2.SIMPLE ELECTRICAL SYMBOLS AND UNITS BY NDAKO MAIMUNA ALIYU(MRS).ppt

8/20/2019 2.SIMPLE ELECTRICAL SYMBOLS AND UNITS BY NDAKO MAIMUNA ALIYU(MRS).ppt

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Simple electrical symbols&units

By

Ndako Maimuna Aliyu

1simple electrical symbols and

units. by Ndako M.A.


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•Electrical current

• Voltage(emf & p.d)

•Resistance

•

Ohm’s law•Capacitance

•Conductors and insulators

2simple electrical symbols and units. by NdakoM.A.


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Introduction

Electron theory; All matter is composed ofatom. The nucleus or centre of an atom hasprotons with a positive polarity or ‘charge’ andneutrons with no charge.

The nucleus is surrounded by one or moreelectrons with a negative polarity. These twopolarities indicate 2 opposite characteristicsthat is fundamental in all physical

applications.



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The arrangement of electrons & protons

are basic particles of electricity thatdetermines the electrical characteristicsof all substances. when a material hasboth electrons and protons in it, No

evidence of electricity is seen because thenumber of electrons equal that of protonsin which case the opposite electrical

forces cancels out making the materialelectrically neutral.



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Note however that, it is the opposing

characteristics that provide a method ofbalancing one against the other.



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THE CHARGE;This is a property that

causes the opposite polarities (+ve & -ve )to attract or repel each other as the casemaybe. See the example below.



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VOLTAGE(potential difference);

potential refers to the possibility of doing work. Any charge has the potential(i.e. ability to dowork)of moving another charge by eitherattraction or repulsion. Therefore potential

difference refers to the difference of electricpotential between the two points of an electriccircuit .as seen below;



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Its symbol is V and the unit of measurement isvolts.The instrument used to measure the potential

difference is called as Voltmeter. The voltmeteris always connected in parallel with the circuitwhose voltage is to be calculated. As shown in asimple circuit below.



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Volt is a measure of the amount of work orenergy needed to move an electric energy.



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The required work to be done or pressure is also

needed to direct the electric charge to flow in auniform direction. i.e. from a low potential to ahigh potential. The force or pressure is referredto as electromotive force. Thus electromotiveforce is simply defined as work done per unit ofcharge.



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For instance, in the figure shown below, a

resistor is connected to the source of energy,here a battery, consider that the wires whichare connecting the resistor and battery areoffering no resistance to the flow of current.

Battery has two terminal +ve and –veterminals. +ve terminal at higher point ascompared to –ve terminal .



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Now if the internal resistance of the battery isneglected, there is a potential difference acrossthe battery. Terminal voltage of battery isequal to its electromotive force. Note howeverthat in practical situations, there would alwaysbe internal resistance of the battery wheneverthere is current flowing in the circuit.



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ELECTRICAL CURRENT;Is simply said

to be a continuous flow of electrons. Electronsmove because protons are 1800 times heavierand it is the movement of free electrons thatprovide s electric current in a metal conductor.

Current is often referred to as charge in motion.That is when the potential difference betweentwo charges forces a third charge to move, thecharge in motion is an electric current.

To produce a movement of an electron, it isnecessary to either have a –vely charged field“push it” or a +vely charged field “pull it”



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Its symbol isIand its unit is Ampere(A).The current in a circuit is measured by the

instrument known as Ammeter. Ammeter isalways connected in series with the circuit.



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AMPERE (A) It is a measurement thatcombines the quantity of electricity and thelength of time it takes the electricity to travel a

certain distance. One amp is the same as onecoulomb per second. I=Q/t.



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ALTERNATING CURRENT;If the

current changes direction after equal intervalsof time, it is called an alternating current,written as ac or AC.



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A frequency associated with the alternatingfrequency (A.C) is called alternating frequency.This frequency is the rate at which currentchanges its direction with respect to time.



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DIRECT CURRENT;If the current alwaysflows in the same direction, it is called Directcurrent and its represented as D.C or d.c. e.g.the current generated from a cell or battery is

D.C because its unidirectional. The +ve and –veterminals are fixed or it is single polarity.



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ELECTRICAL RESISTANCE;Whilediscussing electrical current, we learnt thatcertain materials such as copper have many freeelectrons while substances such as glass, andrubber have fewer free electrons. some othersubstances like mica have no free electrons atall thus making them good insulators. Betweenthe very good conductors such as silver, copper

and very good insulators e.g. glass and rubberlay other conductors of reduced conductingability. They “oppose” or “resist” the flow ofelectrons hence the term resistance.



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Electrical Resistance is the property of an

electrical element to oppose the flow of theelectrical current passing through it whenvoltage is applied across the element.



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Its symbol is R and its unit is ohm . Theinstrument that measures the resistance is

known as ohmmeter.Mathematically it is represented as the ratio ofvoltage to current. i.e. R=V/I



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OHM Ω;

The resistance or opposition to the flow ofelectrical current or electricity. It takes one voltto push one ampere through one ohm ofresistance, this is known as ohms law.



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OHM’S LAW;

The electric potential difference between twopoints on a circuit (V) is equivalent to the∆product of the current between these two points(I) and the total resistance of all electrical

devices present between these two points. (R).

∆ V=I·R

This law can be represented or rearranged as

I=V/R∆



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I=V/RThe equation above indicates the two

variables that would affect the amount ofcurrent in a circuit. The current in a circuit isdirectly proportional to the electric potentialdifference impressed across its end and

inversely proportional to the total resistanceoffered by the external circuit. The greaterthe Emf, the greater the current and the lessthe resistance. The greater the resistance, the

less the current .Charge flows at the greatest rates when thebattery voltage is increased and the

resistance decreased. 29simple electrical symbols and units. by NdakoM.A.


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The ohm’s law equation is often explored in

laboratory using a Resistor, a battery packand a Multimeter. The Multimeter is used tomeasure both current and voltage when setappropriately.

By alternating the number of cells in thebattery pack, the emf across the externalcircuit can be varied.



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CAPACITANCE C; In the topic current we learnt of the unit of

measuring electrical quantity or charge was acoulomb. Now a capacitor (formerly condenser)has the ability to hold a charge of electrons.The number of electrons it can hold under agiven electrical pressure (voltage) is called itscapacitance or capacity. Two metallic platesseparated by a non-conducting substance

between them make a simple capacitor.



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The symbol of a capacitor is seen below in a

simple circuit charged by a battery



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In the circuit above, when the switch is openthe capacitor has no charge upon it, when theswitch is closed current flows because of thevoltage pressure, this current is determinedby the amount of resistance in the circuit. At

the instance the switch closes the emf forceselectrons into the top plate of the capacitorfrom the negative end of the battery and pullsothers out of the bottom plate toward the

positive end of the battery.



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Two points need to be considered here. Firstlyas the current flow progresses more electrons

flow into the capacitor and a greater opposingemf is developed there to oppose further currentflow, the difference between battery voltage andthe voltage on the capacitor becomes less and

less and current continues to decrease. Whenthe capacitor voltage equals the battery voltageno further current will flow.The second point is if the capacitor is able tostore one coulomb of charge at one volt it is saidto have a capacitance of one Farad.



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CONDUCTORS;The behavior of an object that has been charged

is dependent upon whether the object is madeof a conductive or a nonconductive material.conductors are materials that permit electronsto flow freely from particle to particle. An objectmade of a conducting material will permitcharge to be transferred across the entiresurface of the object. If charge is transferred to

the object at a given location, that charge isquickly distributed across the entire surface ofthe object. The distribution of charge is theresult of electron movement.



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Since conductors allow for electrons to be

transported from particle to particle, a chargedobject will always distribute its charge until theoverall repulsive forces between excesselectrons is minimized. If a charged conductor

is touched to another object, the conductor caneven transfer its charge to that object. Thetransfer of charge between objects occurs morereadily if the second object is made of a

conducting material. Conductors allow forcharge transfer through the free movement ofelectrons.



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SIEMENS S

Siemens measures conductance, or how easilyan electric current flows through an object.



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INSULATORS;

In contrast to conductors,insulators arematerials that impede the free flow of electronsfrom atom to atom and molecule to molecule. Ifcharge is transferred to an insulator at a given

location, the excess charge will remain at theinitial location of charging. The particles of theinsulator do not permit the free flow of electrons;subsequently charge is seldom distributed evenly

across the surface of an insulator.



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Conductive objects are often mounted upon

insulating objects. This arrangement of aconductor on top of an insulator preventscharge from being transferred from theconductive object to its surroundings. This

arrangement also allows for the personcarrying out the experiment to manipulate aconducting object without touching it. Theinsulator serves as a handle for moving the

conductor around on top of a lab table



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If charging experiments are performed with

aluminum pop cans, then the cans should bemounted on top of Styrofoam cups. The cupsserve as insulators, preventing the pop cansfrom discharging their charge. The cups also

serve as handles when it becomes necessary tomove the cans around on the table.



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Examples of Conductors and InsulatorsExamples of conductors include metals, aqueous solutions

of salts (i.e.,ionic compoundsdissolved in water),graphite, and the human body. Examples of insulatorsinclude plastics, Styrofoam, paper, rubber, glass and dryair. The division of materials into the categories ofconductors and insulators is a somewhat artificial

division. It is more appropriate to think of materials asbeing placed somewhere along a continuum. Thosematerials that are super conductive (knownassuperconductors) would be placed at on end and the

least conductive materials (best insulators) would beplaced at the other end. Metals would be placed near themost conductive end and glass would be placed on theopposite end of the continuum



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Note however that, The conductivity of ametal might be as much as a million trillion

times greater than that of glass.



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SUMMARYThere are many atoms around us which

carry fundamental particles. Thesefundamental particles; electrons and protonswhich carry –ve and +ve charges are the

reason for electricity. The particles neutronsand protons are present in the nucleus whileelectrons are moving around the outer shell ofthe atom. The movement of electrons is the

reason for electricity in which the effect isseen in our everyday life e.g. lightening whichis a spark or flashlight.



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Electrical current; This is themovement of charge by a potential difference.

If the current changes direction at equalintervals it is said to be an alternatingcurrent(A.C) while it is called a directcurrent(D.C) when it flows in the samedirection. Voltage(emf or p.d); Is the electrical

pressure or force that is required to force a

current through a resistance.Resistance; Because conductors are notperfect, they resist to some degree the flow ofcurrent.



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The resistance of a conductor is its voltage

drop divided by the current flowing throughthe conductor. This is also known as OHM’SLAW. The law is simply represented as R=V/I.measured in .

Ohm’s law can be simplified using what I callthe “WATER DISPENSER ANALOGY”



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Quantity Symbol Equation(s) Standard

Metric UnitOther Units

Voltage(emf

or p.d)∆V

∆V = ∆PE / Q

∆V = I • R

Volt (V) J / C

Current I

I = Q / t

I = ∆V / R

Amperes (A)

Amp or C / s

or V / Ω

Resistance R

R = ρ • L / A

R= ∆V / I

Ohm (Ω) V / A

Table of electrical units and

symbols



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References:1.Fundamentals of electric circuits by Mitchel ESchulz2.Grob’s basic electronics by Charles K. Alexander andMathew N.O. Sadiku.

3. Getting started in electronics by Forrest M Mims III4.www.physicsclassroom.com/current electricity5.physics.tutovista.com/electricity-and-magnetism/electri-current.html6.www.rapidtables.com

7.www.completepowerelectronics.com8.www.electronics-tutorials.com

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2.SIMPLE ELECTRICAL SYMBOLS AND UNITS BY NDAKO MAIMUNA ALIYU(MRS).ppt