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1.0 : UNIT ASSOCIATED

WITH BASIC ELECRICALQUANTITIES

Objectives:

1.1 Understand SI Units1.2 Understand the units andsymbols of electrical potential

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1.1 SYSTEM OF UNITS(SI) SI-

Internationalsystem of unit

Based onmetric system

The basic unitin the SI

system arelisted belowwith theirsymbol:

Quantity Unit

Length meter, m

Mass kilogram,kg

Time Second,s

Electric current Ampere,A

Thermodynamictemperature

kelvin,K

Luminousintensity

candela,cd

Amount ofsubstance

Mole,mol

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1.2 THE SI PREFIXES

Table 1.2 The SI Prefixes

Multiplier Prefixes Symbol

eta E

peta P

tera T

giga G

mega M

kilo k

hecto h

10 deka dadeci da

centi c

mili m

micro

nano n

pico p

femto f

atto a

Q

18

10

1210

1510

910

610

310

210

110

210

310

610

910

1210

1510

1810

SI unit used prefixes based on the power of 10 to relate the larger and

small units to the basic units

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1.2 THE SI PREFIXES (Cont..)

x

10Base

Exponent

000,10.0000.11011044

!!! xMove to the right

0001.0.0001.1011044

!!! x Move to the left

The power of ten is expressed as an exponent of the base of 10

The exponent indicates the number of places that the decimal places

that the decimal point is moved to the right or left to produce the decimal number

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Problem 1 Covert 0.15 miliampere (0.15mA) to

microamperes

[Ans:150A]

Working Steps:

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Problem 2 Convert 4500 microvolts to milivolts

[Ans: 4.5mV]

Working Steps:

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Problem 3 Convert 0.00022 microFarad to

pikoFarad

[Ans:220pF]Working Steps:

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Exercise Convert

1.5000nA=_____A

2.47000pF=____F3.15mA+8000A=______mA

4.0.05mWAtt+8kWatt=_______kWatt

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1.3 Basic Electrical Units

Quantity Symbol Unit Symbol

capacitan C farad F

charge Q coulumb C

conducta G siemens S

current I ampere A

energy W joule J

frequenc f hertz Hz

impedencZ ohm

inductanc L henry H

power P watt W

resistanceR ohm

voltage V volt V

Table 1.3 Electrical Quantities and their

responding units with SI symbol

One symbol is used to represent the name of quantities

It used to represent the unit of the measurement of that quantities

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1.4 Charge Unit : Coulumb (C)

1 Coulumb: 1ampere second

Formula: where I is the currentin amperes and t is the time in seconds

electronsxCoulomb18

1024.61 !

ItQ !

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Problem 4 If current of 5A flows for 2 minutes,

find the quantity of electricity

transferred[Ans:600C]

Working Steps:

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1.5 Electrical potential and

e.m.f The unit of electric potential is volt (V)

One volt is the potential difference(voltage) between 2 points when one joule

of energy is used to move one coulomb ofcharge from one point to the other

A change in electrical potential between twopoints: potential difference

The electromotive force (e.m.f) provide by

source of energy such as a battery or agenerator is measured in volt

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1.6 Resistance Unit: ohm ()

Define: opposition to the current

Formula: where Vis the potential

difference across two points (in Volt) and Iis the current flowing between the two

points (in Amperes)

I

VR !

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1.7 Conductance Define: The reciprocal of resistance is called

Unit: Seimens

Formula: where

Ris the

resistance( in ohm)

RG

1!

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1.7 Current Unit: Amperes (A)

Define : measured by the number ofelectrons(amounts of charge , Q) that flowpast a point in a unit of time

Formula: where ,Q is the

charge of the electrons (in Coulombs) andt is the time (in seconds)

t

QI !

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Problem 5

Q1:Ten coulombs of charge flow past a given point in a wire 2s.

What is the current in Ampere?

[Ans: 5A]

Related Problem

Q2: If there are 8A of direct current though the filament of the light

bulb , how many coulomb have moved through the filament in 1.5s?

Working Steps:

Q3:What is the current in amperes when 20C flow past a point in a wire

in 4s?

Working Steps:

Working Steps:

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1.9 Power & Energy Unit (Power): Watt (W)

Define (Power): the rate of doing work ortransferring energy

Formula (Power): where Wis the work

done (in Joules) and tis the time (in second)

Unit (Energy): Joule (J)

Define (Energy) :Ability to do work Formula (Energy):

t

WP!

PtW !

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1.10 Electrical Power and

Energy Formula (Power):

Formula (Energy): Power x time

VIP!

VIt

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Problem 6 A source e.m.f of 5V supplied a

current of 3A for 10 minutes. How

much energy is provided in this time?[Ans: 9kJ]

Working Steps:

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Problem 7 An electric heater consumes 1.8MJ

when connected to a 250 V supply for

30 minutes. Find the power rating ofthe heater and the current taken fromthe supply.

[Ans: 4A]

Working Steps: