Physics

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Teaching Notes and Answer Keys

Electromagnetism

Subject area: Physics

Topic focus: magnetic properties, magnetic field, theEarth’s magnetic field, magnetic field of an electricwire, the electromagnet, intensity of anelectromagnetic field, electromagnetic motors

Learning Aims:

• polarity of bar magnets: they have two poles, likepoles repel and opposite poles attract each other.

• magnetic fields can be measured and they have adirection

• how to detect a magnetic field• the origin of the Earth’s magnetic field• what electromagnets are and how to measure the

intensity of a magnetic field• how a loudspeaker works.

Skills:

Reading: defining and describing, decoding information,scanning for specific information, interpreting visualsReasoning Skills: problem solving, interpreting,deducing, giving reasons, predicting, testing conclusions

Magnetic Properties, p. 291+2 These activities are meant to make students aware of

the concrete application of abstract concepts. Havestudents try out little experiments on magnetism. Theassignment should be given the day before the lesson.

3 After reading the text they can now understand theexplanation of an abstract concept, with the help ofvisual elements provided.

1 1 a) knife, fork, paper clip, etc. – They contain iron; 1 b) wood,plastic, copper, etc. – They don’t contain iron; 2) Yes; 3) Yes; 4) Yes; for a very short time; 5) Open question.

2 1T, 2F, 3T, 4T, 5T, 6T3 North attracts South, North repels North

Magnetic Fields, p. 304 Make sure you have prepared the necessary materials

for the experiment (magnet bar, iron filings and a sheetof paper). The outcome of the experiment should looklike the picture on page 30. Check glossary on p. 63 for challenging words: to align,dusting, iron filings, playground, to run sth. through

sth.5 Let students analyse either the picture in activity 4 or

the result of the experiment and write their observations.6 Guide the students in observing how similar the two

magnetic fields are and point out how a compass works.You can find more info about the inner and outer core ofthe Earth in the Plate Tectonics module of this book (pp. 47-52). This will help to better understand the originof the Earth’s magnetic field. You might also elicit somepractical uses of a compass (navigation, orientation,detection of magnetic fields, topographic measurements).

7 Thanks to the information gathered from the previousactivities, the students should now be able tounderstand and explain what happens in the proposedexperiments in this activity.

5 Model answerThe filings follow lines in arcs (magnetic curves) from one endof the magnet to the other. They concentrate and appear closertogether where the field is stronger, that is, near the two polesof the magnet and they look weaker as they get further away.

7 1 A attracts, B repels; 2 A & B attracts; 3 A & B: nothinghappens; 4 A repels, B attracts

The Solenoid, pp. 31-32

8 Elicit or pre-teach vocabulary by using pictures A, Band C on p. 30 (see also Challenging words). If possible, have students experiment with making “Asimple electromagnet” like the one shown on p. 31.

9 Point out that the intensity of a magnetic field can bemeasured and elicit ideas about which variables mightbe involved. Check glossary on p. 63 for challenging words: coil,cross-section, wire, to wrap.

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8 1 Figure B, 2 figure A, 3 figure C10 4 gauss; calculation:

10 : 2 = 20 : x20 x 2 : 10 = 4 gauss

Electromagnetic motors at home,

pp. 32-3311 Have students think about things they use in their

everyday life and check the list in this activity.12 Suggest to the students that they look carefully at the

illustration on p.33 as they go through the text.Extension

If possible, organize an experiment following theinstructions in this link. Students will be surprised howsimple it is to make a loudspeaker, and are likely toremember the need for permanent magnets.http://www.practicalphysics.org/go/Experiment_336.html

11 Items which have magnets in them: loudspeakers, Mp3headphones, computer, radio, telephone.Items which don’t have magnets in them: wind-up alarmclock, electric light bulb, electric space heater.

12 A 1, B 3, C 2, D 5, E 4, F 6, G 7

Check your Knowledge, p. 34The Electromagnetism Test (test A) is focused on overallknowledge of the subject, while the crossword activitychecks students’ subject-specific vocabulary.

A 1 Nickel, iron and steel, 2 Iron filings, 3 A compass, 4 Themagnetic field is stronger, 5 The Earth’s magnetic field is similarto that of a bar magnet. The north pole of a compass needle is amagnetic north pole. It is attracted to the geographic North Pole,which is a magnetic south pole (opposite magnetic polesattract), 6 It becomes an electromagnet, 7 A coil of wire used tocreate a magnetic field is called a solenoid, 8 Tesla, gauss.

B Across2 Compass, 5 Core, 6. Tesla, 7. Magnetic field, 8. FilingsDown1 Loop, 2 Curves, 3 Solenoid, 4 Coil, 7 Magnet

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Physics – Electromagnetism

2 After you’ve done this project involving experiments try to decide which statements are true (T) and which onesare false (F).

T F1 Iron, nickel and steel are attracted by magnets. �� ��2 Gold is attracted by magnets. �� ��3 Magnets exert a force at a distance. �� ��4 Magnets can make another object become magnetic. �� ��5 There are temporary and permanent magnets. �� ��6 The refrigerator is not a magnet. �� ��

3 Now let’s find out why the phenomena you’ve just observed occur. Read the text and fill in the labels on thepicture that illustrates the process.A magnetized bar ischaracterized by two oppositepoles, one at each extremity.These are known as its north(N) and south (S) poles,because if the bar is hung byits middle from a string, its N extremity tends to pointnorthwards and its S endsouthwards. The N and Spoles will both repel similarpoles of another magnet, so N will repel N and S will repel S, but N and S will attract each other. The region where this is observed iscalled the magnetic field. Either pole can also attract objects such as pins and paper clips. That isbecause under the influence of a nearby magnet, each pin or paper clip becomes a temporary magnetitself, with its poles arranged in such a way as to cause a magnetic attraction.

magnet

compass needle

1 N_________ ___________S__________

2 N_________ ___________N__________

Physics

1) Do the magnets attract all objects?a) List 10 objects that are affected by the magnets.

Do they have anything in common?b) List 10 objects that are not affected by the magnets.

Do these objects have anything in common?

2) Can you turn an object into a magnet itself?

3) Touch one of the bars with a paper clip then test whether the clip can attract other clips.

4) Once you remove it from the magnet, does the clip still display magnetic properties? For how long?

5) Look around your house for some other magnets and repeat the experiments.Do these magnets display the same characteristics as the ones above?

Magnetic Properties1 Find two bar magnets and do a few little experiments so you can answer the following questions.

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Physics – Electromagnetism

Magnetic Fields4 Can you see a magnetic field? Try out this little

experiment!You can look at a bar magnet’s magnetic fieldusing iron filings. Buy some iron filings, or findyour own iron by running a magnet throughsome playground or beach sand. Put a lightdusting of filings on a sheet of paper and placethe paper over the magnet. Tap the paperlightly and the filings will align with themagnetic field, letting you see its shape!

5 Describe the picture in activity 4 by focussing on the following aspects:• the rotation of the filings and where the lines start and finish• where the filings are concentrated

6 Look at the figures, read the information in the fact box and compare the Earth’s magnetic field with the onecreated by the bar magnet in activity 4.

S N

Object (B) South end of compass(A) North end of compass1 A magnet (south end)

2 A steel bar (any end)

3 A piece of paper

4 A compass (north end)

______________________________

______________________________

______________________________

______________________________

______________________________

______________________________

______________________________

______________________________

7 What happens when you place something near the compass? Experiment to find out!Test the effects of the following objects on your compass. First, slowly bring the object near the northend of your compass. Then, slowly move it towards the south end. Write your observations in this table.

Earth’s Magnetic FieldOut in space there is no magnetic iron, yetmagnetism is widespread. Scientists believe themagnetic field is generated deep inside theEarth where the heat of the planet’s solid innercore causes movement in the liquid outer core ofiron and nickel. The solid inner core is thoughtto be a mass of iron about the size of the moon

that has a temperatureof several thousand

degreesFahrenheit.

The heat ofthis inner core radiates outwards and upwards until it reaches theboundary with Earth’s liquid outer core, causing the fluid there to expand.As it expands, it becomes a little less dense, so it starts to rise. That’scalled convection, which generates an electric current and, as a result, amagnetic field.

The Earth’s magnetic field is similar to that of a bar magnet. The north poleof a compass needle is a magnetic north pole. It is attracted to the geographic

North Pole, which is a magnetic south pole (opposite magnetic poles attract).

S

N

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The Solenoid – Magnetic field of an Electric Wire8

Look at these pictures, read thetext and fill in the blanks with theletter corresponding to the correctpicture. Then listen and check.(1) Figure _____ shows theshape of the magnetic fieldaround an electric wire whereelectric current is flowing. Inthis figure, imagine that youhave cut the wire and arelooking at its end. The greencircle in the figure is the cross-section of the electric wire itself.A circular magnetic fielddevelops around the wire, asshown by the circular lines inthe illustration. The fieldweakens as you move away fromthe wire (so the lines are fartherapart as they get farther fromthe wire). The field isperpendicular to the wire andthe field’s direction depends onwhich direction the current isflowing in the wire. Because the magnetic fieldaround a wire is circular andperpendicular to the wire, aneasy way to amplify the wire’smagnetic field is to coil the wire,as shown in (2) figure _____ .Wrapping the wire in a coilconcentrates and increases themagnetic field, because of theadditive effect of each turn ofthe wire. A coil of wire used tocreate a magnetic field is calleda solenoid, this is representedin (3) figure _____ .

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A simple electromagnetIf you wrap your wire around a nail 10 times,connect the wire to a battery and bring one end ofthe nail near the compass, you will find that it hasa much greater effect on the compass. In fact, thenail behaves just like a bar magnet. What you havecreated is an electromagnet! You will find thatthis magnet is able to pick up small steel thingslike paper clips, staples and thumb tacks.

Figure A

Figure B

Figure C

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9 How can you measure the intensity of a magnetic field? Listen and read to find out.The intensity of the magnetic field generated by a solenoid is determined by the amount of currentflowing through the wine, the number of coils and the distance from the wire. The unit of intensity of amagnetic field is called the tesla (T). An example of a very intense magnetic field is that generated by amagnetic resonance imaging (MRI) machine; it is 1 T. Another unit used is the gauss, where 104 (10,000)gauss equals 1 tesla.

The intensity of the magnetic field is proportional to the current in the wire. If you double the current,the magnetic field is doubled. If you wrap the wire into a coil, you increase the magnetic field inside thecoiled section and it is proportional to the number of coils per unit of length of the solenoid. In otherwords, a coil consisting of 10 loops has 10 times the magnetic field as a single wire with the same currentflowing through it and a coil with 20 loops, over the same length, has twice the magnetic field as one with10 loops.

In general, we can also say that when a magnetic field is generated by a straight wire, the magneticintensity decreases with distance, that is, it is inversely proportional to the distance. For example, themagnetic force at 2 cm from a wire is half that at 1 cm, and the force at 3 cm is one third the force at 1cm.

10 Let’s apply the formula!A coil with 10 loops produces a magnetic field with an intensity of 2 gauss: calculate the intensity of amagnetic field produced by a coil with 20 loops.

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Electromagnetic Motors at Home11 Look at this list of items you can find at home and decide if they have magnets in them. Put a tick next to the

ones you think have magnets.

Item Does It Have Magnets?Loudspeaker

Wind-up alarm clock

Mp3 headphones

Computer

Radio

Telephone

Electric light bulb

Electric space heater

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Physics – Electromagnetism

12 Read the text above and then put these sentences in the right order for the summary. The first and the lastsentences are already done for you.A �� Inside the ear there is a thin membrane called the eardrum and when it vibrates, the brain

“translates” these vibrations into sounds. B �� You can vary the magnetic force of an electromagnet by changing the amount and direction of the

current going through the coils that are wound around it.C �� Rapid changes in air pressure cause the eardrum to vibrate.D �� A loudspeaker consists of a permanent magnet, an electromagnet and a membrane or cone.E �� Loudspeakers and tape recorders are devices that apply this effect.F �� The loudspeaker membrane or cone is attached to an electromagnet which is surrounded by

a permanent magnet. G �� By varying the electric current going through the wires around the electromagnet, we can create

vibrations that go back and forth between the electromagnet and the speaker cone.

How do Loudspeakers work?Remember how the human ear perceives sounds: inside the ear there is a thin membrane called theeardrum and when it vibrates, the brain “translates” these vibrations into sounds. Rapid changes inair pressure cause the eardrum to vibrate. In other words, sound travels in waves of air pressurefluctuation.

One useful characteristic of anelectromagnet is the fact that you canvary its magnetic force by changingthe amount and direction of thecurrent going through the coils thatare wound around it. Loudspeakersand tape recorders are devices thatapply this principle.

A loudspeaker consists of a permanentmagnet surrounding an electromagnetthat is attached to the loudspeakermembrane or cone.

By varying the electric current goingthrough the wires around theelectromagnet, we can createvibrations that go back and forthbetween the electromagnet and thespeaker cone. If the electric currentvariations occur at the samefrequencies as sound waves, the resulting vibration of the speaker cone will generate sound wavesthat include those of the human voice and music.

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Check your knowledgeA The Electromagnetism Test.

1 Name three magnetic substances.2 What sort of material could you use to see a magnetic field? 3 What could you use to find the polarity of a magnet? 4 What does it mean when the magnetic field lines are close together?5 A compass is a magnet and the Earth is a magnet. How does the magnetism of a compass work with

the magnetism of the Earth so that a compass is a useful tool for navigating?6 What happens when a coil is placed around an iron bar and the electrical current is turned on?7 What is a solenoid?8 What are the units used to measure the intensity of a magnetic field?

Across

2 A device used to determine geographic direction.5 Where the magnetic field of the Earth is generated.6 A unit of magnetic field intensity.7 An area around a magnet in which its power of attraction is felt.8 Fragments of iron.

Down

1 Something having a shape or a motion that is circular or curved over on itself.2 The arrangement of iron filings between the poles of a magnet.3 A coil of wire acting like a magnet when a current passes through it.4 A series of connected spirals or concentric rings.7 An object that has the property of attracting iron or steel.

B Crossword 1

2 3

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