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Physics

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Teaching Notes and Answer Keys

Electromagnetism

Subject area:Physics

Topic focus:magnetic properties, magnetic field, the Earth's magnetic field, magnetic field of an electric wire, the electromagnet, intensity of an electromagnetic 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 theintensity of a magnetic field •how a loudspeaker works.

Skills:

Reading:defining and describing, decoding information, scanning for specific information, interpreting visuals

Reasoning Skills:problem solving, interpreting,

deducing, giving reasons, predicting, testing conclusionsMagnetic Properties, p. 29

1+2These activities are meant to make students aware ofthe concrete application of abstract concepts. Havestudents try out little experiments on magnetism. Theassignment should be given the day before the lesson.

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

11 a) knife, fork, paper clip, etc. - They contain iron; 1 b) wood,

plastic, copper, etc. - They don't contain iron; 2) Y es; 3) Yes;

4) Yes; for a very short time; 5) Open question.

21T, 2F, 3T, 4T, 5T, 6T

3North attracts South, North repels North

Magnetic Fields, p. 30

4Make sure you have prepared the necessary materials

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

5Let students analyse either the picture in activity 4 orthe result of the experiment and write their observations.

6Guide the students in observing how similar the twomagnetic fields are and point out how a compass works.You can find more info about the inner and outer core of

the Earth in the Plate T ectonics module of this book (pp. 47-52). This will help to better understand the origin of the Earth's magnetic field. You might also elicit some practical uses of a compass (navigation, orientation, detection of magnetic fields, topographic measurements).

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

5Model answer

The filings follow lines in arcs (magnetic curves) from one end of the magnet to the other. They concentrate and appear closer together where the field is stronger , that is, near the two poles of the magnet and they look weaker as they get further away.

71 A attracts, B repels; 2 A & B attracts; 3 A & B: nothinghappens; 4 A repels, B attractsThe Solenoid, pp. 31-32

8Elicit or pre-teach vocabulary by using pictures A, B

and C on p. 30 (see also Challenging words). If possible, have students experiment with making " A simple electromagnet " like the one shown on p. 31.

9Point 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.

TR9 - CLIL_Electromagnetism_8TR 10 - CLIL_Electromagnetism_9

81 Figure B, 2 figure A, 3 figure C

104 gauss; calculation:

10 : 2 = 20 : x

20 x 2 : 10 = 4 gaussElectromagnetic motors at home,

pp. 32-33

11Have students think about things they use in their

everyday life and check the list in this activity.

12Suggest

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 the

instructions in this link. Students will be surprised how simple it is to make a loudspeaker , and are likely to remember the need for permanent magnets. http://www .practicalphysics.org/go/Experiment_336.html

11Items which have magnets in them:loudspeakers, Mp3

headphones, computer, radio, telephone. Items which don't have magnets in them:wind-up alarm clock, electric light bulb, electric space heater .

12A 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.

A1 Nickel, iron and steel, 2 Iron filings, 3 A compass, 4 The magnetic field is stronger, 5 The Earth' s magnetic field is similar to that of a bar magnet. The north pole of 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), 6 It becomes an electromagnet, 7 A coil of wire used to create a magnetic field is called a solenoid, 8 Tesla, gauss. BAcross2 Compass, 5 Core, 6. Tesla, 7. Magnetic field, 8. Filings Down

1 Loop, 2 Curves, 3 Solenoid, 4 Coil, 7 Magnet

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Physics - Electromagnetism

2After you've done this project involving experiments try to decide wh

ich statements are true (T) and which ones are false (F). TF

1 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. 

3Now let's find out why the phenomena you've just observed occur. Read the text and fill in the labels on the

picture that illustrates the process.

A magnetized bar is

characterized by two opposite poles , one at each extremity.

These are known as its north

(N) and south (S) poles, because if the bar is hung by its middle from a string, its

N extremity tends to point

northwards and its S end southwards. The N and S poles will both repel similar poles 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 is

called the magnetic field . Either pole can also attract objects such as pins and paper clips. Tha t is because under the influence of a nearby magnet, each pin or paper clip b ecomes a temporary magnet itself, with its poles arranged in such a way as to cause a magnetic att raction.magnet compass needle

1N_________ ___________

S __________

2N_________ ___________

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 Properties

1Find two bar magnets and do a few little experiments so you can answer t

he following questions.

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Physics - Electromagnetism

Magnetic Fields

4Can you see a magnetic field? Try out this little

experiment!

You can look at a bar magnet's magnetic field

using iron filings. Buy some iron filings, or find your own iron by running a magnet through some playground or beach sand. Put a light dusting of filings on a sheet of paper and place the paper over the magnet. Tap the paper lightly and the filings will align with the magnetic field, letting you see its shape!

5Describe 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

6Look at the figures, read the information in the fact box and compare th

e Earth's magnetic field with the one created by the bar magnet in activity 4. SN Object(B) South end of compass(A) North end of compass

1 A magnet (south end)

2 A steel bar (any end)

3 A piece of paper

4 A compass (north end)________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

7What happens when you place something near the compass? Experiment to fi

nd out! Test the effects of the following objects on your compass. First, slowly bring the object near the north

end of your compass. Then, slowly move it towards the south end. Write your observations in this table.

Earth's Magnetic Field

Out in space there is no magnetic iron, yet

magnetism is widespread. Scientists believe the magnetic field is generated deep inside the

Earth where the heat of the planet's solid inner

core causes movement in the liquid outer core of iron and nickel. The solid inner core is thought to be a mass of iron about the size of the moon that has a temperature of several thousand degrees

Fahrenheit.

The heat of

this inner core radiates outwards and upwards until it reaches the boundary 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. Tha t's called convection, which generates an electric current and, as a result, a magnetic field . The Earth's magnetic field is similar to that of a bar magnet. The north pole of a compass needle is a magnetic north pole. It is attracted to the geo graphic North Pole, which is a magnetic south pole (opposite magnetic poles att ract). S N

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Introduction to CLIL - Change Up! © ELIPhotocopiable ........................................................................ .........................................................

Physics - Electromagnetism

The Solenoid - Magnetic field of an Electric Wire

8

Look at these pictures, read the

text and fill in the blanks with the letter corresponding to the correct picture. Then listen and check. (1) Figure _____shows the shape of the magnetic field around an electric wire where electric current is flowing. In this figure, imagine that you have cut the wire and are looking at its end. The green circle in the figure is the cross- section of the electric wire itself.

A circular magnetic field

develops around the wire, as shown by the circular lines in the illustration. The field weakens as you move away from the wire (so the lines are farther apart as they get farther from the wire). The field is perpendicular to the wire and the field's direction depends on which direction the current is flowing in the wire.

Because the magnetic field

around a wire is circular and perpendicular to the wire, an easy way to amplify the wire's magnetic field is to coil the wire, as shown in (2) figure _____.

Wrapping the wire in a coil

concentratesand increasesthe magnetic field, because of the additive effect of each turn of the wire. A coil of wire used to create a magnetic field is called a solenoid, this is represented in (3) figure _____.

TR9 - CLIL_Electromagnetism_8

A simple electromagnet

If you wrap your wire around a nail 10 times,

connect the wire to a battery and bring one end of the nail near the compass, you will find that it has a much greater effect on the compass. In fact, the nail behaves just like a bar magnet. What you have created is an electromagnet ! You will find that this magnet is able to pick up small steel things like paper clips, staples and thumb tacks.

Figure A

Figure B

Figure C

Physics

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Physics - Electromagnetism

9How 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 current flowing through the wine, the number of coils and the distance from the wire. The unit of intensityof a magnetic field is called the tesla(T). An example of a very intense magnetic field is that generated by a magnetic resonance imaging (MRI) machine; it is 1 T. Another unit used is the gauss, where 10 4 (10,000) gaussequals 1 tesla. The intensity of the magnetic field is proportional to the current in th e wire. If you double the current, the magnetic field is doubled. If you wrap the wire into a coil, you inc rease the magnetic field inside the coiled section and it is proportional to the number of coils per unit of length of the solenoid. In other words, a coil consisting of 10 loops has 10 times the magnetic field as a single wire with the same current flowing through it and a coil with 20 loops, over the same length, has t wice the magnetic field as one with

10 loops.

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

10Let's apply the formula!

A coil with 10 loops produces a magnetic field with an intensity of 2 ga uss: calculate the intensity of a magnetic field produced by a coil with 20 loops.

TR 10 - CLIL_Electromagnetism_9

Electromagnetic Motors at Home

11Look 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.

ItemDoes It Have Magnets?

Loudspeaker

Wind-up alarm clock

Mp3 headphones

Computer

Radio

Telephone

Electric light bulb

Electric space heater

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Introduction to CLIL - Change Up! © ELIPhotocopiable ........................................................................ .........................................................

Physics - Electromagnetism

12Read the text above and then put these sentences in the right order for

the summary. The first and the last sentences are already done for you. A??Inside the ear there is a thin membrane called the eardrumand when it vibrates, the brain "translates" these vibrations into sounds. B ? ?You can vary the magnetic force of an electromagnet by changing the amoun t 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 mem brane 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 i s surrounded by a permanent magnet. G ??By varying the electric current going through the wires around the elect romagnet, we can create vibrations that go back and forth between the electromagnet and the spea ker cone.

How do Loudspeakers work?

Remember how the human ear perceives sounds: inside the ear there is a t hin membrane called the eardrumand when it vibrates, the brain "translates" these vibrations into sounds.

Rapid changes in

air pressure cause the eardrum to vibrate. In other words, sound travels in waves of air pressure fluctuation.

One useful characteristic of an

electromagnet is the fact that you can vary its magnetic force by changing the amount and direction of the current going through the coils that are wound around it. Loudspeakers and tape recorders are devices that apply this principle.

A loudspeaker consists of a permanent

magnet surrounding an electromagnet that is attached to the loudspeaker membrane or cone.

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. If the electric current variations occur at the same frequencies as sound waves, the resulting vibration of the speaker cone will generate sound waves that include those of the human voice and music. 1 7

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Physics - Electromagnetism

Check your knowledge

A 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 naviga ting?

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 23
4 5 6 7 8

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