Physics
Electromagnetism
Teaching Notes and Answer Keys
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, like
poles repel and opposite poles attract each other.
• magnetic fields can be measured and they have a
direction
• 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 visuals
Reasoning Skills: problem solving, interpreting,
deducing, giving reasons, predicting, testing conclusions
Magnetic Properties, p. 29
1+2 These activities are meant to make students aware of
the concrete application of abstract concepts. Have
students try out little experiments on magnetism. The
assignment should be given the day before the lesson.
3 After reading the text they can now understand the
explanation of an abstract concept, with the help of
visual 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, 6T
3 North attracts South, North repels North
Magnetic Fields, p. 30
4 Make 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.
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 of
the Earth in the Plate Tectonics 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).
7 Thanks to the information gathered from the previous
activities, the students should now be able to
understand and explain what happens in the proposed
experiments in this activity.
28
5 Model 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.
7 1 A attracts, B repels; 2 A & B attracts; 3 A & B: nothing
happens; 4 A repels, B attracts
The Solenoid, pp. 31-32
TR 9 - CLIL_Electromagnetism_8
8 Elicit 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.
TR 10 - CLIL_Electromagnetism_9
9 Point out that the intensity of a magnetic field can be
measured and elicit ideas about which variables might
be involved.
Check glossary on p. 63 for challenging words: coil,
cross-section, wire, to wrap.
8 1 Figure B, 2 figure A, 3 figure C
10 4 gauss; calculation:
10 : 2 = 20 : x
20 x 2 : 10 = 4 gauss
Electromagnetic motors at home,
pp. 32-33
11 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 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
11 Items 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.
12 A 1, B 3, C 2, D 5, E 4, F 6, G 7
Check your Knowledge, p. 34
The Electromagnetism Test (test A) is focused on overall
knowledge of the subject, while the crossword activity
checks students’ subject-specific vocabulary.
A 1 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.
B Across
2 Compass, 5 Core, 6. Tesla, 7. Magnetic field, 8. Filings
Down
1 Loop, 2 Curves, 3 Solenoid, 4 Coil, 7 Magnet
Physics
Magnetic Properties
1
Find two bar magnets and do a few little experiments so you can answer the following questions.
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?
2
After you’ve done this project involving experiments try to decide which statements are true (T) and which ones
are false (F).
T F
1
2
3
4
5
6
3
I
I
I
I
I
I
Iron, nickel and steel are attracted by magnets.
Gold is attracted by magnets.
Magnets exert a force at a distance.
Magnets can make another object become magnetic.
There are temporary and permanent magnets.
The refrigerator is not a magnet.
I
I
I
I
I
I
Now 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
1 N_________ ___________
characterized by two opposite
S__________
poles, one at each extremity.
These are known as its north
compass
(N) and south (S) poles,
needle
because if the bar is hung by
its middle from a string, its
magnet
N extremity tends to point
northwards and its S end
2 N_________ ___________
southwards. The N and S
N__________
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. That is
because under the influence of a nearby magnet, each pin or paper clip becomes a temporary magnet
itself, with its poles arranged in such a way as to cause a magnetic attraction.
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Physics
Magnetic Fields
4
Can 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!
5
N
S
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 one
created by the bar magnet in activity 4.
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
S
iron and nickel. The solid inner core is thought
to be a mass of iron about the size of the moon
N
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. That’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 geographic
North Pole, which is a magnetic south pole (opposite magnetic poles attract).
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 north
end of your compass. Then, slowly move it towards the south end. Write your observations in this table.
Object
(A) North end of compass
(B) South end of compass
1 A magnet (south end)
______________________________
______________________________
2 A steel bar (any end)
______________________________
______________________________
3 A piece of paper
______________________________
______________________________
4 A compass (north end)
______________________________
______________________________
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Physics
The Solenoid – Magnetic field of an Electric Wire
8
TR 9 - CLIL_Electromagnetism_8
Figure A
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 crosssection 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
concentrates and increases the
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 _____ .
Figure B
Figure C
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.
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Physics
9
TR 10 - CLIL_Electromagnetism_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 current
flowing through the wine, the number of coils and the distance from the wire. The unit of intensity of 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 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 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 twice 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 at 3 cm is one third the force at 1
cm.
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 a
magnetic field produced by a coil with 20 loops.
Electromagnetic Motors at Home
11 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
How do Loudspeakers work?
Remember how the human ear perceives sounds: inside the ear there is a thin membrane called the
eardrum and 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.
12 Read 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.
1 Inside the ear there is a thin membrane called the eardrum and when it vibrates, the brain
A I
“translates” these vibrations into sounds.
B I 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 I Rapid changes in air pressure cause the eardrum to vibrate.
D I A loudspeaker consists of a permanent magnet, an electromagnet and a membrane or cone.
E I Loudspeakers and tape recorders are devices that apply this effect.
F I The loudspeaker membrane or cone is attached to an electromagnet which is surrounded by
a permanent magnet.
7 By varying the electric current going through the wires around the electromagnet, we can create
G I
vibrations that go back and forth between the electromagnet and the speaker cone.
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Physics
Check your knowledge
A
The Electromagnetism Test.
1
2
3
4
5
Name three magnetic substances.
What sort of material could you use to see a magnetic field?
What could you use to find the polarity of a magnet?
What does it mean when the magnetic field lines are close together?
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?
B Crossword
1
2
3
4
5
7
6
8
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.
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