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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 Pg.4

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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,

IRV

RI

IRIP2

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Multiples and Submultiples Pg. 4

Prefix Symbol Exponential format

Multiplier

teragigamegakilo

millimicronanopico

TGMk

mnp

1012

109

106

103

10-3

10-6

10-9

10-12

1 000 000 000 0001 000 000 0001 000 0001 000

0.0010.000 0010.000 000 0010.000 000 000 001

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PREFIXES

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PREFIXES

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Standard Measurement Units Previous measuring systems

Imperial SystemEnglish units of pound (mass), foot (length) and

degree Fahrenheit (temperature) CGS Units

Centimetre for length, gram for mass, second for time

MKSA SystemMetre, (length) Kilogram, (mass) Second (time) and Ampere (electric current)

To help understand,

not for examination

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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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BASE SI UNITS Pg. 2

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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 SI UNITS

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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.)

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

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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!)