ELECTRICAL FUNDAMENTALS

TOPIC 1Fundamental And Derived Units

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Learning ObjectivesAt the conclusion of this section, students should be able to: Identify the basic units of measurementDefine and use the SI derived units for force,

pressure, energy, work, temperature and powerConvert units to multiple and sub-multiple units Transpose a given equation for any variable in the

equationPerform basic calculations of electrical and

related mechanical quantities given any combination of units, multiple units or sub-multiple units.

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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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TRANSPOSITION

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TRANSPOSITION addition/subtraction

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TRANSPOSITION multiply/divide

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TRANSPOSITION – multiply/divide

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TRANSPOSITION – mixed operations

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TRANSPOSITION – mixed operations

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TRANSPOSITION – mixed operations

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TRANSPOSITION – roots

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TRANSPOSITION – roots

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SubstitutionTake 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 RSuppose 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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Substitution

Substituting IR for V in the power equation,

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IRV

RI

IRIP2

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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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SI Derived UnitsThe 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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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

Derived QuantitiesVelocity (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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Derived Mechanical Units

Unit Symbol Quantity Symbol

Force F Newton N

Pressure P Pascal Pa

Energy & Work W Joule J

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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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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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Multiples and Submultiples

Tera 1012 T

giga 109 G

mega 106 M

kilo 103 k

milli 10-3 m

micro 10-6 m

nano 10-9 n

pico 10-12 p

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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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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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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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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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Express in correct scientific notation

0.0000568

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64 960 000

0.07085

61 500

PREFIXES

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PREFIXES

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Abbreviations and Conventions

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; 240V i.e. the unit symbol appearing on the next line.)

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(shortened names for things) (agreed standard ways to do or write things)

Abbreviations and Conventions2. 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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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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