ELECTRICITYt

Subject 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.

Magnetic 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).

Skills :

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?

Electromagnetism

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 Magnets exert a force at a distance.

4 Magnets 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

Make 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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