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Fundamental electrical principles2.8.1.2

Direct Current Circuits

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Fundamental units

* 7 Base Units*International Metric System- Systeme Internationale (SI)

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Fundamental SI units

Amount of Substance Mole mol

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ANDRE AMPERE

ELECTRO - MAGNET

1775-1836

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SI Derived units

★ Basic Units are not sufficient for all situations of measurement★Derived units are for non-basic situations★Derived Units use: length, mass and time (various combinations)★Derived Unit Groups: Mechanical, electrical, magnetic

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Derived MechanicalUnits

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Derived ElectricalUnits

Significant FiguresThis is a method of rounding a particular value where

you choose a desired number of figures for measurement.

• Example: The number of electrons per second in a circuit is 6,250,000,000,000,000,000 electrons when 1 amp is flowing. In order to express this number in scientific notation, we must represent the significant digits as a number between 1 and 10.

• The result is 6.25 x 1018.• How did we get this answer? 1. Decide on where you want the decimal place to be (between the 6

and 2)= 6.250000000000000002. Count the numbers including zeros on the right hand side of the

decimal place = 183. The number now becomes 6.25 x 10 18

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Scientific Notation★ Rounding Method for a desired number of figures for measurement.

★ Number between 1-9 (first element), times a power of ten (second element).

★ Ex: 7.85 (first element) x 105 (second element)

In practical cases some SI values are inconveniently large or small, Inorder to choose values that are convenient to handle, multiples orsubmultiples are used. For example, if the resistance of an electricalinstallation is measured at 15 000 000 ohms, it is more convenient to referto this value as 15 megohms; that is, 15 units, each.

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Scientific notation Example If we have measured a voltage reading of 785000 V how

do we write it in scientific notation

Is step is to break it in two parts , one part being a number between 1 and 9 and the second part a power of 10

the first part will be 7.85000

the second part we can work out by adding up the spaces to the right of the decimal point = 5. This will give us 105 ( 10x10x10x10x10)

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Engineering Notation

Tera 1012 T

giga 109 G

mega 106 M

kilo 103 k

milli 10-3 m

micro 10-6 µ

nano 10-9 n

pico 10-12 p

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Multiple or Divide using Scientific Notation:

★Multiple/ Divide decimal numbers with each other★Use exponents to “combine” powers of 10★ In it’s not the correct scientific notation- change it.

Example 1:2.5x 10 2 x 4.0 x 10 2

2.5 x 4 = 10

10 2 + 10 2 = 10 4

Answer = 10 x 10 4

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Example 2:

680000 x 3.5 x 10 4

Step 1: convert to the same notation 680000 = 6.8 x 105

Step 2: multiply the first part of each number together 6.8 x 3.5 = 23.8

Step 3: add the powers of 10 = 104 + 105 = 109

Step 4: put them together 23.8 x 109

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Transposition of Formulae

★FUNDAMENTAL RULE OF TRANSPOSITION:

Whatever you do to one side of the formula, you must do to the other side.

★ ‘sides’ of an equation are either side of the equals sign (=).

V = IRIf we want I on its own we divide both sides by RV/R = IR/RV/R stays as it isWith IR/R the R’s cancel each other outYou are left with: V/R = I You can switch this around to read: I= V/R

something more complicated?

H = IN/LMake l the subject of the equation

Multiply both sides by LHL = INL/L

HL = IN (the L’s cancel each other out on the right hand side)

Now divide both sides by HHL/H = IN/H

L = IN/H (the H’s canceled each other out on the left hand side,

leaving L on its own, the subject of the equation)L=IN/H