8.1 Em 09

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Sek Men Sains Muzaffar Syah

108

An electromagnet is a magnet made by winding a coil of insulated wireround a soft iron core, so that a magnetic field is produced when acurrent is passed through the coil.

o When a current flows through the coil,it produces a magnetic field.

o The soft iron core becomestemporarily magnetized when thecurrent is switched on.

o When the current is switched off, itloses its magnetism.

A magnetic field pattern can be represented by field lines that show theshape of the field.Magnetic field lines which are close together represent a strong field.The field direction is defined as the direction indicated by a compassneedle placed in the magnetic field.

The Right-Hand Grip RuleGrip the wire using the right hand, with your thumb pointing in thedirection of the current. Your other fingers now point round the wire in

the direction of the magnetic field.When the direction of the current is reversed, the magnetic fielddirection also is reversed.

8.1

What isanelectro-magnet?

What is amagneticfield

pattern?

Determinethe directionof themagneticfield arounda current-

carrying wire

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A c t i v i t y 1

Aim Draw the magnetic field pattern due to a current in a 1.straight wire, 2. coil 3.solenoidDetermine the direction of the magnetic field.

Procedure

1. Use small plotting compasses and place them on the cardboard to determinethe direction of the magnetic field.2. Observe and sketch the pattern produced.3. Reverse the current supply connections to see its effect on the compass

needle.4. Remove the compasses and sprinkle iron filings onto the cardboards and tap

the sheets gently with a pencil. Sketch your observation.

StraightWire

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Co

il

Solenoid

Note:The right hand grip rule for a solenoid:The thumb points towards north pole of themagnetic field while the other fingers

indicate the direction of the current in thesolenoid.

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Discu

ssion

1. What information can be obtained from the plotting compasses?

_____________________________________________________________________

2. What happens to the magnetic field when the supply connections are reversed?

_____________________________________________________________________

3. What difference would you observe if the current in the solenoid is increased?

_____________________________________________________________________

Activity 2: Plan and conduct experiments to study factors that affect the strength of themagnetic field of an electromagnet

Aim To determine the factors affecting the strength of an electromagnet

How to measure the strength of the electromagnet?

Hypothesis State the relationship you will expect between the strength of an electromagnetand

(a) the number of turnson the coil

(b) the size of thecurrent carried bythe coil

(c) the material used forthe core.

Manipulated

variableRespondingvariable

Controlledvariable

Apparatus Iron nail (15 cm long), s.w.g 22 insulated copper wire, dry cell, ammeter (0-5 A), rheostat, glass rod, paper clips,

Procedure1. Wrap an insulated copper wire round an iron

nail 20 times to form a solenoid.2. Set up the iron nail.3. Turn on the switch. Count the number of

paper clips attracted by the iron nail.

4. Repeat the experiment with 30, 40 and 50turns of coppers wire round the iron nail.

5. Use a 50 turns solenoid and adjust the currentby adjusting the resistance of the rheostat.Record the number of paper clips attracted bythe electromagnet for different current flowingin the solenoid.

6. Keeping both the number of turns and the current constant, replace the iron nail with a glassrod. Repeat the measurement.

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Tabulate your readings

ConclusionState the factors that affect the strength of the electromagnet.

____________________________________________________________________________

____________________________________________________________________________

____________________________________________________________________________

D e s c r i b e a p p l i c a ti o n o f E l e c t r o m a g n e t

Electric Bell

When the bell push is pressed, acurrent flows in the coils of theelectromagnet, causing it to bemagnetized.

The magnetized electromagnetattracts the soft-iron armature,causing the hammer to strike thegong.

The movement of the armaturebreaks the contact and causes theelectromagnet to lose it magnetism.

The light spring pulls the armatureback, remaking the contact andcompleting the circuit again.

The cycle is repeated so long as thebell push is pressed and continuous

ringing occurs.

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Magnetic Relay

A solenoid switch worked by anelectromagnet. It is used in a circuitwhich controls the operation ofanother circuit, especially if thecurrent is large in the second circuit.

Circuit 1 requires only a smallcurrent.

When the switch is closed, currentflows in the coil, causing the soft-ironcore to be magnetized and attractsthe amarture

The movement of the iron armaturecloses the contacts in the secondcircuit. Circuit 2 is now switched on.

Circuit 2 may have a large currentflowing through it to operatepowerful motors or very bright lights.

The advantage of using a relay is that a smallcurrent (circuit 1) can be used to switch on andoff a circuit with a large current (circuit 2).

This is useful for two reasons:(b) circuit 1 may contain a component such as

a light detecting resistor (LDR), which uses

small currents.(c) Only the circuit with a large current needs

to be connected with thick wire.

Telephone Ear-piece

The varying current from themicrophone flows through the coilsof an electromagnet in the earpiece.

This pulls the iron diaphragmtowards the electromagnet by adistance which depends on thecurrent.

As a result, the diaphragm moves inand out and produces sound wavesthat are replicas of those that enteredthe microphone.

Circuit Breaker

Acts as an automatic switch thatbreaks open a circuit when thecurrent becomes too large.

In a household circuit, the currentmay become excessive when there isa short circuit or an overload.

The strength of the magnetic field of

the electromagnet increasessuddenly.

The soft iron armature is pulledtowards the electromagnet.

This results in the spring pullingapart the contacts. The circuit isbroken and the current flow stopsimmediately.

After repairs have been made, the reset buttonis pushed to switch on the supply again.

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TUTORIAL 8.1

1. Diagram shows a compass is placedhorizontally on a piece of cardboard andit is initially points north.

When the current is switched on, thecompass needle will..A. move to the west

B. move to the eastC. remain to the northD. vibrate at its original position.

2. Diagram below shows two compasses Mand N are placed surrounding a straightwire carrying a current.

What will happen to the compassesneedle when switch S is turned on?A. Both compasses will deflect 90 in an

opposite direction.B. Both compasses will deflect 90 in the

same direction.C. Both compasses will deflect 180D. Compass M will deflect 180 while

compass N will remain at the sameposition.

4. Diagram shows a magnetic field patternwhich is produced from an experiment.The magnetic field pattern is due to ..

A. coilB. straight wireC. solenoidD. magnet bar

5. Diagram below shows a solenoidcarrying current.

Which of the following shows the correctpattern and direction of the magneticfield produced?

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6. The diagram shows an electromagnetPQR.

What is the magnetic polarity at P, Q andR?

P Q R

A North South South

B South North North

C South North South

D North South North

7. Diagram shows a copper wire coiledround a U-shaped soft iron core. The

copper wire is connected to a battery.

When the switch is turned on, themagnetic pole of X, Y and Z are..

X Y Z

A North North South

B North South North

C South North North

D South North South

8. Diagram shows two iron core are coiledround with a wire.

What happen to A and B when currentflows?

A. A and B are stationaryB. A and B attract to each otherC. A and B repel at each otherD. A and B repel first, then attract.

9. The diagram shows two iron nails arehung freely in a solenoid.

When current flows through the solenoid,the nails willA. be at their original positionB. attract to each otherC. repel at each otherD. oscillate with a constant frequency

10. Which of the following shows the correctpolarity of the electromagnet?

11. Soft iron is used as the core of anelectromagnet becauseA. it is a good conductor of electricityB. the melting point of iron is very highC. it loses all its magnetism when the

current is switched offD. it retains all its magnetism, when the

current is switched off.

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12. Diagram 22 shows the apparatus used tostudy the effect of an electromagnet.

Which change will increase the numberof paper clips attracted to the iron rod?(2008)A. Use a thinner wire to form the coilsB. Use a non-insulated wire to form the

coilsC. Reduce the number of coilsD. Increase the magnitude of the

current.

13. The diagram shows an electromagnet.

Which factor does not influence thestrength of the electromagnet? (2005)A. Material used for the wireB. Material used for insulator of the wireC. Number of turns of wire

D. Current flowing in the wire

14. The diagram shows an experiment tostudy the suitable type of core for anelectromagnet. 4 different metal rods isput inside copper coils.

Table below shows the number of paperclips attracted to the metal rod whenswitch is turned on and then turned off.

Metalrod

Number of paper clipsattracted to the metal rod

Switchturn on

Switchturn off

A 40 35

B 38 5

C 25 4D 1 0

Which of the metal rods A, B, C and D issuitable to be used as the core of anelectromagnet?

Structured Questions1. The diagram shows 4 compasses are

placed on a horizontal cardboard. Astraight wire passes through the centreof the cardboard. When the current isswitched off, all the compasses needles

point north.

(a) On the diagram below, draw arrowsto show the direction of the needleswhen the current flows through the

straight wire into the cardboard.

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(b) Current passes through a coil PQ asshown below. The circles representthe compasses.

(ii) Draw arrows in the circle to show thedirection of the compass.

(iii) The coils PQ act as a magnet. State the

magnetic polarity at :P : Q :

(c) On the diagram below, sketch magneticfield line around and inside the coilsPQ. Show the direction of the magneticfield.

2. Diagram 2.1 shows a simpleelectromagnet used for lifting andreleasing a small matter ball.

(a) In Diagram 2.1, mark the direction ofthe current flow in the solenoid when

the switch is on. [1 mark]

(b) (i) Name the magnetic pole at theend of the core P when the switchis on.

[1 mark]

(ii) State the rule used to determinethe pole of the magnetic field.

.[1 mark]

(iii) Based on the solenoid inDiagram 2.1, draw the magneticfield lines that is formed aroundthe solenoid when the switch ison. In your diagram shw thedirection of the magnetic fieldlines.

[2 marks]

Material usedfor the core

Shape of the core

Steel

Soft iron

Aluminium

(c) Based on Table 2, state the suitableproperties to be chosen to built anelectromagnet and state the reasonfor your choice.

(i) Material used for the core.

[1 mark]

(ii) Reason

[1 mark]

(iii) Shape of the core

[1 mark]

(iv) Reason

[1 mark]

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(d) The electromagnet in 2(c) is used inan electric bell as shown in Diagram2.2.

Explain the working principle of the

electric bell.

[3 marks]

(e) State 2 ways to increase theloudness of the sound of the electricbell.

________________________________

3. The structure of a relay and its relatedconnections is shown is diagram below.

(i) What are the parts X, Y and Z shownin the diagram?X ____________________________

Y ____________________________

Z ____________________________

(ii) The relay coil is wound on a metalcore M. State a suitable material forthis metal core M.

_______________________________

(iii) Explain why this material is suitableto be used.

_______________________________

(iv) Give a reasons why the high voltageis switched in this way rather than byinserting a switch directly in the high

voltage circuit._______________________________

_______________________________

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Soalan SPM 20064. Diagram (a) and Diagram(b) show wire

coils connected to the ammeters,switches and d.c. power supply.

When the switch is on and iron filings ofthe same amount is spread on each ofthe cardboard surface, the pattern of theiron filings is formed as shown in thediagrams.(a) What is meant by magnetic field?(b) Using Diagram(a) and Diagram(b) ,

compare the number of turn of thecoils, the pattern of the iron filingsand the angle of deflection of theammeter indicator.

(c) State the relationship between thestrength of the magnetic field and(i) the pattern of iron filings,(ii) the number of turn of thecoils.

8.1 Em 09

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