Download Magnetic field of a bar magnet

Magnets &
Motors
Index
 Magnetic and non-magnetic materials
 Properties of magnetic materials
 Magnetic Fields
 Electromagnetism
 Electric Motors
 Induced currents
 Transformers
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Magnetic and non-magnetic materials
Magnetic materials
A magnetic object is a material which can be magnetized, and is
attracted to magnets. All strongly magnetic materials contain iron,
nickel, or cobalt. For example steel is mainly iron. A material that
contains mainly iron and is a very strong magnetic material is iron.
The lodestone is a naturally magnetized piece of the
mineral known as magnetite. The ancient Greeks
used the lodestone as a compass, by attaching it to a
string, and then it would point towards the North
Pole.
Strong magnetic materials are also called ferromagnetic. These are
divided in two categories hard and soft, this depends on how well they
keep their magnetism when they are magnetized.
Name an example of Hard Ferromagnetics; steel.
These materials are difficult to magnetize but do
not lose their magnetism readily. A practical use
for these materials would be: permanent magnet.
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Name an example of Soft Ferromagnetics;
iron.
These
materials
are
easy
to
magnetize but their magnetism is easily
lost.
They
are
used
in
cores
of
electromagnets because their magnetic
effect can be switched on/off.
Non-Magnetic materials
These include metals such as brass, copper, zinc, tin and aluminum as
well as all other non-metals.
Magnetic Metals
Non-magnetic Metals
Iron
Brass
Steel
Copper
mercury
Zinc
Nickel
Tin
cobalt
Aluminium
Where does magnetism come from?
Inside the atom, the electrons orbit around the central nucleus. This
orbiting and spinning of the electrons creates a magnetic effect within
the material. In many types of atoms, the magnetic effect is cancelled
but in some others it is not. This is where magnetism comes from. In a
material which is not magnetized the atomic dipoles inside the material
point in random directions.
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What do you think happens to dipoles when the material becomes
magnetized?
___________________________________________________________
___________________________________________________________
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Properties of Magnetic Materials
Magnetic poles
When a magnet is dipped into iron filings, the small
iron filings are attracted to its ends. This shows that
the magnetic acts at two points of the magnet. These
points are called ‘the poles’. Every magnet has two
poles; the north and south pole.
Assume that you have two similar magnets, what will happen when you
bring;
Two South poles next to each other
_________________
A south and a north pole next to each other
_________________
Two north poles next to each other
_________________
This is because like poles repel, and unlike poles attract each
other. Therefore, the north pole is attracted to the south pole,
and vice versa.
Magnetic poles always exist in pairs. It is
not possible to produce just an isolated
north pole or isolated south pole. If a bar
magnet is broken into two, each piece
will have a north and south pole.
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If you had to tie a bar magnet to a piece
of string, and you suspended it in mid-air,
it swings around and stops with its north
pole facing the Earth’s North, and the
south of the magnet facing the South of
the Earth. This means that the Earth
exerts a weak magnetic force on the
poles of the magnet.
Induced magnetism:
Which materials are attracted to magnets? Why is this so?
When these materials are brought close to magnets, magnetism is
induced in them as shown in the diagrams. This means that iron acts as a
temporary magnet, while steel acts as a permanent magnet.
Making a magnet:
There are various ways how to make a magnet, as mentioned a piece of
steel can by magnetized if it is simply put close to a magnet however,
this makes steel a weak magnet. The following are two ways in which
steel can be made a permanent magnet.
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1. Stroking: This method consists of
stroking with a magnet a piece of
steel from end to end in the same
direction for several times.
2. Electrically: take the magnetic
material and place it inside a
coil of wire which has numerous
turns of copper wire which is
connected to a low voltage, high
current supply. The alignment of
the dipoles depends on the direction of the current supplied. By
which
rule
is
the
direction
of
the
current
given?___________________.
How can a stronger magnet be produced electrically?
___________________________________________________________
___________________________________________________________
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Demagnetization of magnets
There are 3 main ways to demagnetize a magnet which are:
1. Hammering: this is done by hammering
the magnet which would cause the
dipoles to lose their alignment.
2. Heating: heating the magnet would
cause the atoms to move rapidly which
in turn would cause the dipoles to lose
their alignment.
3. Electrically: this is the most effective
method of demagnetization. The magnet
is placed inside a solenoid connected to
an ac supply this alternating current
changes current direction rapidly. The
magnet is then slowly withdrawn in the
east west direction with the alternating current still flowing
through the solenoid.
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Magnetic Fields
List 3 cases were a magnetic field is present: ____________________,
____________________, or _______________________. In these cases
a magnetic field can be observed around the magnets and this exerts
forces on magnetic materials in it.
Magnetic field of a bar magnet:
The magnetic field lines run from the north pole to the south pole. The
field direction is represented by an arrowhead, which indicates the
direction of the magnetic field..
Draw the field lines on this magnet:
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Magnetic fields can also be investigated using a small compass. When a
compass is placed inside the magnetic field it points in same direction as
the arrowhead of the magnetic field How can you trace the magnetic
field lines of a magnet using a compass?
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There is a way in which we can
see a representation of the
magnetic field lines of a bar
magnet, this is by using iron
filings.
Iron
filings
are
sprinkled on a paper which is
placed over a bar magnet, the filings will now become tiny magnets each
having their own north and south and these are pulled into position
from the poles of the magnet.
Can you now draw the magnetic fields of two magnets when they are
placed next to each other in the following ways?
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When two magnets are placed next to each other their magnetic fields
combine to produce a single field as you have displayed above. There is
what is called a neutral point this is where the field of one magnet
exactly cancels the field of the other, this occurs only when there are
two like poles facing each other. Can you mark with an X on one of the
above diagrams where there is a neutral point?
Magnetic field of a straight wire carrying a current
When a current is passed through a wire a magnetic field is produced.
The magnetic field is in the form of circles around the wire and the field
is the strongest _______________ to the wire. What happens to the
field
lines
if
you
increase
the
current?
_______________________________.
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There is a way how we can find the direction
of the magnetic field around the wire and this
is called the right hand grip rule. For this rule
imagine
grabbing
the
wire
with
you
_________hand, so that your thumb points in
the direction of conventional current. So it
follows that your fingers now point in the same
direction as the field lines around the wire.
Finding the shape of the magnetic field around a wire:
A wire is passed through a piece of
cardboard with a hole in the middle,
iron filings are then sprinkled over the
cardboard. A large current is passed
through the wire WITH CAUTION, while
tapping the card so that the iron filings
show the shape of the magnetic field
around the wire.
How can you determine the direction of the magnetic field lines?
___________________________________________________________
___________________________________________________________
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How does the current change the strength of the magnetic field in a
straight wire?
Magnetic field of a solenoid:
When a current is passed through a solenoid
it also produces a magnetic field in the same
shape as that produced by a bar magnet. The
field inside the solenoid is very strong and
uniform. This magnetic field also has
magnetic poles at the ends of the coil. If the current in increased, the
strength of the field ________________.
What happens if you increase the number of turns of the coil?
___________________________________________________________
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Why does this happen?
___________________________________________________________
___________________________________________________________
Here we can once again use the right hand grip rule to find the poles
produced inside the solenoid.
Can you explain how you can use this rule to find the north/south poles
of the coil?
___________________________________________________________
___________________________________________________________
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Finding the shape of the magnetic field around a solenoid:
A solenoid is passed through a piece of cardboard with holes in it. Iron
filings are then sprinkled over the cardboard.
A large current is passed through the wire
WITH CAUTION, while tapping the card so
that the iron filings show the shape of the
magnetic field around the wire.
How can you determine the direction of the magnetic field lines?
___________________________________________________________
___________________________________________________________
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The strength of the magnetic field can be increased by; increasing the
current in the circuit, using more turns on the wire of the coil. Also using
a soft iron core aids to produce a stronger field. The iron core helps he
solenoid by acting as a temporary magnet.
How does the current affect the strength of the magnetic field in a
solenoid?
Draw the field lines of the solenoids below:
Current = 10A
Current = 30A
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Electromagnetism
What do you think is the difference between a permanent magnet and
an electromagnet?
___________________________________________________________
___________________________________________________________
An electromagnet is made up from a coil which is wound round an iron
core (much like that of a solenoid) but the difference is that it can be
switched on and off using a simple switch. When a current flows through
the coil, this produces a magnetic field.
A DIY solenoid using an iron nail
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Consider this circuit below;
What happens to the core when the
circuit is switched on?
__________________________________
__________________________________
__________________________________
Why is it important that the core is made from iron and not steel?
___________________________________________________________
___________________________________________________________
The strength of the electromagnet can be increased by;
 Increasing the current in the circuit,
 Using more turns on the wire of the coil,
 And by using a bigger iron core.
An electromagnet picking up iron
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HW! Uses of electromagnets:
Three uses of electromagnets in everyday life are:
1. Circuit breaker
2. Electric bell
3. Loudspeaker
4. Electromagnetic door lock
5. Electromagnetic for lifting
Describe in detail one of the above: It is suggested that you use diagrams
in the space below:
___________________________________________________________
___________________________________________________________
___________________________________________________________
___________________________________________________________
___________________________________________________________
___________________________________________________________
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Electric Motors
Magnetic Force on a Current:
Let us assume that a copper wire is placed
perpendicular to a magnetic field. The wire only Is copper a
magnetic material
feels a force from the magnet when the current is
or not?
passing through it. This happens because the current
produces its own magnetic field which acts on the poles of the magnets.
The force is increased if:
 The current is increased
 A stronger magnet is used
 The length of the wire in the field is increased
Discuss: Do you think that the wire will move when there is a current
passing through it?
This force experienced by the wire is called The Catapult Effect.
The ‘’catapult effect’’
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Fleming’s Left Hand Rule:
The direction of motion of the copper wire can be found by using
Fleming’s Left hand rule.
Using the diagram on the right, and representing the direction of the
field with the first finger, and the direction of
the current with the second finger, through
Fleming’s left hand rule, we can see that the
wire moves upwards (the direction of the
thumb). What needs to be altered for the wire
to move downwards? N.B. The wire is not
attracted to either pole.
In applying this rule it is important to remember that
1. The field direction of the magnet is from the North Pole to the
South Pole.
2. The current direction is from the positive to the negative;
following conventional current direction.
3. This rule only applies if the current and the field are at right
angles.
N.B. If the current and the field are in the same direction (parallel to
each other, then there is no force).
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Turning Effect on a coil:
If a coil is carrying a current in a magnetic field, the forces on it produce
a turning effect. Many electric motors use this principle
As you can see from the diagram, on opposite sides of the coil, current
flows in opposite directions. So if we apply Fleming’s left hand rule on
the coil next to the North side of the magnet, this wire is pushed up.
Likewise on the other side, it is pushed down. This creates a turning
effect.
What happens to this turning effect if the coil has more turns?
___________________________________________________________
___________________________________________________________
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Test your knowledge:
There is a force on the wire in the diagram on the right;
a. Give two ways in which the force
could be increased.
b. Use Fleming’s left hand rule to work
out the direction of the force.
c. Give two ways in which the direction
of the force can be reversed.
A simple DC Motor
The simple DC motor runs on
direct current. What do you
understand by direct current?
_________________________
_________________________
_________________________
The coil is made up of insulated copper wire, and is allowed enough
room to rotate between the poles of the magnet. Attached to the coil
there is a split ring, also known as the commutator. There are two
brushes that act as contacts which rub against the commutator and keep
the coil connected to the battery. Suggest a suitable material for these
brushes: __________________
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A simple DC motor
The maximum turning effect on the coil is when it is horizontal, this is
because the forces here are the furthest apart. If there was no change to
the forces, the commutator would come to rest in the vertical position,
however when the coil just passes the vertical position, the commutator
changes the direction of the current through it, and so do the forces.
This continues to push the coil further round until it is once again
vertical. And so on. Thus, the coil is rotating clockwise, half a turn at a
time. What would happen if the battery or the poles of the magnet,
were reversed?
___________________________________________________________
___________________________________________________________
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List 4 ways in which the turning effect can be increased:
a.__________________________________________________________
b._________________________________________________________
c._________________________________________________________
d._________________________________________________________
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Induced Currents
Induced EMF
A current produces a magnetic field, is it possible for a magnetic field to
produce a current?
If a bar magnet is pushed into a coil when there is no battery attached to
the coil, an emf is induced in the current. This effect is called
electromagnetic induction. This is because the field lines of the magnet
are being cut by the wire and therefore it makes a complete circuit.
This induced emf can be increased:
 Moving the magnet faster
 Increasing the number of turns
 Using a stronger magnet
The above statement is summed up by Faraday’s Law of
Electromagnetic Induction, which states that;
“The emf induced in a conductor is directly proportional to the rate at
which the magnetic field lines are being cut by the conductor“
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An application of Faraday’s law is the electric guitar; the pick up under the
string are tiny coils with magnets inside them the steel string become
magnetized. When they vibrate, current is induced in the coils boosted by an
amplifier to produce sound.
Lenz’s law
If a magnet is moved in or out of a coil the direction of the current can
be predicted using Lenz’s law which states that an induced current
always flows in a direction such that it opposes the changes which
produces it.
From the diagram above, the induced current turns the coil into a weak
electromagnet. The North pole of this electromagnet opposes the North
pole of the actual magnet. This creates repulsion between the two.
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As the magnet is being pulled out of the coil, the induced current
changes direction, and so the poles of the coil are reversed, this time
there is attraction between the coil and the magnet as it is being pulled
away.
What can we deduct from both cases? (Refer to Lenz’s law)
___________________________________________________________
___________________________________________________________
Lenz’s law and the law of conservation of energy
Lenz’s law is a direct example of the law of conservation of energy.
Energy is used when a current flows round a circuit, so energy must be
used to induce this current in the first place. As it can be seen above,
energy was being used to move the magnet against the opposing force.
Eddy Currents
What if the conductor you're moving through the magnetic field isn't a
wire that allows the electricity to flow neatly away? You still get electric
currents, but instead of flowing off somewhere, they swirl about inside
the material. These are what we call eddy currents. They're electric
currents generated inside a conductor by a magnetic field that can't flow
away so they swirl around instead, dissipating their energy as heat.
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One of the interesting things about eddy currents is that they're not
completely random: they flow in a particular way to try to stop whatever
it is that causes them. This is an example of another bit of
electromagnetism called Lenz's law
Here's an example. Suppose you drop a coin-shaped magnet down the
inside of a plastic pipe. It might take a half second to get to the bottom.
Now repeat the same experiment with a copper pipe and you'll find your
magnet takes much longer (maybe three or four seconds) to make
exactly the same journey. Eddy
currents are the reason. When the
magnet falls through the pipe, you
have a magnetic field moving through
a stationary conductor (which is
exactly the same as a conductor
moving through a stationary magnetic
field). That creates electric currents in
the conductor—eddy currents, in fact,
which cause the magnet to slow
down.
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Transformers
From what we have discussed previously we know that a magnet pushed
into a coil induces an emf. We that if the _____________ is complete, a
current is also induced.
Mutual Induction
If an electromagnet is placed next to coil, as shown above, and the
electromagnet is switched on an emf is induced in the coil next to it for a
fraction of a second.
What happens to the galvanometer?
___________________________________________________________
When the electromagnet is switched off, an emf is produced in the
opposite direction for a fraction of a second. This is like pulling the
magnet away from the coil.
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List two methods of how the induced emf in the second coil can be
increased:
 ______________________________________________________
 ______________________________________________________
When coils are magnetically linked, so that a change in current in one
causes an induced emf in the other this is called mutual induction.
A heart pacemaker uses mutual induction.
Pulses of current through a coil in the
pacemaker unit include pulses in a coil fitted
in the patients’ chest. These trigger
heartbeats.
A Simple Transformer
Insert diagram of a transformer.
AC Voltages can be increased or decreased by using a simple
transformer. A simple transformer works by mutual induction. What is
the
difference
between
AC
and
DC?
___________________________________________________________
When alternating current flows in the primary coil this sets an
alternating magnetic field which induces an alternating current in the
secondary coil. This is because the magnetic field lines from the primary
coil are ‘cutting’ the secondary. Provided that no energy is lost the
following equation applies for transformers
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𝑜𝑢𝑡𝑝𝑢𝑡 𝑣𝑜𝑙𝑡𝑎𝑔𝑒
𝑖𝑛𝑝𝑢𝑡 𝑣𝑜𝑙𝑡𝑎𝑔𝑒
=
𝑡𝑢𝑟𝑛𝑠 𝑜𝑛 𝑜𝑢𝑡𝑝𝑢𝑡 𝑐𝑜𝑖𝑙
𝑡𝑢𝑟𝑛𝑠 𝑜𝑛 𝑖𝑛𝑝𝑢𝑡 𝑐𝑜𝑖𝑙
𝑉2
In symbols:
𝑉1
=
𝑛2
𝑛1
Draw the field inside the transformer using the diagram below:
Questions:
1. In the experiment on the right: what happens when
a. The switch is closed (turned on)
b. The switch is left closed (in the on position)
c. The switch is opened again
2. A transformer has a turns ration of 1:4. Its input coil is connected
to a 12 volt AC supply. Assuming there are no field line or energy
losses.
a. What is the output voltage?
b. What turns ratio would be required of an output voltage of
36V?
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Step Up and Step Down Transformers
How is a transformer able to step up or step down the voltage supplied?
___________________________________________________________
___________________________________________________________
What is the difference between a step up and a step sown transformer?
___________________________________________________________
___________________________________________________________
Power through a transformer
If no energy is wasted in a transformer, the input power will delivered
equally to the output power. So:
𝒊𝒏𝒑𝒖𝒕 𝒗𝒐𝒍𝒕𝒂𝒈𝒆 × 𝒊𝒏𝒑𝒖𝒕 𝒄𝒖𝒓𝒓𝒆𝒏𝒕 = 𝒐𝒖𝒕𝒑𝒖𝒕 𝒗𝒐𝒍𝒕𝒂𝒈𝒆 × 𝒐𝒖𝒕𝒑𝒖𝒕 𝒄𝒖𝒓𝒓𝒆𝒏𝒕
In symbols:
𝑽𝟏 𝑰𝟏 = 𝑽𝟐 𝑰𝟐
Since Voltage x Current is the same on either side of the transformer
increases the voltage and respectively reduces the current in the same
proportion or vice versa.
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National Grid
Power for the AC mains generated in power stations and transmitted
through long distance cables, and then distributed to consumers. The
power supplied by the power station is first passed through a step up
transformer to raise the voltage and reduce current to minimize loss of
electricity.
This power from is then distributed by a series of substations. These
contain step down transformers which reduce the voltage to the level
needed by the consumers. In Europe and Malta, this is 230V.
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