024 (3 places to the right)
– Positive exponent move decimal to the right
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26. Examples
• Express in standard form
• 1.09 x 10 3
• 4.22715 x 10 8
• 3.078 x 10 – 4
• 9.004 x 10 – 2
• 5.1874 x 10 2
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27. To change from scientific notation
to standard form:
• Move decimal point to right for positive
exponent of 10
• Move decimal point to left for negative
exponent of 10
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31. 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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32. 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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33. 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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