Chapter 8 Electromagnet Teacher's Guide 2009

JPN Pahang Teacher ’s Guide

Physics Module Form 5 Chapter 8: Electromagnetism Electromagne tism

CHAPTER 8: ELECTROMAGNETIS ELECTROMAGNETISM M

8.1: MAGNETIC EFFECT OF A CURRENT-CARRYING CONDUCTOR

Electromagnets current 1. Conductor Conductor is is a material material that that can can flow flow ……………… …………………….. …….. electricity 2. Electromagnet Electromagnetism ism is the the study of of the relations relationship hip between between ……………….a ……………….and nd ………… magnetism …….. Electric fields 3. …………………… ………………………. …. are are produced produced by electricall electrically y charged charged objects objects.. magnetic fields 4. A bar magnet produces the ………………………around it. Draw the pattern of the

magnetic fields produced around a bar magnet below 5. Show the deflect deflection ion of of the the pointer pointer of of the the compasses compasses..

N

S

6. Magnetic Magnetic fields fields also also can be produce produce by an electric electric current current in in a wire. wire. temporary 7. Electromagnet Electromagnet is is a ………………… ………………… magnet. magnet. It is is made by winding winding a coil coil of insulated insulated wire round a soft iron core.

1



8. Using the diagram, diagram, complete complete the steps steps to switch switch on on the magnetis magnetism m effect. effect.

Soft iron core

Soft iron core

switch

switch Insulated wire

DC supply

DC supply

Insulated wire

pins

Switch is closed

Current flows

pins

Electroma gnetic filed produced

Soft iron core is magnetise

Attracts pins

Magnetic Field due to a Current in Straight Wire

1. Using Right-hand Grip Rule , draw the direction of current flows and pattern of magnetic fields formed

Direction of magnetic fields

2. Right-hand Grip Rule states that the thumb of the right hand points to the direction of magnetic fields current flow and the other four curled fingers points to direction of its ………………….

2



3. Draw the the same patterns patterns from from the top views views as follow followss together together with with the direction direction shown shown by the compasses.

into Means that a wire carrying current ………….. the plane o f paper out Means that a wire carrying current …………... of the plane of paper

Exercise 8.1

1. Two Two small plotting compasses P and Q are placed near a long straight wire carrying a current as shown. P

Q

Show the directions shown by the pointers of the compasses

3



4. In the the diagra diagram m below below,, the compa compasse ssess are put put under the conductor. Can you draw the direction shown by compasses and what happen if the compasses are put on the conductor?

The compass is put UNDER the conductor

The The compass is put UNDER the conductor

Magnetic Field due to a Current in a Circular Coil

5. Draw the the direction direction of current current flows flows and pattern pattern of magnetic magnetic fields fields formed. formed. 6. The direction direction of magnetic magnetic field also can can be determi determined ned using using the Right–Hand Grip Rule.

* Notes: The field strength is the strongest at the center area because combination from two sources

4



Magnetic Field due to a Current in a Solenoid

7. Solenoid is combination of coils of wire wounded around on some surface or wounded around an iron core. 8. Draw the the direction direction of current current flows, flows, pattern pattern of magneti magneticc fields fields formed formed and direction direction by the compasses. 9. To check the poles poles formed, formed, see from from beside beside of the solenoi solenoid. d. If the current current flow flow is South clockwise , the polarity on that side is …………………… but, if the current flow is North anti-clockwise , the polarity on that side is ……………………

S

N

Factors that affects the strength of an electromagnet

10. To study this, we need to carry out experiments as follows. The higher the number of paper greater clips attracted to the solenoid shows the …………..….the strength of that electromagnet.

Cell Rheostat

Iron core Paper clips

5


Manipulated Number of turns Current Types of core


Responding Number of paper clips attracted Numb Number er of paper paper clip clipss att attra ract cted ed Number of paper clips attracted

Fixed Current, types of core Numb Number er of turn turns, s, type typess of of cor coree Number of turns, Current

Factors that affects the strength of an electromagnet

11. As a conclusion, conclusion, fill the the table below below Factors

Conditions

Number of turns Electric current Use of normal iron-core Use of soft-iron core

Number of paper clips

Affects on the strength of

attracted

electromagnetic fields

-nil-

Applications of Electromagnet

12. There are many applications of electromagnets. Label and describe the figure figure of each apparatus stated in the table below with its simple work-frame. 1) Electric bells

batteries

switch

1. When the switch is closed, the current flow will magnetized the soft-iron core

springs

2. The soft iron armature is pulled toward electromagnet and hit the gong 3. At 3. At the same time, the contact will open and stop the current flows. No electromagnet.

soft-iron core

contact

soft-iron armature

gong

4. The spring mechanism brings the armature back to its original position 1. When the switch A is closed, a small 5. The contacts close again and similar current flows in input circuit will process is repeated. magnetized the soft-iron so ft-iron core 2. The soft iron armature is attracted toward electromagnet

2) Electromagnetic Relays

3. At 3. At the same time, the spring contact closed 4. A 4. A large current flows to high voltage 6

circuit to operate the electrical apparatus.


soft-iron core


to high voltage circuit

3) Telephone earpieces//speakers

1. A 1. A varying current received from the Alloy diaphragm

caller in telephone line 2. The varying current passes through solenoid and magnetized the soft-iron core 3. The electromagnet varies in magnetic

Soft-iron core

Permanent magnet solenoid

strength according to the verifying current 4. The alloy diaphragm will attract to Its operate as ………………………… electromagnet by varying force ………………………………………… 5. Sound produced as compression and ………………………………………… rarefaction of air particles 1. When the current becomes high (ex: short circuit) the electromagnet strength increase suddenly

4) Circuit breakers

2. The iron catch is pulled toward automatic switch to breaks electromagnet circuit to open when current becomes 3. The spring pulling apart the contacts 7 too large 4. The circuit will break and the current flows stop immediately


spring


contacts

to household appliances

from main supply

5) Lifting Magnet

move heavy objects or steel Its function to ………………………… … from place to place ……………………………………… 1. Opposite poles on the surface will produces a stronger magnetic field 2. When the current is switched on, the soft iron core is magnetized to become a very powerful magnet North Pole

3. Its 3. Its lifts up iron and steel 4. When the crane has moved to new location, the current is switched off and

South Pole

the soft-iron core is demagnetized 3. The objects will fall

Exercise 8.1.2

8



1. A magnetic field is produced by the current in a long straight wire. The strength of the magnetic field increases if. A

The ma magni gnitude ude of the current ent incre creases

B

The di distance fr from th the wi wire in increases

C

The le length of of the wi wire increases

2. Which of the following is true about the magnetic field produced by a current in a flat circular coil?. A

It is uniform inside the coil

B

It is the weakest at the centre

C

It is the strongest at the centre

D

There is is no no mag magn neti etic fi field ou outside the coil

3. What is the effect of placing a piece of soft iron core inside a solenoid carrying current? A

The re resistance nce of of th the so soleno enoid de decre creases

B

The current in the sole olenoi noid increas eases

C

The ma magnetic fi field be becomes st stronger

4. An electromagnet is used in these apparatus except A

a compass

B

an a.c dynamo

C

a moving coil ammeter

D

a telephone earpiece

5. Which electromagnet is the strongest? A

B

+2V-

C

+2V-

D

+4V-

9

+4V-



8.2: Force Acting on a Current-carrying Conductor in a Magnetic Fields

1. If a current-carry current-carrying ing conductor conductor is placed placed in a magnetic fields fields as shown shown in the experiment, experiment, the the force conductor will experiences a …………………..

Magne t

CShape

Power Supply

Freely conductor Long conductor

2. Draw Draw the the comb combin inat atio ion n (called (called catapult field ) of two electromagnetic fields below and show the direction of movement of the conductor.

N

S

N

+

S

=

3. Fleming’s left-hand rule can be use to determine the direction of the force acting or the motion of the conductor.

First Finger ( Field ) Thumb

( Thrust force ) N

Centre Finger ( Current )

10



Effect of Two Parallel Current-carrying Coils in a Magnetic Field

4. If two current-c current-carry arrying ing conductor conductor is placed in a magnetic magnetic fields fields as shown shown in the experiment, experiment, repel attract the conductor will experience ………………… or ………………….. force between them. 5. Complet Completee the diagram diagram for curr current ent with with opposite direction flows in two parallel conductor below:

repel The two conductor will ……………….. to each other

6. Complet Completee the diagram diagram for curr current ent with with same direction flows in two parallel conductor below:

11



attract The two conductor will ……………….. to each other Turning Effect of a Current-carrying Coil in a Magnetic Field

7. Consider Consider a current-ca current-carryi rrying ng coil ABCD placed placed between between the poles poles magnet as as shown in in the figure below. upward 8. As the current flows through through the coil coil from A to D, an …. ….…………….. (1) force acts on the downward arm CD whereas a ……………………… (3) force acts on the arms AB according to Fleming’s Left Hand rule.

Coil

N

S

1

B

4

Carbon brush

A

C

2 3

Magnet D

current

commutator

Electrical energy

Kinetic energy

9. Draw the the catapult catapult filed formed formed and draw draw arrows arrows to show show the movement movement of arms arms AB and CD.

AB

CD F

N

S

12



F Direct Current Motor

10. Complete the four stages of the the motion of DC motor below

N

Coil 00 Degree : ………………………… Through carbon brushes Downwards Current flow : Yes / No

S

1

3

2

Upwards …………….………………….. Anti-clockwise Arm AB : ……………………….

current

Arm CD : ………………………… Rotation : ………………………… …………………………..

N

900 Degree : ………………………… Circuit broken at carbon brushes right Current flow : Yes / No

Coil S

left …..…………….………………….. Anti-clockwise due to inertia Arm AB : ………………………. current

Arm CD : ………………………… Rotation : ……………

N

1800 through brushes Degree carbon : ………………………… Upward Current flow : Yes / No downward …..…………….………………….. Anti-clockwise Arm AB : ……………………….

Coil S

current

……….

13

Arm CD : ………………………… Rotation : ……………



N

Coil S

2700 Degree : ………………………… Circuit broken at carbon brushes left Current flow : Yes / No / No right …..…………….………………….. Anti-clockwise due to inertia Arm AB : ……………………….

current

Arm CD : ………………………… Rotation : ……………

Comparison with Alternating Current Motor

11. Complete Complete the diagram diagram of AC motor below:

14

……….



a.c supply

Factors that affect the speed of rotation of an electric motor

12. The magnitude of force force acting on a conductor in a magnetic magnetic increases by: i.

Incr Increas easin ing g the the curre current nt flow flow

ii. ii. Stro Strong nger er mag magnet net used used 13. In conclusion, the speed of rotation rotation of the coil also can increased increased by: Increasing the current i. ………………………………… Using a stronger magnet ii. ii. ………… ……………… ………… ………… ………… ……… … Increasing the number of turns on the coils iii. …………….…… …………….……………… …………………… ………………… ………

Moving Coil Ammeter

15



The angle of deflection is directly proportional to the current flows in the coil The hair spring will restore the pointer back to its original position. The force acting causes the coil to rotate and lead the pointer to the deflection

When current flows in moving coil, magnetic field of radial magnet will interacts with magnetic field produce by the coil

Exercise 8.2

1. The motion motion of a conduct conductor or carr carryi ying ng current in an magnetic fields can be determine by A

Right hand Grip Rule

A current carrying carrying wire is in between the poles

B

Fleming’s Right Hand Rule

of a magnet. The direction of the force on the

C

Fleming’s Left Hand Rule

wire is

D

Direction of current flow

A

2.

N

N

S

16

North

B

East

South

D

West

C



3. Two paral parallel lel wire wiress are carr carryi ying ng equal equal

A

the hair spring is harder

currents in opposite directions. Which

B

the magnet is stronger

diagram shows the forces F on the

C

the scale is shorter

wires? A

B

6. Choose Choose the best best patt pattern ern formed formed.. F

F

F

C

F D

F

F

F

F

4. The functi function on of of the the comm commutat utator or in in a

F

F

d.c. motor is A

to re reverse th the cu current in in th the co coil at at

every half rotation B

to en enable th the co coil to to be be in in el electrical

contact with carbon brushes C

to pr prevent th the wi wires fr from en entangling

5. A moving moving coil coil-am -ammet meter er is is less less sensitive if 8.3: Electromagnetic Induction solenoid 1. As we know, know, a steady magnetic magnetic field field can be be produced by a …………… …………… or ………………… ………………… wire carrying electric current , permanent magnet. ……………….

2. When we move move a coil without without any current current flow is is in a magnetic magnetic field, an induced e.m.f (electromotive force) is produced. induced current 3. An ………………… …………………………… …………… … flows flows through through the conductor conductor and this phenomenon phenomenon called called electromagnetic induction ………………………………… 4. So, elec electro tromag magnet netic ic induct induction ion requi requires res relative motion between magnet and the coil to produce an induced current.

17



5. Complete Complete the table table below below as experime experiment nt data for the the diagram diagram below: below:

Induced e.m.f by a moving conductor

G

Action The The wir wiree is is move moved d upw upwar ards ds The wire is moved downwards The wire is move horizontally Magnet is moved upwards

Obseravtion Gal Galvano vanome mete terr def deflect lect to left eft Galv Ga lvan anom omet eter er de defl flec ectt to ri righ ghtt No deflection Galv Ga lvan anom omet eter er de defl flec ectt to ri righ ghtt

Inference Current flows in wire Curr Cu rren entt flo flows ws is re reve vers rsed ed No current flows Curr Cu rren entt flo flows ws is re reve vers rsed ed

Right-hand -hand rule can be use to determine the direction of the induction current 6. Fleming’s Right

produced. Thumb ( motion )

First finger ( Field )

Center finger ( induced current )

18



Induced e.m.f by coil

No deflection on the galvanometer No current is induced

Magnetic field lines are being cut. Current induced

Moving the coil towards a magnet also induces current Current induced in opposite direction

Lenz’s Law

7. Lenz’s Lenz’s Law also also can be use to determin determinee the direction direction of induced induced current current produced. produced. 8.

Lenz’s Law states that the direction of the induced e.m.f is such that its magnetic effects oppose always …………………….the change producing.

19


When the N pole is moved towards the coil, end of coil becomes N pole.


When the N pole moved away from the coil, end of coil becomes S pole.

Faraday’s Law of electromagnetic induction directly proportional 9. Faraday’ Faraday’ss Law states states that the magnitude magnitude of the induced induced e.m.f. e.m.f. is …………………… ……………………………. ………...

to the rate of change of magnetic flux experienced by b y the conductor. 10. The magnitude magnitude of the e.m.f in in a wire increases when: The wire is moved faster i. ……… ………………… ……………… ………… ……… ……… A stronger magnet is used ii. ii. ………… ……………… ………… ………… ………… ………… …….. The length of wire in e.m field is increased iii. ………………..… ………………..…………… …………………… ………………. ……. 11. The magnitude magnitude of the the e.m.f in in a coil/solenoid increases when: The relative motion between magnet and coil is increased i. ………… ……………… ………… ………… …….… .……… ………… ………… ………… ………… ………… ……… … The number of turns on coil is increased ii. ……………… ……………………… ……………… ……………… ……………… ……………… ……….. The cross-sectional area of the coil is increased iii. ……….…………… ……….………………..…… …..……………… …………………… ……………. ….

Applications of electromagnetic induction

20



12. A generator is basically the inverse of a motor. There are many coils of wire wound that can falling water rotate in a magnetic field. The axle is turned by some mechanical such as ………………….., steam ………………………. or wind turbine. 13. The dc generator and ac generator make use of electromagnetic induction to produce ……… output voltage ………………

Handle to rotate

DC Generator

Split rings commutator

21



14. Draw the graph of output current from the dc generator above.

When coils is at its horizontal position 900 ………………… 2700 ...........................

When coils is at its vertical position 00 1800 ……………… …………….. 3600 ………………

22

•

Change of rates of magnetic flux is maximum

•

Induced e.m.f is maximum

•

No changes of magnetic flux

•

No induced e.m.f is induced



AC Generator

15. Draw the graph of output current from the ac generator above

16. The magnitude of the output voltage/current increases when: Number of turns of the coil is increased iv. iv. …………………. ………………….………… …………………… …………………… ……………… …… The strength of the permanent magnet is increased v. ……………… ……………………… ……………… ……………… ……………… ……………… ……….. The speed of rotation is increased vi. ……….…………… ……….………………..…… …..……………… …………………… ……………. ….

23

Handle

When Nocoils changes is at of itsmagnetic vertical position flux • ……………… …………….. No e.m.f induced • JPis NP ahang Teacher ’s Guide ………………


When coils is at its horizontal position 900 ………………… 2700 ...........................

When coils is at its vertical position 00 1800 ……………… …………….. 3600 ………………

24

•

Change of rates of magnetic flux is maximum

•

Induced e.m.f is maximum (+ve /-ve)

•

No changes of magnetic flux

•

No induced e.m.f is induced



Alternating and direct current (a.c/d.c)

17. Complete the table of comparison below. below. AC Current

DC Current

Graphs Current, I/A

Current, I/A Time, t/s

Time, t/s

Current, I/A

Current, I/A

Time, t/s

Time, t/s

Direction variable 1. ac generator

constant

Examples of sources and symbols 1. dc generator

2. dynamo

2. dry cells // batteries

3. home plugs

3. electrolyte cells

+

25

-



Exercise 8.3

1. Which Which of the follow following ing is an an exampl examplee of induced magnetism? A. A compa compass ss needle needle pointi pointing ng north north B. A north pole attracting attracting iron fillings fillings C. A nort north h pole repel repellin ling g a north pole pole D. A coil turning turning in magnetic magnetic field field 3. 2.

The diagram shows a coil in magnetic field. If we want to make a a.c generator,

4. Which Which of the follow following ing repres represent ent the

what should to be con nected to X and Y?

output voltage of an a.c generator?

A. d.c. d.c. supp supply ly

A.

B. Slip Slipss ring ringss C. Soft Soft-i -iro ron n core core D. Split Split rings rings commuta commutator tor B.

C.

26



D.

8.4: Transformers 240 V 1. In Malaysia, Malaysia, our our electricit electricity y for domestic domestic supplie supplied d at a voltage voltage of ……………… ……………… a.c. a.c.

2. However, However, most most of home home appliances appliances at home home use lower lower than or higher higher than 240V 240V.. laptops televisions 3. Transfor Transformers mers are are found in in many devices devices such such as ………………., ………………., …………… …………………., ……., ……… ……… mobile phones ………….. and etc. larger small 4. The main use of transfo transformer rmer is to to convert ……………… ……………………. ……. AC Voltage Voltage to …………… …………….. one or vice-versa.

Operating principle of a transformer

5. Compl Complet etee the the diag diagram ram below below Laminated soft-iron core INPUT

OUTPUT

Primary coil

Secondary coil

Symbol of transformer magnetic field 6. When an alternati alternating ng current current flows flows in primary primary coil, coil, and ………………… …………………………. ………... is produced produced

in the soft iron core. secondary 7. Magnetic Magnetic flux linkage linkage to …………… ……………………. ………. coil coil and will will cut the magnetic magnetic fields fields lines. lines. current

27



8. The secondary secondary coil coil experiences experiences the the rate of change change of magnetic magnetic field field and …………… …………………. ……. is induced in secondary coil. (induced (induced e.m.f ) collapses 9. When the current current in in primary primary coil decreas decreases, es, the magnetic magnetic field field will will ………………… ………………… and and

again cut the secondary coil. 10. An e.mf acting in the opposite opposite direction is induced in the secondary secondary coil. alternating e.m.f 11. Hence, an ……………………….. ……………………….. of the same frequency is induced in the secondary coil. ratio 12. The output voltage is depends on the ………………. ………………. of number of turns of primary and secondary coils. Step-up and step-down transformers

13. The relationsh relationship ip between the the voltages and the ratio of the turns in primary and secondary coils can be write as below. Primary voltage Secondary voltage Vp Vs

=

Number of turns in primary coil Number of turns in secondary coil

=

Np Ns

OR

Vp Np

=

Vs Ns

greater 14. If Ns f Ns is greater than Np than Np,, then Vs is ………………… than Vp. Vp. The type of transformer is … step-up transformer ………………………. less 15. If Ns f Ns is less than Np than Np,, then Vs is ………………… than Vp. Vp. The type of transformer is ……… step-down transformer …………………. 50 16. For example, if the turns turns ratio is 1:50, the output voltage is stepped up ……….. ……….. times.

Step-down transformer

Step-up transformer

17. If we we consider consider an ideal transformer , there is no loss of energy.

28



Power supplied to the primary coil VpIp

Is Ip

=

Power used in the secondary coil

=

VsIs

=

Np Ns

Is Ip

=

Vp Vs

Comparing with the transformer equation

Energy losses in a transformer 100% 18. As we know, know, an ideal transformer has ………….. effiency. less than 19. But in practice, the efficiency efficiency of a tranformer is …………….. 100%.

20. The effiency of a transformer is expressed expressed as follows:

 



Pout Pin VsIs VpIp





100%

100%

21. Complete Complete the table table below: below: Type of losses Eddy current

•

Causes Changing of magnetic field induced

•

Way to reduce Use laminated core

•

Use low resistance copper

current in soft iron core •

Heat loss

•

Eddy current produced and generate heat As the number of turn increases, the

resistance of conductor also increases. •

/aluminium wires

Heat produced by electrical energy to oppose the resistance

Hysterisis

•

The core is magnetized and demagnetized

•

alternately due to a.c current in primary

•

coil

Use soft-iron core It is able to be magnetized and demagnetized easily

29


Physics Module Form 5 Chapter 8: Electromagnetism Electromagne tism •

Flux leakage

•

Energy loss as heat

Leakage of magnetic flux in the primary

•

Proper core design

coil

Exercise 8.4

number of turns in the primary coil and secondary coil are N1 are N1 and N2 and N2.. Which

1. Soft Soft iro iron n core core is use used d as the the transformer core because

has the highest output voltage?

A. Soft iron core has low resistance resistance

N1 A B C D

B. Soft Soft iron iron becomes becomes a perman permanent ent magnet magnet C. Soft Soft iron iron is easily easily magn magneti etized zed and and demagnetized

12000 60 6000 1200

N2 24000 1200 2000 200

D. Soft iron is is better better conductor conductor than steel steel 4. The numb number er of of turns turns betwee between n each each pair pair

2. Whic Which h of the the foll follow owin ing g is correct

of output terminals of a transformer is

about step-up transformer

shown in the diagram. Between which

A. The output output voltage voltage is higher than input input

terminal the output is 12 V?

voltage B. the outpu outputt power power is great greater er than than input input power C. The outpu outputt current current is great greater er than than the input current

A. P and Q 3. All the four four trans transfor former mer given given below below

B. Q and R

have the same input voltage. The

30



C. R and S

C. 6.67 A

D. P and R

D. 66.7 A

5. When When prim primary ary coil coil of an ideal

6. A transf transform ormer er only only can work work with with

transformer is connected to 12 V a.c

A. d.c d.c sup suppl ply y

supply, supply, the current is 2.0 A. If the

B. a.c a.c suppl upply y

output voltage is 400 V, V, what is the

C. dry dry cel cellls

current in secondary coil?

D. elect electro roly lyte te cells cells

A. 0.06 A B. 0.60 A 8.5: Generation and Transmission of Electricity Sources of Energy

1. The generation generation of of electrici electricity ty comes comes from many sources sources such as : Hydro a. ………………………… Gases b. b. ………… ……………… ………… ………. …... .... Nuclear c. ………………………… Diesel d. ………………………… Coal e. ………………………… Biomass f. ………………………… Solar g. ………………………… Wind h. …………………………

Transmission of Electricity

d.c 2. Electricity Electricity can be supply either from ………….. ………….. or a.c or a.c source.

3. The circuit circuit diagram diagram below shows shows a sample of the the transmission transmission of electri electricity city from from ………… d.c … source. Draw the current flow on the circuit diagram.

bulb

31



a.c 4. The diagram diagram below shows shows a sample sample of the the transmissi transmission on of electricity electricity from …………… ……………

source.

National Grid Network pylons 5. National National Grid Grid Network Network is a network network of undergro underground und cables or …………… …………… which which connect all all

the power stations and substations for the whole country co untry.. 6. This network network starts starts at electrical electrical power power plant like like Chenderoh Chenderoh Lake Power Stati Station on which is hydroelectric power station, and end at our o ur houses. 7. Complete Complete the diagram diagram below below by showing showing the cables cables of transm transmissi ission on and suitable suitable users. users.

transmission

400 kV

132 kV 33 kV Heavy industr

11 kV

450 V

Light industr

My house Buildin s

Step-up transfor

Stepdown

Step-down transformers

33 kV 32 Power plant

240 V



8. Electrical Electrical energy energy is transmi transmitted tted from from the power power station station to the consume consumerr using long long transmission cables. heat 9. This will will bring to power power loss as ………… ………… energy energy.. Power loss loss can be calculate calculate as follow: follow:

P heat



2

I R

I = current flows in in the cable R = resistance of the cable

10. The power loss loss can reduce reduce by: resistance i. Reduc Reducin ing g the the ………… ……………… ………… ……… … of the the cabl cables es increasing ii. Reducing Reducing the current current or …………………… …………………….. the voltage voltage in in the cable. cable.

Renewable energy

11. Energy plays a very important role in economic development but the reserves of fossil fuels such as oil and gas are very limited. 12. Hence, there is modern trend of the nations that is to harness the renewable energy . 13. Renewable energy sources are continually continually replenished naturally means they are sustainable. 14. Give the example example of renewable energy: Hydroelectric i. …………………………… Solar ii. ii. ………… ……………… ………… ………… ……… … Wind iii. iii. ……………… ……………………… …………… …… Geothermal iv. iv. ………… ……………… ………… ………… ……… … Biomass v. ……… ………………… ……………… ………… …… Wave vi. vi. ………… ……………… ………… ………… ……… … Tidal vii. …………………… …………………………… ………

15. Give the example example of non-renewable energy: Fossil fuel such as: Oil Gas Coal

33


i.


…………………….. a) ………………….. b) …………………. c) …………………..

16. Give the benefits of using the renewable energy to to our nation: Avoid depletion of fossil fuels i. ………… ……………… ………… ………… ………… ………… ………… ………… ………… ………… ………… ………… ……… … Cleaner sources for little pollution ii. ……………… ……………………… ……………… ……………… ……………… ……………… ……………… …………… ……… … Avoid harming flora and fauna iii. …………………… ……………………………… …………………… …………………… …………………… ………………… ……… Avoid the disruption of ecological balance iv. iv. …………………… ……………………………… …………………… …………………… …………………… ………………… ………

- End of Chapter 8 -

Exercise 8.5

1. Whic Which h of the the foll follow owin ing g is not a

A. Its availabl availablee all the time time

renewable energy source?

B. Its Its eas easy y to to use use

A. Wind

C. Its Its che cheap ap to use use

B. Solar

D. It’s It’s a renewa renewable ble ener energy gy

C. Coal D. Bioma omass

4. Whic Which h of the the fol follo lowi wing ng is is an advantage of National Grid Network?

2. The major major source source of energy energy for

A. Electr Electrica icall power power breakdo breakdown wn can be

electrical generation in Malaysia is

countermeasure immediately

A. Natu Natura rall gas gas

B. Loss Loss of power power can can be be reduc reducee

B. Wind

C. Its Its usin using g less less cabl cables es

C. hydro

D. It’s It’s a renewa renewable ble ener energy gy

3. Whic Which h of the the fol follo lowi wing ng is is an advantage of using wind energy for generation of electricity?

34



5. In the the transmi transmissi ssion on of electr electrici icity ty,, the

A. 1:22

most effective mean to reduce power

B. 22:1

loss in the transmission cable is by

C. 1:220

A. Usin Using g coppe copperr cabl cables es

D. 220:1

B. Transm Transmis issio sion n at high high voltag voltagee C. using using alte alternat rnating ing curren currentt

7. If the the resis resistan tance ce of 1 km of the cabl cablee is

D. Usin Using g thic thick k cabl cables es

2 Ω, what is the power loss from each km length of cable?

For question 6 and 7

A. 2.00 W

6. A power power stat station ion generat generates es 500 500 kW of

B. 14.34 W

power at 600 V. V. An ideal transformer

C. 26.54 W

steps up the voltage to 132 kV for

D. 28.69 W

transmission. What is the ratio of the number of turns in the secondary coil of the transformer to that in primary coil? Reinforcement Chapter 8 Part A: Objectives Questions

1. Which Which diagr diagram am best best show showss the patter pattern n of field lines around a bar magnet?

D.

A.

B.

2. Which Which mater material ialss are are suitab suitable le to to make make the core of an electromagnet? A

C.

35

Core of an electromagnet Iron


B C D


Steel Wood Cuprum

wire

cardboard B.

W N E compass

3. Refer Refer to to the the diagr diagram am above above,, when when the the C.

switch is close, the pointer of the compass will point to A. Stay Stay at Nort North h B. East C. West D. Vibrate ibrate at its its place place

D. . 4. A stra straigh ightt wire wire carryi carrying ng a current current produ produces ces

a magnetic field. Which diagram shows .

the correct shape of the field?

5. A stud student ent carri carries es out out an exper experime iment nt to see see

A.

the effect of a magnetic field on a wire carrying a current. The wire moves upwards as shown. What should the student do to make the wire move downwards?

36



What is the voltmeter reading? A. 1.2 V B. 12 V A. Change Change the direc directio tion n of the curre current nt

C. 120 V

B. Move Move the the pol poles es clo close ser r

D. 1200 V

C. Send Send a smaller smaller curre current nt through through wire wire 8. Which Which two two elect electric rical al quan quantit tities ies are

D. Use Use stro stronge ngerr magnet magnet

measured in volts? A. curr current ent and e.m.f e.m.f B. current current and resist resistance ance

6. When When electri electricit city y is trans transmit mitted ted over over long long

C. e.m.f e.m.f and and potenti potential al diff differe erence nce

distances, energy is wasted. How can the

D. potential potential difference difference and resis resistance tance

wasted energy be kept as small as possible?

9. The figur figuree shows shows an elect electrom romagne agnet. t.

. A. Keep the the current current in in the transm transmissi ission on lines lines as large as possible

● R

B. Keep Keep the power power suppl supplied ied to the the transmission lines as large as possible C. Keep the the resistan resistance ce of the transm transmissi ission on

The strength of the magnetic field can be

lines as large as possible

increase at point R by

D. Keep Keep the volta voltage ge suppl supplied ied to to the

I

transmission lines as large as possible

increasing the number of turns of the coil

II reducing the resistance of the rheostat III bringing the electromagnet closer to P

7. The diagra diagram m show showss a transf transforme ormer r

A. I and and II II onl only y B. I and and III III only only C. II and and III III only only D. I, II and II only only

37



C. 200 V D. 800 V

. 10. The diagram shows shows a solenoid connected connected 12. Which graph graph shows the the output voltage voltage

to a sensitive voltmeter v oltmeter.. Which of the

from a simple a.c. generator?

following would give a zero reading on the A.

voltmeter?

B.

C.

A. Holding Holding the magnet magnet statio stationary nary inside inside the solenoid B. Moving Moving the the magnet magnet away from from the the solenoid

D.

C. Moving Moving the the magnet magnet towards towards the the solenoid solenoid D. Moving the solenoi solenoid d towards towards the magnet magnet

. 11. The diagram shows shows a transformer transformer with an

. 13. A transformer has 50 turns turns on its primary

alternating voltage of 100 V applied to the primary coil. What is the output voltage?

coil and 100 turns on its secondary second ary coil. An a.c. voltage of 25.0 V is connected across the primary coil. What is the voltage across the secondary coil?

A. 50 V B. 100 V

38



N

PQRS is a cuprum wire put horizontally on a

A. 12.5 V

table. X and Y are two compasses put on the

B. 50.0 V

wire. What will happen to the compasses

C. 175 V

when switch S is closed?

D. 200 V

A. Both Both compas compasses ses point point to to East East

. 14. Two circuits are set up as shown. The iron

B. Both Both compass compasses es point point to West West

rods are placed close together, and are free

C. Both Both compass compasses es point pointer er unchan unchange ge

to move.

D. Compas Compasss X pointe pointerr unchang unchanged ed but compass Y point to East.

16. What happens to the size of the gap at X when switch S is closed? A. It decr decrea ease sess

Diagram shows an electromagnet PQR. The

B. It decr decreas eases es then then incr increas eases es

poles for PQR are

C. It incr increa ease sess D. It does does not not chang changee A B C D

15.

39

P North South South North

Q South North North South

R South North South North



17. Which of the followi following ng can be use to

19. What is the the current in the the cable?

determine the direction of magnetic field

A. 2.0 A

produced by current-carrying conductor?

B. 3.0 A

A. Lenz Lenz’’s law law

C. 6.0 A

B. Fara Faraday day’’s law law

D. 24.0 A

C. Flemin Fleming’ g’ss Left-h Left-hand and rule rule D. RightRight-hand hand Grip Grip rule rule

20. What is the the power loss loss in the cable? A. 6 W B. 12 W C. 18 W

18. Electromagnet Electromagnetic ic strength can be influence influence

D. 24 W

by these factors except A. Magnit Magnitude ude of curren currentt B. Directi Direction on of curren currentt flows flows C. Numb Number er of of tur turns ns D. Types ypes of core core used used

For questions 19 and 20

In a model of the transmission of electricity, electrical power of 36 W is generated at 12 V and transmitted using cables of total resistance 2.0 Ω. Part B: Structure Questions

1. Figure 1 shows an electromagnet connected co nnected to a 6.0 V d.c power po wer supply. supply. switch Insulated wire

6.0 V dc

DC supply

pins X

40



a) (i) (i) What What is the the cor coree made made of? of? Soft iron ………………………………………………………………………… (ii) State the polarity at end X when the switch is closed? North ………………………………………………………………………… b) When the switch switch is is closed, closed, it was found found a number of pins was was attracted attracted to end end X. (i) Suggest one way to decreasing the number of pins attracted to end X? Reduce the dc supply to reduce current ………………………………………………………………………… (ii) What will happen to the pins at end X when the switch is open? Explain your answer. answer. Pins drop into the container. Soft iron core losses its magnetism …………………………………………………………………………

2. Figure 2 shows an alternating current generator.

a) (i) Name Name the the phenomenon phenomenon used used to to produced produced an e.m.f e.m.f in the coil. Electromagnetic induction ……………………………………………………………...... (ii) What is the effect on the current produced if the magne ts are moved further apart from each other? Give a reason. Current, Current decreases. The magnetic flux becomes weaker. A lower rate of change of ……………………………………………………………………………………………. I/A 4 magnetic flux results a smaller induced current ……………………………………………………………………………………………. b) Figure Figure 3 shows graph of the output current current against against time. time. 2 the graph 0.04 0.08 -2 -4

Time, t/s 41


i.


Stat Statee the the valu valuee of the the peak peak curre current nt.. 2A ……………………………………………….

ii. Calculate Calculate the the frequency frequency of the the alternat alternating ing current current.. f = 1 / 0.08 = 12.5 Hz ……………………………………………….. iii. On the same axes, sketch the graph output current against time if the coil is now rotated at twice the speed. c) What are are the changes changes that need need to be made to convert convert this this generator generator to to a direct current current motor? Replace the slip rings commutator to a split-ring commutator. ………………………………………………………………………………………………… Add in a direct current power supply …………………………………………………………………………………………………

3. Figure shows a magnet which is pushed p ushed towards a solenoid.

42



Q

P

Galvanometer

a) State the the magnetic magnetic poles of of the two ends, ends, P and Q of the solenoi solenoid d when the magnet magnet is moved moved towards the solenoid, stationary inside the solenoid and then move away from the solenoid. Move toward P is North pole and Q is South pole, Stationary no pole for P and Q …………………………………………………………………………………………………… Move away P is South pole and Q is North pole ……………………………………………………………………………………………………

…… b) b) Stat Statee the the Lenz’ Lenz’ss Law Law Lenz’s Law states that the direction of the induced e.m.f is such that its magnetic effects …………………………………………………………………………………………………… always oppose the change producing. producing. …………………………………………………………………………………………………… …… c) Suggest Suggest three ways to to make the deflection deflection angle angle of the Galvanomete Galvanometerr to become bigger bigger.. The relative motion between magnet and coil is increased ………………….……………………………………………. The number of turns on coil is increased ………………….…………………………………………….. The cross-sectional area of the coil is increased ………………….…………………………………………….. d) Name four four essent essential ial parts of an a.c generat generator or Magnet ……………………………………………… Coils of wire …………………………………………….. Slip-rings ………………………………………………. Carbon brushes ………………………………………………..

43



Part C: Essay Questions

1. The transmission of electrical energy from a power station to factories and houses is by means of a system called National Grid Network. a) State the the transforma transformation tion of energy energy in in a hydroelectr hydroelectric ic power statio station. n. b) Explain Explain what is meant meant by National National Grid Grid Network Network and state state three three advantages advantages of its its distribution of electrical energy. c) Figure Figure 4 below shows a model for for transmiss transmission ion of electrical electrical power power using using source of 12 V a.c. a.c.

12 V a.c source

240 V

240 V

Transmission wire

12 V

12 V bulb is bright

You are supposed to set up the model by using the information in the table below: Number of turns of coils 1200 600 60 40 i. 1.

Types of transformers Solid copper core Laminated soft-iron core Laminated copper core Wooden core

Material of transmission cables Constantan Copper Nichrome Tungsten

Choose the number number of turns turns of primary primary and secondary secondary coil for for step-up step-up transfor transformer mer.. Give a reason.

energy energy  electrical  kinetic a) Gravitational ii. Choose the the type ofpotential core of the step-up step-up and step-down step-down transf transformer ormer.. energy Give a reason. reason. b) -Nati -National onal Grid Network Network is is a network of under undergrou ground nd cables and and pylons cover cover iii. Choose the type type of material material of transmission cables. Give a reason. the whole country. -It enables the transmission of electricity to be distributed to various regions continuously. d) If the resistance resistance of the transmi transmission ssion cable in in the model model is 20 20 Ω, calculate -If any breakdown, the electric can be supply from another areas from another pylons i. the curre cu rrent nt that that flows flows thro through ugh the the transm transmiss ission ion cabl cablee -It has good electrical energy consumption such as transferring the energy from ii. the power powe r loss loss due to heating heating effe ct of the telectrical he transmiss transmission ion cable cable low needed energy to higheffect needed energy c) i. 60 60:12 :1200 00 becau because se 60:1 60:1200 200 = 12:2 12:240 40 ii. Lamina Laminated ted soft-ir soft-iron on core core because because its easy to to magnetized magnetized and demagnetized iii. Copper because because it has smaller smaller resista resistance nce and can can reduce reduce the power power loss loss d) i. V=IR IR,, I = 12 A ii. P = I 2 R = (12)2 (20) = 2880 44 W



2. Diagrams show wire coils connected to the ammeters, switches and d.c power supply

When the switch is on and iron filings of the same amount is spread on each of the cardboard surface, the pattern of the iron fillings is formed as shown in the diagrams. a) What What is is meant meant by magneti magneticc field field?? b) Using diagrams diagrams above, above, compare compare the number of turn turn of the coils, coils, the pattern pattern of the iron fillings and the angle of deflection of the ammeter indicator.

45



c) State the relations relationship hip between between the the strength strength of the magnetic magnetic field field and and i.

the the pat patte tern rn of iron iron fill fillin ings gs

ii. the number number of turn turn of the coils coils d) Diagram Diagram shows two two thin copper strips strips,, PQ and RS, connected connected via circuit. circuit. Explain Explain what happens to PQ and RS, when the switch is on.

P

Q

_

+

Q

S

+

_

2 (a) Magnetic field : magnetic force region (b) -

The number of turns of the coil in Diagr Diagram am 10.2 is more // vice versa The arrangem arrangement ent patterns patterns of the iron fillin fillings gs in Diagr Diagram am 10.1 10.1 is furth further er apart apart // vice versa. The angl anglee of of defl deflect ection ion Dia Diagra gram m 10.2 10.2 is big bigger ger..

(c) (i) - The closer the pattern og iron filling filling , the greater strength of magnetic field / vice versa (d) -

When curre current nt flows flows,, the the magneti magneticc field field is formed // Diagr Diagram am The dir direct ection ion of the cur curren rentt in the thi thin n coppe copperr : The flo flow w of cur curren rentt is is in in the the opposite direction // Diagram The direct direction ion of magneti magneticc field field betwee between n two two thin copper stri strips ps : same direct direction ion The two pieces of thin copper repulse against one another / diagram.

46



3. A student used the apparatus shown in figure below to investigate electromagnetic induction. The magnet will be drop inside the coil. He observed that the angle that the pointer of the galvanometer deflects become increase when he increase the number of turns on the solenoid.

G

G

a) From the above observation, observation, make a suitable suitable infere inference nce b) State one appropriat appropriatee hypothesis hypothesis that could could be investi investigated gated c) Design Design an experiment experiment to investigate investigate the hypothesi hypothesiss that you stated stated in (b). (b). In your descripti description, on, state clearly the following: i.

Aim Aim of expe experrime iment

ii. Variabl ariables es in in the exper experime iment nt iii. iii. List List of the appar apparatu atuss iv. iv. Arrange Arrangemen mentt of the apparat apparatus us v. Procedures Procedures of of the experime experiment nt on controlli controlling ng the manipula manipulated ted and respondi responding ng variables vi. Tabulation abulation of of the the result result vii. Analysis Analysis of the result viii.State one precaution in the experiment

47



3. Inference: The e.m.f induced// angle of deflection deflection in the solenoid influenced influenced by the number of turns in the solenoid. f) Hypothesis: The e.m.f induced// angle of deflection in a solenoid increases when the number of turn on the solenoid increases g) To investigate investig ate the relationship relations hip between betwee n the e.m.f. induced// indu ced// angle ang le i. Aim : To of deflection in a solenoid and the number of turns turns of the solenoid ii. Manipulated variable : Number of turns of the solenoid Responding Variable: Variable: e.m.f induced in the solenoid // Galvanometer reading// angle of deflection Fixed Variable: Variable: The strength of the magnet mag net used // the speed of magnet movement into the solenoid iii. Magnet bar bar,, Galvanomet Galvanometer er,, copper copper wire wire iv.. Se iv Seee abov abovee pi pict ctur uree v. 1. Wind 50 turns turns of copper wire wire to make make a solenoid, solenoid, then connect connect to a galvanometer 2. Release a strong strong magnet magnet bar bar from from the top into into the the solenoid solenoid and take the reading of galvanometer 3. repe repeat at the the experiment experiment using 100, 150, 150, 200 200 and 250 turns turns of copper wire e)

vi. Tab vi. able le of of res resul ult t Number of turns, N

Galvanometer reading, V //divisions// angle of deflection

50 100 150 200 250

Galvanometer reading, V/div

vii. vi i. An Anal alysi ysiss of of re resul sults ts

Number of turns / N

48

Chapter 8 Electromagnet Teacher's Guide 2009

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