Nota Electromagnetism

7/24/2019 Nota Electromagnetism

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http://www.one-

school.net/Malaysia/UniversityandCollege/SPM/revi

sioncard/physics/physics.html

1.1 Introduction to Magnetism !evision"

You should have learned magnetism in form 3. In form 3, you learned that:

1. A magnet can attract certain type of metal.

2. The metals that can be attracted by a magnet are called the magnetic materials!

of ferromagnetic materials!. "#amples of magnetic materials are iron, steel,nic$el and cobalt.

%e also learned that a magnet has 2 poles&the 'orth (ole and the )outh (ole. There is a

magnetic field surrounding the magnet. A magnetic field is a region in the surrounding of

a magnet *hich a magnetic material e#periences a detectable force.

Magnetic #ield $ineThe magnetic filed of a magnet is represented by the magnetic field line. The magnetic

field line flo*ing out from the 'orth pole and flo*ing into the )outh pole.

The distance bet*een the

field lines represent the strength of the field, the closer the field line, the stronger thefield. In the diagram, the magnetic field A is stronger than magnetic field + because the

line in magnetic field A is closer.

Compass in a magnetic #ield
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The pattern and the direction of a magnetic field can be determined by a compass. The

pointer of the compass is al*ays in the direction of the magnetic field. In figure b-

above, *e can see that *hen a fe* compasses are put near to a bar magnet, the pointer ofthe compasses are all in the direction of the magnetic field.

If a compass is placed near to a current carrying *ire, the pointer of the compass *ill

point along the direction of the magnetic field generated by the current. This *ill discussin electromagnetism.

%.1 &hat is 'lectromagnetism

%hen current passes through a conductor, magnetic field *ill be

generated around the conductor and the conductor become a magnet.This phenomenon is called electromagnetism. )ince the magnet is

produced electric current, it is called the electromagnet.

An electromagnet is a type of magnet in *hich the magnetic field isproduced by a flo* of electric current. The magnetic field disappears

*hen the current ceases.

In short, *hen current flo* through a conductor, magnetic field *ill

be generated. %hen the current ceases, the magnetic field disappear.

%.% Magnetics '((ects o( a Current Carrying Conductor

- Straight &ire
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Magnetic #ield Pattern

/igure a-0

The magnetic field forms by straight *ire are concentric circles around the *ire as sho*n

in figure a- above. Ta$e notes that *hen the direction of the current is inversed, the

direction of the magnetic field line is also inversed.

The direction of the magnetic field line can be determined by the a#*ells )cre* ule

or the ight 4and 5rip ule.
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/igure a-0

)ometime, the magnetic field pattern may be given in plan vie*, as sho*n in figure b-.

In plan vie*, a dot in the *ire sho*s the current coming out from the plane *hereas across in the *ire sho*s the current moving into the plane.

)irection o( the Magnetic #ield

The direction of the magnetic field formed by a current carrying straight *ire can bedetermined by the ight 4and 5rip ule or the a#*ell )cre* ule.

!ight *and +rip !ule

5rip the *ire *ith the right hand, *ith the thumb pointing along the direction of the

current. The other fingers give the direction of the magnetic field around the *ire. This is

illustrated in

/igure c-0

,he Mawells Screw !ules

The a#*ell )cre* ules sometime is also called the a#*ells 6or$scre* ule.
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Imagine a right handed scre* being turn so that it bores its *ay in the direction of the

current in the *ire. The direction of rotation gives the direction of the magnetic field.

Strength o( the Magnetic #ield

The strength of the magnetic field form by a current carrying conductor depends on the

magnitude of the current. A stronger current *ill produce a stronger magnetic field

around the *ire as sho*n in /igure e- belo*.

The strength of the fielddecreases out as you move further out. This is illustrated in figure f- belo*. Thus, you

must be very careful *hen you are as$ed the dra* the magnetic field in your e#am. The
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distance of the field lines must increase as it is further out form the *ire.

%. Magnetic '((ects o( a Current-Carrying Conductor -

Coil

#ield Pattern/igure a- belo* sho*s the field pattern produced by a current flo*ing in a circular coil.

In )(, you need to $no* the field pattern, the direction of the field and the factors

affect the strength of the field.

The direction of the field can be determined by the ight 4and 5rip ule. 5rip the *ireat one side of the coil *ith your right hand, *ith thumb pointing along the direction of the

current. Your other fingers *ill be pointing in the direction of the field.

/igure a-0
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/igure a-0

)etermining the Pole o( the Magnetic #ieldThe pole of the magnetic field of a solenoid can be determined by the ight 4and 5rip

ule. Imagine your right&hand gripping the coil of the solenoid such that your fingerspoint the same *ay as the current. Your thumb then points in the direction of the field.)ince the magnetic field line is al*ays coming out from the 'orth (ole, therefore the

thumb points to*ards the 'orth (ole.

/igure b-0

There is another method can be used to determine the pole of the magnetic field forms by

the solenoid.

Try to visualise that you are vie*ing the solenoid from the 2 ends as illustrated in figure

c- belo*. The end *ill be a 'orth pole if the current is flo*ing in the a'ticloc$*ise, or a

)outh pole if the current is flo*ing in the cloc$*i)e direction.
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Strength o( the

Magnetic #ieldThe strength of the magnetic field can be increased by

1. Increasing the current,

2. Increasing the number of turns per unit length of the solenoid,

3. 7sing a soft&iron core *ithin the solenoid.

Physics nimationpplet6ompass in a )olenoid

'ercises

The diagram above sho*s the apparatus set&up to

investigate the factors affect the strength of an electromagnet. %hen the s*itch is closed,

the pin are attracted to the soft iron rod. The number of pins attracted to the soft iron rod

can be increased by

A Increasing the current in the circuit
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+ Increasing the resistance in the rheostat

6 8eceasing the number of turns in the solenoid

8 eplacing the battery *ith alternating current.

%.2 Uses o( 'lectromagnet - 'lectric 3ell

(osted by "lectromagnetism 9 :1; ( 9 electric bell9< comments =

%hen the s*itch is on, the circuit is completed and current flo*s. The electromagnet

becomes magnetised and hence attracts the soft&iron armature and at the same time pullthe hammer to stri$e the gong. This enables the hammer to stri$e the gong.

As soon as the hammer moves to*ards the gong, the circuit is bro$en. The current stopsflo*ing and the electromagnet loses its magnetism. This causes the spring to pull bac$

the armature and reconnect the circuit again.

%hen the circuit is connected, the electromagnet regain its magnetism and pull the

armature and hence the hammer to stri$e the gong again. This cycle repeats and the bellrings continuously.

youtube : "lectric +ell

%.4 Uses o( 'lectromagnet - Circuit 3rea5er
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(osted by "lectromagnetism 9 :1> ( 9 < comments =

The figure sho*s the structure of a circuit brea$er. A circuit brea$er is an automatic

s*itch that cut off current in a circuit *hen the current become too large.

%hen the current in a circuit increases, the strength of the electromagnet *ill increase in

accordance? this *ill pull the soft iron armature to*ards the electromagnet.

As a result, the spring pulls apart the contact and disconnects the circuit immediately, and

the current stop to flo*.

%e can reconnect the circuit by using the reset button. The reset button can be pushed to

bring the contact bac$ to its original position to reconnect the circuit

%.6 Uses o( 'lectromagnet - ,elephone 'arpiece

(osted by "lectromagnetism 9 :1@ ( 9 3 comments =
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The fore finger, middle finger and the thumb are perpendicularly to each other. The

forefinger points along the direction of the magnetic field, middle finger points in the

current direction and the thumb points along the direction of the force.

The strength of the force can be increased by:

1. Increase the current

2. 7sing a stronger magnet

,he Catapult #ieldThe interaction of the t*o magnetic fields produces a resultant field $no*n as catapultfield as sho*n in the figure belo*.

#orce 8etween % Current-Carrying Conductors
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The direction of the force can be determined

by /lemings left hand rule. )ince the current in both sides of the coil flo* in opposite

direction, the forces produced are also in opposite direction. The 2 forces in oppositedirection constitute a couple *hich produces a turning effect to ma$e the coil rotate.

"#amples of electric euipment *hose operation is based on this turning effect are

1. the direct current motor2. the moving coil meter.

)irect Current Motor

(osted by "lectromagnetism 9 :2< ( 9 motor9 < comments =

. "lectric motor converts electricalenergy to $inetic energy. 8iagram above sho*s the structure of a simple direct current

motor 86 motor-. It consist a rectangular coil of *ire placed bet*een 2 permanent

magnets. The coil are soldered to a copper split ring $no*n as commutator. 2 carbonbrushes are held against the commutator.
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$en9s $aw


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http://upload.wikimedia.org/wikipedia/commons/c/c7/Emil_Lenz.jpg


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%hen a magnet is moved into and out of a coil, the induced current that flo*s throu

gh the coil can be determined from EenFs Ea*. EenFs Ea* states that t

induced current al*ays flo*s in the direction that opposes the change in magnetic

flu#.EenFs Ea* obeys the principle of conservation of energy. %or$ is done to move the

magnet against the repulsive fo

e. This *or$ done is converted to electric energy *hich manifests as an induced current

./or a conductor in a closed circuit moving perpendicular to a magnetic field andhence cutting its magneti

u#, the direction of the induced current is determined from /lemings ight&4and ule.

/lemings ight&4and ule is used to determine the direction of the induced current

http:HHyoutube.comH*atchvJs(Ea*6KvLmg

http:HHyoutube.comH*atchvJM86g#N@o'c

)irect Current +enerator

(osted by "lectromagnetism 9 :2O ( 9 generator9< comments =
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Period nd #reuency

The time ta$en for one complete cycle is $no*n as the period, T. The freuency f is

defined as the number of complete cycles in 1 second. The relationship bet*een thefreuency and the period is:

f = \frac{1}{T}

,he e((ective voltage (or a sinusoidal alternating current

The ma#imum potential difference supplied by an a.c source is $no*n as the pea$ voltage

Q(. The effective potential difference for an a.c is eual to the potential difference of aalternating current if both results in the same heating effect. The effective potential

difference for a.c is $no*n as the root mean suare voltage r.m.s- of the a.c. and is given

y the follo*ing euation:V_{rms} = \frac{{V_p }}{{\sqrt 2 }}

The root&mean&suare r.m.s- value of an alternating current is the value of the steady

direct current *hich produces the same po*er in a resistor as the mean po*er produced

by the alternating current. The r.m.s current is the effective value of the alternating

current.

,rans(ormer

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This image is original released underGNU Free Documentation license0

&hat is trans(ormer;Transformer is a device that is used to raise or lo*er do*n the potential difference of an

alternating current.

#unction:The function of a transformer is to increase or decrease the potential difference of an

alternating current supply.

Structure and ,echnical ,erms1. A transformer consist of 3 parts, namely

1. The primary circuit

2. The core

3. The secondary 6ircuit
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This image is released under the 5'7 /ree 8ocumentation Eicense.0

Primary Circuit:

The primary circuit is the circuit that connected to the input energy source. The current,

potential difference and coil *inding- in the primary circuit are called the primary

current Ip-, primary potential difference Qp- and primary coil respectively.

Core:

The core is the ferromagnetic metal *ound by the primary and secondary coil. The

function of the core is to transfer the changing magnetic flu# from the primary coil to the

secondary coil.

Secondary Circuit:

The secondary circuit is the circuit that connected to the output of the transformer. The

current, potential difference and coil *inding- in the secondary circuit are called the
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secondary current Is-, secondary potential difference Qs- and secondary coil

respectively.

&or5ing Principle o( ,rans(ormer

1. A transformer consists of a primary coil and a secondary coil *ound on a soft ironcore.

2. %hen an alternating current flo*s in the primary coil, a changing magnetic flu# is

generated around the primary coil.

3. The changing magnetic flu# is transferred to the secondary coil through the ironcore.

O. The changing magnetic flu# is cut by the secondary coil, hence induces an e.m.f.

in the secondary coil.;. The magnitude of the output voltage can be controlled by the ratio of the number

of primary coil and secondary coil.

,ypes o( Current in ,rans(ormerThe current in the primary circuit must be alternating current because alternating current

can produce changing magnetic flu#. A changing magnetic flu# is needed to induce e.m.f.in secondary coil.

The induced current in secondary is also an alternating current. The freuency of the

alternating current in secondary coil is same as the freuency of the primary current.

The alternating in the secondary circuit can be converted into direct current by using apair of diode.

Sym8ol o( ,rans(ormer

The figure on the left sho*s the symbol of a transformer.

The 2 lines in bet*een the coil denote the core.
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,ypes o( ,rans(ormer

Step-up trans(ormer

A step&up transformer is one *here the e.m.f. in the secondary coil is greater than the

e.m.f. in the primary coil. It is used to increases the potential difference. The number of*indings in the secondary *inding is greater than the number of *indings in the primary

coil. The current in the primary coil is greater than the current in the secondary coil.

Step-down trans(ormer6onversely, a step&do*n transformer is one *here the e.m.f. in the secondary coil is lessthan the e.m.f. in the primary coil. It is used to reduce the potential difference. The

number of *indings in the primary *inding is greater than the number of *indings in the

secondary coil. The current in the primary coil is lesser than the current in the secondarycoil.

Calculation o( Potential )i((erence Change

{{V_s } \over {V_p }} = {{N_s } \over {N_p }}

Qp J input primary- potential difference

Qs J output secondary- potential differenceIp J input primary- currentIs J output secondary- current

Calculation o( Current ChangeIdeal ,rans(ormerV_p \times I_p = V_s \times I_s


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/actors that affect the efficiency

,he heating e((ect o( current in a coil.(o*er is lost as heat I2 *hereby I is the current flo*ing through the coil and is theresistance of the coil

Methods to increase the e((iciency7se thic$ copper *ires of lo* resistance. 7se coolant to decrease the temperature of thetransformer.

*eating e((ect o( induced eddy currentsIn the iron core. %hen the magnetic field in the iron core fluctuates, eddy currents are

generated in the iron core.

Methods to increase the e((iciency

7se a laminated iron core *hereby each layer is insulated *ith enamel paint to preventthe flo* of eddy currents. The high resistance bet*een layers of the iron core decrease

the prevalence of eddy currents and heat.

Magneti9ation o( the iron core.The energy used in the magnetiFation and de&magnetiFation of the iron core each time

current changes its direction is $no*n as hysterisis. This energy is lost as heat *hichsubseuently heats up the iron core.


7se a soft iron core that is easily magnetiFed and de&magnetiFed.

#lu lea5age.)ome of the induced magnetic flu# from the primary coil is not transmitted to the

secondary coil, therefore the e.m.f in the secondary coil is decreased.The secondary coil*indings- are intert*ined tightly *ith the primary coils. The iron core

should form a closed loop.


The secondary coil *indings- is intert*ined tightly *ith the primary coils. The iron core

should form a closed loop.

+eneration o( 'lectricity

(osted by "lectromagnetism 9 :2@ ( 9 electricity generation9 < comments =

!enewa8le and


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energy and the advantages of using rene*able energy.

ene*able energy sources are the energy sources *hich can becontinuously replaced. "#amples of rene*able energy are:

1. 4ydro energy2. )olar energy

3. %ind energyO. 5eothermal energy

;. +iomass>. Tidal energy

dvantages o( Using !enewa8le 'nergy1. 6lean and do not pollute the environment.

2. "asily obtain.

3. 6an be replenished once it has been used.O. 6an prevent energy crisis.

,he dvantages and )isadvantages o( )i((erent ,ypes o( Sources

o( 'nergy

#ossil #uel.

The advantages of using fossil fuel such as petroleum, natural gasand coal as a source of energy to generate electricity are.

1. It is relatively easy to be collected as they are present in largeamount in one place.

2. It contains high energy capacity.

The disadvantages of using fossil fuel as a source of energy are.1. It releases a lot carbon dio#ide *hen it is burned. Earge amount

of carbon dio#ide presence in the atmosphere *ill cause theincrease of the temperature of the atmosphere, $no*n as global
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*arming. 5lobal *arming is the most serious environmentalproblem that *e are facing no*.

2. /ossil fuels are non&rene*able energy.3. (ollute the air. +urning of fossil fuel *ill release particles and

some haFardous gases such as sulphur dio#ide and nitrogendio#ide *hich *ill pollute the air.

*ydroelectric

The advantages of using hydroelectric as a source of energy togenerate electricity are.

1. It is a rene*able energy.2. +uilding a dam does not pollute the environment.

3. In a lot of country, *ater can be easily obtained and is free.

O. +uilding a hydroelectric plant does not involve very hightechnology as nuclear po*er plant.
http://upload.wikimedia.org/wikipedia/commons/d/de/Stwlan.dam.jpg


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The disadvantages of using hydroelectric as a source of energy are.1. +uilding a dam *ill cause a large area flooded *ith *ater, and

hence seriously destroys the ecosystem nearby.2. The flooded area causes the loss of *ild life habitat and

agriculture land.3. 8am failure happens *ill cause a disaster to the lo*er reaches

area of the river.O. The cost to build a dam is very high.


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&ind Power


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This image is shared by

gner 6hristian. This fileis licensed under the 6reative 6ommon

s Attribution )h

1. It is a rene*able energy.

2. It does not pollute the environment.3. It is suitable for mountain area because the *ind speed in

mountain area is relatively high.

The disadvantages of using *ind as a source of energy are.

1. A large area is needed to build the *indmills.

2. The *indmills *ill produce high level of noise.3. The po*er generated is inconsistent due to the inconsistent of

the *ind speed and *ind direction.
http://upload.wikimedia.org/wikipedia/commons/e/e0/Windenergy.jpghttp://en.wikipedia.org/wiki/Creative_Commonshttp://en.wikipedia.org/wiki/Creative_Commonshttp://creativecommons.org/licenses/by-sa/2.5/http://upload.wikimedia.org/wikipedia/commons/e/e0/Windenergy.jpghttp://en.wikipedia.org/wiki/Creative_Commonshttp://en.wikipedia.org/wiki/Creative_Commonshttp://creativecommons.org/licenses/by-sa/2.5/


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Solar power


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The advantages of using s1. It is a rene*able energy.

2. )unlight is free and can be obtained easily.3. It does not pollute the air.

The disadvantages of using solar energy as a source of energy are.1. A large area is needed to install the solar plate.

2. The amount of sunlight at non&tropical area depends on theseason.

3. The energy collected need to be store in a cell so that it can beused at the time sunlight is not present.

O. Its capacity is limited by the capacity of the cell. 'ormally acell has relatively lo* capacity.

,ransmission o( 'lectricity

(osted by "lectromagnetism 9 :2@ ( 9 electricity transmission9 < comments =

%hy the voltage is stepped up before transmitting electricity overlong distance

)tepping up the voltage can reduces the current in the cable. This

reduces the po*er loss in the cable during transmission.

This image is shared by its author Aatu Eiimatta in %i$ipedia Image.This fileis licensed under theCreative

CommonsAttribution ShareAlike 2.5License.0

(roblems Involving "lectricity Transmission.1. (o*er Eoss 8uring Transmission.

2. The high voltage transmission cable is very dangerous.

3. The cost of the cables are high.O. 6harge lea$age may happen bet*een cables and earth.

;. (ylons may be struc$ by lightning.>. (ylons and cables may be struc$ by light aircraft.


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&hat is a national +rid

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