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ALTERNATING CURRENT 

After reading this section you will be able to do the following:

• Define what AC stands for and what it means.

• Explain how AC is created and delivered to different places.

• Discuss the differences between AC and DC.

AC is short for alternating current. This means that the direction of

current flowing in a circuit is constantly being reversed back and forth.

This is done with any type of AC current/voltage source.

The electrical current in your house is alternating current. This comes

from power plants that are operated by the electric company. Those big

wires you see stretching across the countryside are carrying AC current

from the power plants to the loads which are in our homes and

businesses. The direction of current is switching back and forth !" times

each second.

This is a series circuit using an AC source of electricity. #otice that thelight bulb still lights but the electron current is constantly reversing

directions. The change in direction of the current flow happens so fast

that the light bulb does not have a chance to stop glowing. The light bulb

does not care if it is using DC or AC current. The circuit is delivering

energy to the light bulb from the source which in this case is a power

plant.

Review

1. AC or alternating current means the electrical current is

alternating directions in a repetitive pattern.

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2. AC is created by generators in power plants and other sources. This

AC current is delivered to our homes and businesses by the power

lines we see everywhere.

3. The fre$uency of repetition of this current is !" %ert&. This meansthe direction of the current changes sixty times every second.

IRECT CURRENT 

After reading this section you will be able to do the following:

• Explain what DC stands for and what it means.

•Define what a good source of DC would be.

#ow that we have a fairly good understanding of basic electricity terms

and concepts let's take a closer look at some more details of the

electrical current itself.

The battery we have been using for a current/voltage source generates

direct current which simply means the current flows in only one direction.

A( )*#+ A( E)ECT,*#( A,E -)*#+ T%,*0+% T%E AT*1( of the circuit work is being done.e can see that work is being done in this circuit because it lights the light bulb.

The actual amount of electrons that are flowing is determined by the type and si&e

of the battery as well as by the si&e and type of the light bulb. e could reverse

the polarity of the battery by switching the contacts 2wires3 and the current

would flow in the opposite direction and the bulb would still light.

Either way the battery is connected to the circuit current can only flow in one

direction. irect current !C" can also be generated by means other than

batteries. (olar cells fuel cells and even some types of generators can provide DCcurrent.

C or direct current means the electrical current is flowing in only

one direction in a circuit.

1. 4atteries are a good source of direct current 2DC3.

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In electricity, alternating current (AC) occurs when charge carriers in a conductor

or semiconductor  periodically reverse their direction of movement. ousehold utility

current in most countries is AC with a fre!uency of "#hert$ ("# complete cycles per

second), although in some countries it is %# $. &he radio'fre!uency () current in

antennas and transmission lines is another e*ample of AC.

An AC waveform can +e sinusoidal, s!uare, or sawtooth'shaped. ome AC

waveforms are irregular or complicated. An e*ample of sine'wave AC is common

household utility current (in the ideal case). !uare or sawtooth waves are produced

 +y certain types of electronic oscillators, and +y a low'end uninterrupti+le power

supply (-) when it is operating from its +attery. Irregular AC waves are produced

 +y audio amplifiers that deal with analog voice signals and/or music.

&he voltage of an AC power source can +e easily changed +y means of a power

transformer. &his allows the voltage to +e stepped up (increased) for transmission and

distri+ution. igh'voltage transmission is more efficient than low'voltage

transmission over long distances, +ecause the loss caused +y

conductor resistance decreases as the voltage increases.

&he voltage of an AC power source changes from instant to instant in time.

&he effective voltage of an AC utility power source is usually considered to +e the 0C

voltage that would produce the same power dissipation as heat assuming a pure

resistance. &he effective voltage for a sine wave is not the same as the peak voltage .

&o o+tain effective voltage from pea voltage, multiply +y #.#. &o o+tain pea

voltage from effective voltage, multiply +y 1.1. or e*ample, if an AC power

source has an effective voltage of 11 4, typical of a household in the -nited tates,

the pea voltage is 1"% 4.

3

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It is understandable why most people cannot visualize what is happening inside common conductors and cables;even trying to explain that something is moving through copper, a metal, goes against common sense. At the mostbasic level electricity is not all that different from water, hence its basic terms are fairly easy to understand if youcompare an electric circuit to a system of water pipes. The basic difference between water and electricity is that water will fill anything if it somehow manages to burst out of the pipes, while electricity needs a conductor to send electronsto. By visualizing a model system of pipes most people will understand several terms more easily.

 

Voltage = Pressure !oltage is pretty much the "pressure" of the electrons and indicates how strongly and #uic$lythey move through the conductors %cables&. !oltage and pressure are e#ual by many respects, including thepipe'cable strength; too much pressure would burst a pipe, too high a voltage would destroy or ignore a cable(sshielding.

Current = Flow - )urrent is the "flow rate" of electrons, indicating how many electrons move through a cable. Thehigher the current, the more electrons move through the cables. *uch li$e large #uantities of water re#uire thic$erpipes, high currents re#uire thic$er cables.

 

By $eeping the water circuit model in mind a lot of other terms can be explained as well. +or example, powergenerators can be visualized as water pumps, power loads can be pictured as water mills which need flow andpressure of water to rotate and even electronic diodes can be thought of as water valves which allow the water to flowtowards one way only.

irect current is, much li$e the name suggests, the flow of electrons towards a single direction. It is very simple to

visualize direct current using the "water circuit" model; simply thin$ of water flowing towards one direction inside apipe. )ommon devices producing direct current are solar cells, batteries and dynamo generators. Almost everythingcan be developed to be powered by a ) current source and it is almost exclusively used in any low voltage, mobileand electronics applications.

) current is pretty straightforward and almost everything is based on -hm(s aw %! / I 0 1&, while the power of a) load is measured in 2atts and e#uals 3 / ! 0 I %2atts&.

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Because of the simple e#uations and its behavior, ) current is relatively easy to comprehend. The first electricpower transmission systems, developed by Thomas 4dison bac$ in the 56th century, were using direct current. Thelong range distribution of ) however is problematic and it was soon replaced a few decades later by the significantlyadvantageous %at the time& A) current developed by 7icola Tesla.

4ven though the commercial power grids of the entire planet nowadays are using A), technology advancementsironically made high voltage ) transmissions more efficient over very long distances and'or extreme loads, such aswhen interconnecting separate A) systems li$e entire countries or even continents. 8owever, A) continues to be thechoice for low voltage commercial grids, for reasons explained later in this article.

 

 Although A) is far easier to generate by using $inetic energy through a generator, batteries can only produce ) andthis is why ) dominates low voltage and electronics applications. Batteries can be charged only by ) as well,which is why all A) power is instantly transformed to ) when a battery is a main part of a system. A very commonexample of this would be any automotive application, such as motorcycles, cars and truc$s. The alternator %also$nown as dynamo& present in vehicles generates A) current which is instantly transformed to ) through a devicecalled "rectifier", because a battery is present and most electronics need ) voltage to operate. 9olar cells and fuelcells can also only produce ), which can then be transformed to A) if necessary through a device called "inverter".

*uch li$e the name suggests once again, alternating current is the flow of electrons which constantly changes

direction. 4ver since the late 56th century, nearly all home and business power grids worldwide are using sine wave A) current because it is easier to generate and much cheaper to distribute, with the exception of very few longdistance applications which benefit from the lower power losses of the newly developed very high voltage )systems.

 A) has another great advantage, it allows for transfer of energy from the consumption point bac$ into the grid as wellas from the grid to the consumption point. This is very beneficial for buildings and installations that now produce moreenergy than they consume, which is #uite possible when using alternate energy sources such as solar panels and

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wind turbines. The fact that A) allows the twoway flow of energy is the main reason why alternate power sources arebecoming extremely popular and affordable.

2hen things come down to the technical level, unfortunately A) current complicates things dramatically for anamateur to clearly understand how it wor$s and ma$es the "water circuit" model obsolete; however it can still bevisualized as water rapidly changing the direction of its flow, even though nobody would ever understand how water

would accomplish anything useful by doing this. A) current and voltage constantly changes direction; how #uic$ly isdefined by the fre#uency of the application %measured in 8z& and for residential power grids it usually is : ' <8z,which means that the voltage and current will change direction 50 / 60 times per second . )alculating the active%1*9& voltage and current is fairly easy with sine wave systems; simply divide the pea$ by =>. In layman(s terms,when A) current changes directions : times per second %:8z&, it means that the incandescent lights of your houseare being turned on and off : times per second. The human eye cannot perceive it and your brain simply believesthat the lights are constantly turned on.

In the above graph you can see a random, imaginary A) power load connected to a >?! A) outlet. As you can see

not only the current %i& and voltage %v& are constantly alternating, but they also are out of phase %unsynchronized&.The vast ma@ority of A) power loads will cause a phase difference. This means that you need to apply vectormathematics even for the most simple of calculations; it is not possible to simply add, subtract or perform any otherscalar mathematics operations when wor$ing with vectors. 2ith ) current we would say that if :A were transferredto a point from one cable and >A were transferred to the same point from another cable, that would e#ual Adelivered to that point; with A) current that would not be true because the end result would depend on the direction of the vectors.

 

Comparison chart

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Improve this chart AlternatingCurrent

Direct Current

Amount ofenergy that

can be carried:

Safer to transfer overlonger city distances and can

provide more power

Voltage of DC cannot travelvery far until it begins to lose

energy

Flow of

Electrons:

Electrons keep

switching directions- forward

and backward

Electrons move steadily in one

direction or 'forward'

Cause of the

direction of

ow

of electrons:

otating magnet along the

wire

Steady magnetism along the

wire

Frequency:  !he fre"uency of alternating

current is #$%& or

$%& depending upon the

country(

 !he fre"uency of direct

current is &ero(

Direction: It reverses its direction while

)owing in a circuit

It )ows in one direction in the

circuit

Current: It is the current of magnitude

varying with time

It is the current of constant

magnitude

Types: Sinusoidal* !rape&oidal*

 !riangular* S"uare

+ure and pulsating

Obtained

from:

,(C enerator and mains Cell or .attery

assi!e

arameters:

Impedance esistance only

ower Factor: /ies between $ 0 1 it is always 1

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Improve this chart AlternatingCurrent

Direct Current

 !he voltmeter can be connected by 2oining two wires to where the voltage is( 3ne

wire is the positive one* and the other the negative( 4ith some voltmeters* one

must make sure that the wires are connected to the right spots5 the positive

connection on the voltmeter to the more positive 6part6 of the voltage source* and

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the negative to the more negative 6part6( !his way* the voltmeter is parallel to the

electrical circuit(

Connect a simple circuit so that the current will )ow through the ammeter( Connect

the positive probe of the ammeter to the positive terminal of the power supply(

Connect the negative probe of the ammeter to one end of a resistor( 7inally connectthe other end of the resistor to the negative terminal of the power supply( !he

ammeter is now connected so that the current that )ows through the resistor will

also )ow through the ammeter* also know as an in-series connection(

+ut the ammeter in the circuit in the place you want to measure the current( !his

will mean cutting a wire and attaching the ammeter leads to the two cut ends(

8(9 V3/!,E

 !he )ow of charge described in the previous section is established byan e:ternal;pressure< derived from the energy that a mass has by virtueof its position5

potential energy(Energy* by de=nition* is the capacity to do work( If a mass >m? is

raised to some height >h? above a reference plane* it has a measure of potential

energy e:pressed in 2oules >@? that is determined by

4 >potential energy? A mgh> 2oules* @? >8(#? where g is the gravitational acceleration

>B(# ms8?( !his mass now has the ;potential< to do work such as crush an ob2ect

placed on the ref-erence plane( If the weight is raised further* it has an increased

measure of potential energy and can do additional work( !here is an obvious

diFerence in potential between the two heights above the reference plane(

In the battery of 7ig( 8(* the internal chemical action will establish>through an

e:penditure of energy? an accumulation of negative charges >electrons? on one

terminal >the negative terminal? and positive charges >positive ions? on the other

>the positive terminal?( , ;positioning< of the charges has been established that will

result in a potential diFerence between the terminals( If a conductor is connected

between the terminals of the battery* the electrons at the negative terminal have

suGcient potential energy to overcome collisions with other particles in the

conductor and the repulsion from similar charges to reach the positive terminal to

which they are attracted(

Charge can be raised to a higher potential level through the e:penditure of energyfrom an e:ternal source* or it can lose potential energy as it travels through an

electrical system( In any case* by de=nition5

, potential diFerence of 1 volt >V? e:ists between two points if 1 2oule >@? of energy

is e:changed in moving 1 coulomb >C? of charge between the two points(

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 !he unit of measurement volt was chosen to honor ,lessandro Volta >7ig(

8(B?(+ictorially* if one 2oule of energy >1 @? is re"uired to move the one coulomb >1 C?

of charge of 7ig( 8(1$ from position : to position y* the potential diFerence or

voltage between the two points is one volt >1 V?( If the energy re"uired to move the

1 C of charge increases to 18 @ due to additional opposing forces* then the potential

diFerence will increase to 18 V( Voltage is therefore an indication of how muchenergy is involved in moving a charge between two points in an electrical system(

Conversely* the higher the voltage rating of an energy source such as a battery* the

more energy will be available to move charge through the system( Hote in the

above discussion that two points are always involved when talking about voltage or

potential diFerence( In the future* therefore* it is very important to keep in mind

that a potential diFerence or voltage is always measured between two points in the

system( Changing either point may change the potential diFerence between the two

points under investigation(

+otential5 !he voltage at a point with respect to another point in the electrical

system( !ypically the reference point is ground* which is at &ero potential(

+otential diFerence5 !he algebraic diFerence in potential >or voltage? between two

points of a network(

Voltage5 4hen isolated* like potential* the voltage at a point with respect to some

reference such as ground >$ V?(

Voltage diFerence5 !he algebraic diFerence in voltage >or potential? between two

points of the system( , voltage drop or rise is as the terminology would suggest(

Electromotive force >emf?5 !he force that establishes the )ow of charge >or current?

in a system due to the application of a diFerence in potential( !his term is not

applied that often in todays literature but is associated primarily with sources of

energy(

In summary* the applied potential diFerence >in volts? of a voltage source in an

electric circuit is the ;pressure< to set the system in motion and ;cause< the )ow of

charge or current through the electrical system(

, mechanical analogy of the applied voltage is the pressure applied to the water in

a main( !he resulting )ow of water through the system is likened to the )ow of

charge through an electric circuit( 4ithout the applied pressure from the spigot* the

water will simply sit in the hose* 2ust as the electrons of a copper wire do not have a

general direction without an applied voltage(

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If we consider the wire to be an ideal conductor >that is* having no opposition to

)ow?* the potential diFerence V across the resistor will e"ual the applied voltage of

the battery5 V >volts? E >volts?(

 !wo elements are in series if 

1( !hey have only one terminal in common >i(e(* one lead of one is connected to

only one lead of the other?(

8( !he common point between the two elements is not connected to another

current-carrying element(

 !wo elements* branches* or networks are in parallel if they have two points in

common(

an open circuit can have a potential diFerence >voltage? across its terminals* but thecurrent is always &ero amperes(

a short circuit can carry a current of a level determined by the e:ternal circuit* but

the potential diFerence >voltage? across its terminals is always &ero volts(