ELECTRICITYt
Electromagnetism
S ubject area : Physics
Topic focus : magnetic properties, magnetic field, the Earth’s magnetic field, magnetic field of an
electric wire.
Learning Aims :
Polarity of bar magnets ( they have two poles : like poles repel, opposite poles attract each
other.
M agnetic field (How to detect a magnetic field - experiments )
The origin of the Earth’s magnetic field.
What electromagnetism is.
The origin of the electromagnetism
Oersted’s experiment (Laboratory report).
S kills :
Reading : decoding information; scanning for specific information, interpreting visuals.
Speaking: defining and describing.
Listening : listening to CD-ROM .
Writing : fill a schedule.
Method :solve problems, interpret phenomena, ask questions , tests. Role play .
Assessment: CLIL assessment, assessment grid for an oral presentation, assessment rubric to
evaluate communicative skills, assessment rubric for experimental studies.
1-Magnetic properties
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.
1. Use 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 magnet itself ?
3. Touch one of the bars with a paper clip then 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 Iron , nichel, and steel are attracted by magnets.
2 Gold is attracted by magnets.
3 M agnets exert a force at a distance.
4 M agnets can make another object become magnetic.
5 There are temporary and permanent magnets.
5 The refrigerator is not a magnet.
3-After reading the text they can now understand the explanation of an abstract concept, with the
help of visual elements provided.
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 characterized
1- N…………….S
2-N…………….N
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 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 becomes
temporary magnet itself, with its poles arranged in such a way as to cause a magnetic attraction .
2.Magnetic fields
M ake sure you have prepared the necessary materials for the experiment (magnet bar, iron filings .
and sheet of paper).The outcome of the experiment should look like the picture below.
2.1-Brainstorming
Look at this picture and describe what you see
2.2 Read the text and underline the most
important information.
TEXT 1
A magnet is an object made of certain materials which create a magnetic field. Every magnet
has at least one north pole and one south pole. By convention, we say that the magnetic field
lines leave the North end of a magnet and enter the South end of a magnet. This is an example
of a magnetic dipole ("di" means two, thus two poles). If you take a bar magnet and break it into
two pieces, each piece will again have a North pole and a South pole. If you take one of those
pieces and break it into two, each of the smaller pieces will have a North pole and a South pole.
No matter how small the pieces of the magnet become, each piece will have a North pole and a
South pole. It has not been shown to be possible to end up with a single North pole or a single
South pole which is a monopole ("mono" means one or single, thus one pole). The Earth is like
a giant magnet! The nickel iron core of the earth gives the earth a magnetic field much like a bar
magnet.
2.3- Answer the following questions .
a) 1-What is magnetism?
b) 2-What is a magnet?
c) 3-What is a dipole?
d) 4- What is a monopole ?
e) Has Earth a magnetic field ?
2.4 Little experiment !
You can look at a bar magnet’s magnetic field, using iron filings. 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.
2.5 Describe the picture in activity 2.4 by focusing on the following aspects:
The rotation of the filings and where the lines start and finish.
Where the filings are concentrated.
3 Earth’s Magnetic Field.
In the space there is no magnetic iron, yet magnetism is widespread. Scientists believe the magnetic
field is generated deep inside the Earth, were the heat of 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. That’s called convection, which generates an electric current and, as 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).
The Solenoid
(Listen and read to find out)
A solenoid is a long coil of wire wrapped in many turns. When
a current passes through it, it creates a nearly uniform magnetic
field inside.
Solenoids
can convert
electric
current
to
mechanical
action, and
so are very
commonly
used as switches. The magnetic field within a solenoid depends
upon the current and density of turns. In order to estimate
roughly the force with which a solenoid pulls on ferromagnetic
rods placed near it, one can use the change in magnetic field
energy as the rod is inserted into the solenoid. The force is
roughly
Force on rod = change in magnetic field energy
distance rod moves into solenoid
The energy density of the magnetic field depends on the
strength of the field, squared, and also upon the magnetic
permeability of the material it fills. Iron has a much, much
larger permeability than a vacuum. Even small solenoids can
exert forces of a few newtons. The intensity of the magnetic
field generated by a solenoid is determinate by the amount of
current flowing through the wire, the number of coils and the
distance from the wire. The unit of intensity of a magnetic field
is called the tesla (T). Another unit used is the gauss, where
104(10,000) gauss equals 1 tesla. An example of a very intense
magnetic field is that generated by a magnetic resonance
imaging (MRI) . 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 than
at 1 cm, and the force at 3 cm is one third the force at 1 cm.
Magnetic Field Due to a Solenoid
A solenoid is a long coil (shaped like a cylinder) containing a
large number of close turns of insulated copper wire.
The figure above shows a solenoid SN whose ends are connected
to a battery B through a switch X. When a current is passed
through a solenoid, it produces a magnetic field around it. The
magnetic field is shown in the fig. It is along the axis of the
solenoid and is almost constant in magnitude and direction. The
magnetic lines of force inside the solenoid are nearly parallel to
each other and parallel to the axis of the solenoid. A solenoid
when suspended freely, aligns itself in the north-south direction,
thus behaving like a bar magnet. One end of the solenoid acts like
a north pole and the other end the south pole.
The polarity of the solenoid can be changed by reversing the
direction of the current.
The strength of the magnetic field produced by a current carrying
solenoid depends on:
The number of turns - larger the number of turns, greater is
the magnetism produced.
The strength of the current - when current increases,
magnetism also increases.
Nature of 'core-material' used in making the solenoid - if we
use soft-iron as a core for the solenoid, then it produces the
strongest magnetism.
Magnets and Electromagnets
An electromagnet is a magnet that runs on electricity. Unlike
a permanent magnet, the strength of an electromagnet can
easily be changed by changing the amount of electric current
that flows through it. The poles of an electromagnet can
even be reversed by reversing the flow of electricity.An
electromagnet works because an electric current produces a
magnetic field. The magnetic field produced by an electric
current forms circles around the electric current, as shown in
the diagram below:
If a wire carrying an electric current is formed into a series
of loops, the magnetic field can be concentrated within the
loops. The magnetic field can be strengthened even more by
wrapping the wire around a core. The atoms of certain
materials, such as iron, nickel and cobalt, each behave like
tiny magnets. Normally, the atoms in something like a lump
of iron point in random directions and the individual
magnetic fields tend to cancel each other out. However, the
magnetic field produced by the wire wrapped around the
core can force some of the atoms within the core to point in
one direction. All of their little magnetic fields add together,
creating a stronger magnetic field.
As the current flowing around the core increases, the number
of aligned atoms increases and the stronger the magnetic
field becomes. At least, up to a point. Sooner or later, all of
the atoms that can be aligned will be aligned. At this point,
the magnet is said to be saturated and increasing the electric
current flowing around the core no longer affects the
magnetization of the core itself.
1-Do you know what an electromagnet
is?
An electromagnet is a magnet that is
created when electricity flows through a
conductor. The magnetic field goes
around the conductor. The typical way
electromagnets are built is to wrap many
coils of wire around a ferromagnetic core
(almost always iron). Ferromagnetic
materials, such as iron, nickel or cobalt
have the property of concentrating
magnetic fields through them, making the
field strength higher than without the
core. When electricity passes through the
coils of wire, a magnetic field develops
around it, which is caught in the
ferromagnetic core. The magnetic field
goes through the core, out one end of the
core and back in the other end.
2-Which devices use electromagnets?
Electromagnets are used everywhere.
Electric motors, speakers, and power door
locks are some devices that use
electromagnets.
3-How do you make an electromagnet?
It is fairly easy to build an electromagnet.
All you need to do is wrap some insulated
copper wire around an iron core. If you
attach a battery to the wire, an electric
current will begin to flow and the iron
core will become magnetized. When the
battery is disconnected, the iron core will
lose its magnetism. Follow these steps if
you would like to build the
electromagnet.
A Simple electromagnet
Can you make a magnet from a nail, some batteries and
some wire?
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.
Answer the following True or False questions about magnets
and electromagnets:
T/F
1. Heating or hitting a permanent magnet can ruin it.
2. Iron is a good metal to use to make an electromagnet.
3. The north pole of one magnet will attract the north pole of
another magnet.
4. Magnets and electromagnets are used in many devices.
I think that increasing the voltage will.....................the
electromagnet’s strength.
(increase, decrease or not change)
I think that increasing the wire coils will…………… the
electromagnet’s strength.
(increase, decrease or not change)
Questions to think about:
1. What happens to the strength of the electromagnet when
more turns of wire are used?
2. What happens to the strength of the electromagnet when
more volts are used?
3. Where can you find electromagnets in your home?
4. How can you make a permanent magnet lose its
magnetism?
5. How can you measure the intensity of a magnetic field
generated by a solenoid ?
Writing about magnetism
[Think about what you know about magnets]
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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.
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