Transcript magnetic field - Rosehill

What are
Magnets ?

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.

Like poles (eg. N & N; S & S) repel each other and
opposite poles (eg. N & S) attract each other.

Activity 1 – Drawing the 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.
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.
S
N
S
N
S
N
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).
S
N
Note: Some theorists believe that magnetic monopoles
may have been made in the early Universe. So far, none
have been detected.
Magnetic field lines describe the structure
of magnetic fields in three dimensions
Field lines converge where the magnetic
force is strong, and spread out where it is
weak.
For instance, in a compact bar magnet or
"dipole," field lines spread out from one
pole and converge towards the other.
A magnet has a ‘magnetic field’
distributed throughout the surrounding
space
the magnetic force is strongest near the
poles where they come together.

The nickel iron
core of the earth
gives the earth a
magnetic field
much like a bar
magnet.
Equipment:
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Bar Magnet
Iron fillings
Sheet of white paper
Greylead
Questions:
1. Where is the magnetic field the strongest? How do
you know?
2. What happens to the magnetic field as you move
further away from the magnet?
Magnetic substances like iron,
cobalt, and nickel are composed of
small areas where the groups of
atoms are aligned like the poles of
a magnet.
 These regions are called domains.
 All of the domains of a magnetic
substance tend to align themselves
in the same direction when placed
in a magnetic field.
 These domains are typically
composed of billions of atoms.
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Permanent magnets have a north and south
pole
Within a permanent magnet there are a
number of smaller ‘mini-magnets’
 These mini-magnets are all aligned so the north
poles all face the same direction, and all of the
south poles face in the same direction
N
N
N
S
N
N
S
S
N
S
N
S
N
S
S
S
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In materials that aren’t permanent magnets,
these mini-magnets still exist, however, they
are all out of alignment
N
S
S
S
N
N
S
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S
N
N
In temporary magnets, these mini-magnets
can be manoeuvred so that they become
aligned
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Temporary magnets can be stroked
by a permanent magnet to align the
poles
Temporary magnets can also be
created by using electricity =
electromagnetism
Activity 2 – Making Temporary
Magnets
Number of Strokes
0
?
?
?
?
Number of Paperclips
The magnetic field produced around a current
carrying wire is circular
 If the wire is looped, several circular magnet fields
are produced, therefore producing a stronger
magnetic effect
 When several of these loops are placed together, a
solenoid is created, and the magnetic effect is even
stronger

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An electromagnet is a solenoid with an iron core
that further concentrates the field
Solenoid
Iron core
List all of the different uses of electromagnets
you know...
 Practical 1 – Magnetism and Electric Current
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Speakers in television sets, telephones and
sound systems need to use magnetic fields
produced by electric currents to convert
electrical energy into sound energy
Copper wire is wound
around the base of the
cone.
Cone
Solenoid
Base
Permanent
magnet
The coil produces its own
magnetic field because of
the current flowing
through it.
In a speaker, the electric
current is rapidly changing
direction.
Because of this, the coil
moves up and down
alternately repelled and
attracted to the base of
the magnet.
The cone then vibrates as
the coil moves, causing the
air nearby to vibrate,
producing sound.
Cone
Solenoid
The higher the pitch of a
note, the more rapidly the
Base
electric current changes
Permanent direction.
magnet
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An electric motor is a device that converts
electrical energy into kinetic energy

An electric motor turns because it contains
coils that produce a magnetic field when
electric current flows through them
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DC motors
The armature is the
turning point of the
motor on which coils are
wound
Field coils are
magnets that
do not move
The commutator
turns as the
armature turns
The shaft turns as the
armature turns
The rotor coils are wound on
the armature. When electric
current flows they produce a
magnetic field
The brushes connect to the power
supply
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Practical 2 – Making a simple electric motor
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Electric current is generated by running a coil
of wire through a magnetic field
The size of the current generated can be
increased by:
 Number of turns of the wire in the coil
 Strength of the magnet
 Speed of the relative movement between the coil
and the magnetic field
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Practical 3 – Making an electric generator
Electric power generated at power stations is very
large
 Wires carrying this electricity would get very hot if
the size of the current stayed the same from station
to home
 Transformers allow us to increase or decrease the
voltage of the electricity

 Step-up transformers increase the voltage, and therefore
decrease the size of the electric current
 Step-down transformers decrease the voltage, and
therefore increase the size of the electric current

Where might step-up and step-down
transformers be used in following diagram?

E = 16.5 kV
D=
B = 220kV-500kV
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C=
F = 66 kV
A = 240 V
In step-down transformers, the primary coil has more turns.
In step-up transformers, the secondary coil has more turns.
A changing electric current produces a changing magnetic field inside
the secondary coil.
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There are two main types of waves
 Longitudinal
▪ Particles vibrate backwards and forwards
 Transverse
▪ Particles vibrate up and down
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Reading wavelengths

Light consists of electromagnetic waves
 Because light is considered to be made up of a
series of changing magnetic and electric fields
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Light waves travel at 300 000 000 metres per
second
They may travel a long way before enough
energy is lost to destroy them

The electromagnetic spectrum
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Your task – In groups of 3-4, take the
envelope and remove the cut up pieces of
information. Try and place the information in
the correct order. Once you have completed
the task, show the teacher and have them
check you are correct. You are then to
summarise the information in your group as
present it to the class.