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Electricity and magnetism 10: Electromagnetism
solenoid coils - design & uses of electromagnets
Doc Brown's
school physics revision notes: GCSE physics, IGCSE physics, O level
physics, ~US grades 8, 9 and 10 school science courses or equivalent for ~14-16 year old
students of physics
This page will help you answer questions such
as ... How do you make a magnet using electricity? What factors affect the strength of an
electromagnet? What do we use electromagnets for?
Sub-index for this
page.
1.
Introduction to
electromagnetism
(See other
page for
introduction to magnets and magnetism)
2.
The solenoid and
electromagnet applications
3.
How can you increase the magnetic field strength of a solenoid?
4.
Electromagnet uses: magnet, relay
switch, bell, loudspeaker, microphone, Maglev train
1.
Introduction to
electromagnetism
This is all about the connection between
electricity and magnetism.
When a current flows through a wire (or any conductor) a
magnetic field is created around the wire.
The field (of the magnetic flux) can be imagined as a series
of concentric circles at right-angles (perpendicular) to the wire - which is
at the centre of the magnetic field (see the diagrams below).
You can demonstrate the presence of the
magnetic field with iron filings and thickish wire carrying a relatively
high current.
The direction of the magnetic field can
be predicted using Fleming's left-hand rule (illustrated above).
For the above diagram - imagine the
current flowing through a straight wire and the magnetic field can be
envisaged as a series of concentric rings about the axis of the conducting
wire.
The direction of the magnetic field can be predicted from
the 'right-hand thumb' rule.
If the current is flowing 'up' through
the wire, the magnetic field runs anticlockwise and perpendicular to the
wire.
You can show the direction of the field with a small
plotting compass - two shown on the diagram above - and you can trace out the
circular pattern of the magnetic field.
A few simple rules (apart from the
right-hand thumb rule)
(i) If you reverse the direction
of current flow, you also reverse the direction of the magnetic
field.
(ii) The strength of the magnetic
field is increased overall by increasing the current.
(Don't say by 'increasing the
p.d.' without saying to increase the current!)
(iii)
For any current carrying wire,
the closer you are to the wire, the greater the strength of
the magnetic field.
 The magnetic field flux lines get closer and closer
together the nearer you are to the wire - meaning the magnetic field
strength increases the nearer you are to the wire.
The strength of the magnetic
field falls away quite rapidly at first as you get further from the
wire, then the reduction rate slows down with increasing distance.
Its a non-linear graph.
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2.
The solenoid and
its
electromagnet applications
We have seen that a single current
carrying wire produces a magnetic field of concentric lines of force.
This in itself is of little use, but,
there are ways of increasing the magnetic field effect to produce something
of use in many 'electromagnetic' applications.
Left: The magnetic field produced a solenoid coil,
note the linear and denser concentration of the lines of force down the
centre of the coil.
Right diagram above: A solenoid coil containing a soft iron core,
around which is coiled insulated copper wire.
An effective solenoid needs to consist of
hundreds of coils of finely wound insulated copper wire.
Such an iron-cored solenoid coil can act as a
temporary magnet.
 The
principles of a functioning solenoid
If you coil the wire in a compact way (as
in the diagrams above) you can greatly intensify the magnetic field effect.
The stretched out resulting current carrying coil
is called a solenoid and can act as an electromagnet which
can be switched on and off depending whether current is flowing or not
i.e. acts as a temporary magnet.
You can 'construct' this magnetic
field diagram using a plotting compass to map the magnetic field of a
steel bar permanent magnet.
Inside the coils, the increase in field strength is due
to all the lines of force lining up with each other and close
together too - intensifying the magnetic field effect at what is
effectively another north-south pole situation.
Remember - the closer the lines of
force the greater the strength of the magnetic field at that point.
So, note the uniformity and
intensity of the magnetic field inside the coil, which is much
weaker outside the coil because lots of overlapping lines of force
around each coil cancel each other out.
The magnetic field is overall
weak except at the ends of the solenoid where it is very strong.
Note: The magnetic field pattern
outside the solenoid is just the same as a
bar
magnet, with a north and south pole and the magnetic flux
lines flowing from north to south.
Polarity of a solenoid coil
You can work out the polarity of a
solenoid by viewing the end of the solenoid and observing the way the
current is flowing.
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3.
How can you increase the magnetic field strength of a solenoid?
As we have seen, due to the alignment of the
parallel lines of
force, the magnetic field inside the solenoid is very uniform and very
strong, but only in the coil and the poles at the ends of the solenoid,
so how can we increase the magnetic effect i.e. make a stronger
electromagnet?
(a)
Increasing the current flow
The strength of the magnetic field
is increased overall by increasing the current.
Any stream of moving electrically
charged particles naturally creates a magnetic field.
The more charged particles moving
through the wire, the greater the magnetic field effect.
(Don't say by 'increasing the
p.d.' without saying to increase the current!)
(b)
Increasing the number of coils of
wire
Many solenoids consist of hundreds of
turns of thin insulated copper wire.
The more coils packed tightly
together, the greater the strength of the magnetic field.
You can do this by (i) increase the
number of coils using the same length of wire OR (ii) you can both
increase the number of coils AND the total length of wire.
(c)
Using an soft iron core - this
makes a practical electromagnet
If you place a rod of magnetic
material like iron, inside the solenoid, the iron becomes an induced
magnet and the magnetic lines of force are intensified through it.
As with the 'empty' solenoid its
self, the magnetic flux is greatest at the ends of the solenoid, which
now coincides with the ends of the magnetically 'soft' iron rod (left
diagram).
The solenoid plus the iron rod
are effectively make a strong 'bar magnet' (right diagram).
As long as the current is flowing the
electromagnetic effect will work.
Switch off the current and the
magnetic effect goes.
This means you can use this system as
an on/off temporary electromagnet that has many useful
applications.
(d)
Decreasing the length of the
solenoid
If you can compact the solenoid
to a shorter length for the same number of insulated coils of wire you
increase the intensity of the magnetic field.
I don't consider this an
important factor since the insulated coils of wire are usually
packed as tightly together as possible and length might be
determined by how it fits into some device.
In most applications it is factors (b)
and (c) that are employed to increase the effectiveness of the solenoid.
Coil 1. Just a plain solenoid coil,
producing a relatively weak magnetic field.
Coil 2. This solenoid produces a much
greater strength of magnetic field due to the addition of the iron rod.
Coil 3. Using two iron rods, or one
thicker one, the filed strength is increased.
Coil 5. Unlike coils 2. and 3., which are
temporary electromagnets (on/off with current), coil 5 would make a steel
rod a permanent magnet.
Coil 5. could not be used as an
on/off electromagnet, but it is a way of making permanent magnets.
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4.
Uses of
electromagnets
As we have seen, due to the alignment of the
parallel lines of
force, the magnetic field inside the solenoid is very uniform and very
strong, but only at pole at the ends of the solenoid.
Therefore in the applications of a
solenoid electromagnet, the mechanical actions is centred on the poles.
(a)
An electromagnet for picking up
things
An electromagnet that can be switched on and
off has many uses.
A good examples is picking up scrap
iron or steel in a recycling yard.
The 'magnetic' crane can pick up
these items and dump them down wherever you want by switching the
current to the electromagnet on and off. Pressing the 'on' switch
induces a magnet field in the iron 'pickup'. You move the scrap iron
or steel to another location, then switch 'off' the current. The
iron 'pickup' loses its magnetism and the scrap falls to the ground.
(b)
Relay switch system
You can use an electromagnet in one
circuit to operate another circuit.
A switch in a primary circuit
automatically controls the 2nd circuit via an electromagnetic switch
system.
Consider the relay system in
the diagram below.
When you switch on the input circuit (closing switch (1))
the current flows through the solenoid (2).
Inside the solenoid coil is a soft iron core which becomes
magnetised only when the current flows.
The solenoid electromagnet attracts the soft iron armature
(the pivoted 'rocker') which is rotated anticlockwise.
When the 'rocker' rotates it pushes the contacts at (4)
together to close the output circuit.
In this case the output circuit drives an electric motor, but could
be anything you want to switch on remotely using a low
voltage-current circuit.
Uses of a two circuit relay
system
(i) This system is used where
the output circuit might be operating with a potentially
dangerous high p.d. or current.
This is how the starter
motor of car is operated. You don't want the high current
needed by the starter motor moving through a circuit where you put the ignition key in!
(ii) The output circuit might
be in a hazard zone e.g. remote control systems in a nuclear
power plant where machinery is operating where there i
potentially or actually, radioactive materials - obvious
dangers!
(c)
Electric bell
The electric bell circuit
The d.c. power supply is not
shown in the diagram, but the terminal connections are on the
left.
When you press the doorbell
you close a circuit that allows current to flow to magnetise the soft iron core of
the
solenoids.
The magnetised soft iron core
of the solenoid attracts the striker to hit and ring the bell.
In moving, the striker also
breaks the circuit switching off the current and so the electromagnetism of the
solenoid.
Therefore the 'sprung' striker then
returns to close the circuit, re-magnetising the solenoid soft
iron core, so the striker is attracted again to strike the bell.
This happens quite quickly to
give a continuous ringing sound.
As long as you press the
doorbell, the circuit keeps on being opened and closed to give
the bell ringing effect.
(d)
Magnetic separators
in a recycling plant
A magnet can be used to
pick out scrap iron and steel from a conveyer belt of rubbish.
The items might be cans or
steel grills etc. An electromagnet is used for the process.
(e)
Maglev trains
(maglev is shorthand for 'magnetic levitation' but not of the spirit
world!)
Maglev trains use magnetic
repulsion to literally float a train a short height above the
guidance track. A magnetic field can be
manipulated to move the train along at high speeds with
virtually no friction except for air resistance.
Maglev (derived from magnetic
levitation) is a system of high train transportation that uses
two sets of magnets: one set to repel and push the train up off
the track, and another set to move the elevated train ahead,
taking advantage of the lack of friction.
With maglev technology, there is just one moving part: the train
itself. The train travels along a guideway of magnets which
control the train's stability and speed.
Since the propulsion and
levitation require no moving parts Maglev trains are quieter and
smoother than conventional trains and have the potential for
much higher speeds.
Note that electromagnets do not produce a permanent magnetic
force. The magnetism they produce is temporary, and can be
switched off and on depending upon what is required. Engineers
use electromagnetism in the design and construction of maglev
trains.
(f)
Loudspeakers and
microphones
Loudspeakers and microphones
use an oscillating electromagnet system running off alternating
current (a.c.).
For more details see
Motor effect of an electric
current including the loudspeaker
gcse
physics
Generator effect - applications
- including the microphone
gcse
physics revision
(g)
MRI scanners magnetic
resonance imaging
MRI scanners use powerful
electromagnets to create detailed images of the inside of your
body.
It is a relatively safe
technique that does not use ionising radiation, instead it uses
safer EM radio waves.
The high frequency radio
waves resonate with protons (H atoms) in you body and this
resonance is detected and used to build up an image based on
where the protons are and their density or concentration - and
there are a lot of them in your body e.g. water, fat, protein
etc.
(h)
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What next?
Electricity and
magnetism revision
notes index
1.
Usefulness of electricity, safety, energy transfer, cost & power calculations, P = IV = I2R,
E = Pt, E=IVt
2.
Electrical circuits and how to draw them, circuit symbols, parallel
circuits, series circuits explained
3. Ohm's Law, experimental investigations of
resistance, I-V graphs, calculations V = IR, Q = It, E = QV
4. Circuit devices and how are they used? (e.g.
thermistor and LDR), relevant graphs gcse physics revision
5. More on series and parallel circuits,
circuit diagrams, measurements and calculations
gcse physics
6. The 'National Grid' power supply, environmental
issues, use of transformers
gcse
physics revision notes
7.
Comparison of methods of generating electricity
gcse
physics revision notes (energy 6)
8. Static electricity and electric fields, uses
and dangers of static electricity gcse
physics revision notes
9.
Magnetism
- magnetic materials - temporary (induced) and permanent magnets - uses gcse
physics
10.
Electromagnetism, solenoid coils, uses of electromagnets gcse
physics revision notes
11. Motor effect of an electric current,
electric motor, loudspeaker, Fleming's left-hand rule, F = BIL
12.
Generator effect, applications e.g. generators
generating electricity and microphone
gcse
physics
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