You can demonstrate this electromagnetic
induction in the following ways ...
Demonstration 1 Stationary coil and
moving magnet.
Left: Moving a magnet in and out of a coil. Right:
Moving a wire through a magnetic field.
Both simple demonstrations induce a tiny
current in the wire -
electromagnetic induction.
There are several ways you can increase the
induced potential difference (and hence the current flow)
- Moving the wire or the magnet faster -
increase rate of the magnetic field passing through the wire in
which the current is induced.
- Using a more powerful magnet - increase
in the magnetic flux passing through the wire.
- Using a multi-coil of wire - the same
magnetic field is interacting with a greater length of wire.
These factors are considered when designing
electric motors or electrical generators.
You set up a circuit consisting of an insulated copper wire
coil connected to a very sensitive ammeter.
You can use a 'modern' digital mA ammeter or a
galvanometer - a 'pointer' dial version of a very sensitive ammeter from
before the days of digital instruments! You also need a permanent
magnet.
If you bring the magnet near the coil, but stationary,
nothing appears to happen.
BUT, if you move the permanent magnet 'in and out' of
the coil, a p.d. and current flow are induced in the coil.
It is the change in the magnetic field the wire
experiences that induces the pd and current flow in it.
So, to start with, keeping the poles of the magnet
pointing in the same direction ...
when the magnet 'goes in' you should get a 'blip' of
an ammeter reading above 0.0 A in one direction and when you pull
the magnet out, you get a 'blip' of an ammeter reading of less than
0.0 A.
The reason for these two opposite, but numerically equal
ammeter readings, is that the coil and magnet are constants BUT if
you change the direction of motion you change the direction of the p.d.
and induced current.
Similarly, if you swap the poles of the magnet around,
the two readings are reversed - the +ve ammeter reading becomes -ve and
the -ve reading becomes +ve.
In other words, if you change the direction of
the magnetic field you change the direction of the induced p.d. and
current.
So, reversing one thing reverses another and you only
get an induced p.d. with movement!
Note: If you keep on moving the magnet in and out of the
coil you produce a continuous alternating current (a.c.) - this
is the principle by which an alternator works, but you keep the magnet
stationary and move the wire.
See the second demonstration of electromagnetic
induction described below.
Demonstration 2 A moving coil and
stationary magnet.
You need a U shaped permanent magnet or two permanent
magnets.
The coil of wire is connected to a sensitive ammeter.
While the coil is stationary the ammeter reading stays at
zero (0.0 A)
However, as you move the coil in and out of at 90o
to the magnetic field you induce a p.d. and current.
It is the change in the magnetic field the wire
experiences that induces the pd and current flow in it.
So, again, reversing one thing reverses another
and you only get an induced p.d. with movement!
When you move the coil in one direction the induced
current flows one way (e.g. ammeter reads >0.0 A) and if you move the
coil in the other direction, the induced p.d. and current are also
reversed (e.g. ammeter reads <0.0 A).
Here the magnetic field has a constant direction
but the motion is continually reversed, once again producing an
alternating current as the induced p.d. is also reversed.
Demonstration 3. Other ways of moving the magnetic field
and coil relative to each other
There are all sorts of other demonstrations to show
electromagnetic induction e.g.
(i) You can rotate a magnet inside a larger coil
The coil is stationary, but the magnetic field is
constantly changing and 'cutting' through the wire.
For every half-turn of the magnet, the direction of
the magnetic field reverses and so the p.d. reverses too and the
current flows in the opposite direction.
Therefore, with continuous rotation, you produce an
alternating current.
(ii) You can rotate a coil in a stationary magnetic
field
Here the wire is continually 'cutting' through a
magnetic field.
This is a more controlled variation of demonstration
2 and also see the
design of simple generators.
REMEMBER: It is the change in the magnetic field the
wire experiences that induces the pd and current flow in it.
So, both these demonstration amount to the same thing -
conducting wire and magnetic field moving relative to each other,
and ....
... the rotation of the magnet or the coil is actually how
generators work to produce either ...
(i) an alternating current (ac), in which the
current direction periodically reverses,
or (ii) a direct current (dc) which only
flows in one direction.
(3) Two other things
to consider before looking at examples of generators ...
(a) An induced
current opposes the change that caused the induction!
We have seen that a change in magnetic field induces a
current in a conductor e.g. a copper wire.
BUT, when a current flows through a wire, a magnetic
field is created around the wire.
So, we are now dealing with two magnetic fields.
The magnetic field produced by the induced current
in the wire always acts against the change that made it,
AND, it doesn't matter whether the induction is due to
the movement of the wire or the movement of the magnetic field.
It's as if the 'system' is trying to change back to
where it started i.e. the induced current opposes the change that
made it.
At first you might think - how can we continuously
extract electrical energy from the induction system?
BUT, remember, a generator requires a constant
input of kinetic energy from e.g. a diesel engine or steam/water
turbine.
You are building up an electrical energy
store from another energy store!
You can't get energy for nothing!
(b) How can
you increased the induced potential difference
It is important to know how change the size of the
induced p.d. or current flow.
To change the size of the induced pd you must change
the rate at which the magnetic field changes.
You are usually interested in increasing the pd or
current or both at the same time e.g.
Increasing the speed of rotation (motion)
- increasing the kinetic energy input
Increased rotation of the
coil or magnet means more magnetic lines of force are cut per unit
time.
For a given strength of magnetic field the
density of the lines of force are constant, but with increased
motion you move through them faster.
Increasing the strength of the magnetic field
with a more powerful magnet
The greater magnetic flux
density means more magnetic lines of force are cut per unit time.
Remember - the greater the strength of the
magnetic field from a stronger magnet, the closer together are the lines of force.
Increasing the number of turns
of wire on the coil
The greater the density of
the coils, the more of the conductor the magnetic lines of force
cut through.
TOP OF PAGE
and sub-index
(4)
Practical
GENERATORS
A generator is a practical way of producing a
continuous supply of electricity.
There are two main types -
(a) the a.c. alternator - the
current direction is changing,
(b)
the d.c. dynamo - the current only flows in one direction.
Both types make use of the generator effect to induce
current, and the two simple generators described below both involve
rotating coils of wire in a fixed magnetic field cutting through lines
of force.
They both involve a similar circuit construction BUT
there are some differences, so take care!
Both types described here involve rotating the coil in a
fixed magnetic field, so the coil is continuously cutting through the
magnetic flux lines of force.
The generator effect
All generators must have a source of
power to rotate the coil of wire.
As the coil spins, it cuts through
the magnetic field and a current is induced in the coil.
Dynamos are d.c. generators and
alternators generate an a.c. current.
Electromagnetic induction - inducing
a current in a rotating coil cutting through a magnetic field.
The direction of rotation can be
predicted from Fleming's right-hand rule (NOT on the GCSE
specification)
Consider the right side of
the coil for clockwise motion (could be part of simple dc dynamo generator)
The thuMb represents the direction the
force acts
-
direction of motion - downwards (emphasise the M).
The
First Finger represents the direction of the
magnetic field N => S (phonetically emphasise the F).
The SeCond finger
predicts the direction of the
induced convention current (emphasise the 'hard' C).
Repeat for the left side of the
coil, moving upwards, and you should predict the current will be
flowing in the opposite direction.
You should eventually appreciate
the following:
(i) a d.c. motor and a d.c.
generator are essentially of the same construction, and,
(ii) an a.c. motor and an a.c.
generator are essentially of the same construction,
because they are constructed
of similar components, so,
in an electric motor the
electrical current energy is converted into kinetic energy, and,
in an electrical generator,
the kinetic energy is converted into electrical energy.
TOP OF PAGE
and sub-index
5. The
alternator a.c. generator - producing an
alternating current/p.d.
A simple ac ALTERNATOR
Reminders: All generators
must have a source of power to rotate the coil of wire.
As the coil spins, it cuts
through the magnetic field and a current is induced in the coil.
Dynamos are d.c. generators and
alternators generate an a.c. current.
Here, for this simple design of
an a.c. generator, the direction of rotation is predicted from
Fleming's right-hand rule.
Explaining how a simple ac
alternator generator works
The construction is very similar to that of a
simple electric motor.
The coil is rotated through the magnetic field by some
external power source of kinetic energy.
As the coil rotates, cutting through the magnetic
field, a current is induced in the coil.
The current will change direction after every
half-turn.
To 'extract' the electrical current i.e. connect with
the external circuit, ac generators use a system of slip rings and
brushes.
This means the contacts don't swap every half-turn
and so an alternating current (ac, alternating p.d.) is
produced.
See the oscilloscope traces
below of p.d. versus time - note the full oscillating wave shape
of the trace.
This is different from the split-ring commutator
used in the
simple electric motor and the
dynamo generator described next.
Brush contacts allow continuous electrical
connection without inhibiting the movement of the commutator.
Comparing the
output from an a.c. alternator and a d.c. dynamo generator
(repeated in section 6.)
CRO oscilloscope traces from generators
An oscilloscope can show how the p.d. across the coil of
a generator varies with time.
Three examples of oscilloscope traces from generators
are shown above (x axis = time, y axis = pd).
1. This trace shows an alternating current i.e. the
p.d. is changing from +ve to 0 to -ve values in a continuous cycle.
You can tell its an a.c. trace because it goes up
and down of the horizontal axis of p.d. 0 V.
The height of the trace above 0 V at any point
tells you the p.d. generated at that point.
Note the full oscillating
wave shape of the trace.
2. This is also a trace from an alternator
generator, but the rotation of the coil is greater than for 1.
The higher the peak from the 0 V horizontal
axis, the greater the potential difference generated.
Note the full oscillating
wave shape of the trace.
Note the full oscillating
wave shape of the trace.
3. This is a trace from a dc dynamo generator
You can tell it is not an alternating current
because the trace consists of a succession of half-cycles.
The a.c. generator describe above will produce
traces 1. and 2.
Large a.c. alternator generators are
used in power stations producing electricity for power lines of a national
grid system.
See
Energy resources: uses, survey, trends,
comparing renewables, non-renewables, generating electricity
and
The 'National Grid' power supply, environmental
issues, use of transformers
TOP OF PAGE
and sub-index
6. The
dynamo dc generator - producing a direct
current/constant p.d
A simple DYNAMO
Reminders: All generators
must have a source of power to rotate the coil of wire.
As the coil spins, it cuts
through the magnetic field and a current is induced in the coil.
Dynamos are d.c. generators and
alternators generate an a.c. current.
Explaining how a simple dc
dynamo generator works
A dynamo works like an alternator in that the coils are
rotated through the magnetic field by some external source of kinetic
energy.
However, unlike the alternator, but like the electric
motor, it uses a split-ring commutator and NOT slip rings, to
connect with the external circuit.
The split ring commutator swaps the connections
every half-turn so the current keeps flowing in the same direction -
direct current generation (dc).
However, because of the rotation
of the dynamo coil, you do not get a constant p.d. and you get peaks
as with the output of an a.c. alternator, but no change from a +ve
to a -ve p.d.
See the oscilloscope traces
below of p.d. versus time - note the lack of oscillating wave
shape of the trace, its just a series of oscillating 'humps' or
half-waves.
Brush contacts allow continuous electrical
connection without inhibiting the movement of the commutator.
Comparing the
output from an a.c. alternator and a d.c. dynamo generator
(repeat from section 5.)
CRO oscilloscope traces from generators
An oscilloscope can show how the p.d. across the coil of
a generator varies with time.
Three examples of oscilloscope traces from generators
are shown above (x axis = time, y axis = p.d.).
1. This trace shows an alternating current i.e. the
p.d. is changing from +ve to 0 to -ve values in a continuous cycle.
You can tell its an a.c. trace because it goes up
and down of the horizontal axis of p.d. 0 V.
The height of the trace above 0 V at any point
tells you the p.d. generated at that point.
Note the full oscillating
wave shape of the trace.
2. This is also a trace from an alternator
generator, but the rotation of the coil is greater than for 1.
Therefore trace 2. shows a
greater a.c. frequency than 1.
The higher the peak from the 0 V horizontal
axis, the greater the potential difference generated.
The maximum p.d. is greater
in CRO trace 2 than trace 1 - you can tell from the greater
amplitude.
Note the full oscillating
wave shape of the trace.
3. This is a trace from a dc dynamo generator
You can tell it is not an alternating current
because the trace consists of a succession of half-cycles.
The d.c. generator describe above will only
produce trace 3.
7. How can you
increase the p.d. and power output from a dynamo?
As already mentioned, you need to increase the rate of
cutting through the magnetic flux - the lines of force.
There are four ways to do this based
on the simple ac and dc generators described above ...
(i) Increasing the number of turns
of wire in the coil.
The magnetic lines of force
'cut' through more wire per unit time.
(ii) By winding the coil on a soft-iron armature to
increase the magnetic flux through the coil.
Iron concentrates the lines
of force, so more lines of force are 'cut' through per unit
time.
(iii) Increasing the rate of rotation of the coil or
magnet.
More magnetic lines of force
are 'cut' through per unit time.
(iv) By making the field magnet as strong as
possible.
The magnetic flux is more
dense, the lines of force are closer together, so more lines of
force are 'cut' per time.
(v) Making the distance between
the coil and the magnet as small as possible.
These factors apply to any
electric generator design.
TOP OF PAGE
and sub-index
8. How does a
microphone work?
a microphone works due to electromagnetic
induction
A microphone works in the opposite
way to a
loudspeaker.
The diagram above illustrate the principle of a
microphone e.g. for a vocalist or a telephone mouthpiece.
A microphone converts the energy of
the pressure variation of sound waves into an
electrical energy signal in an ac current.
The oscillation of the sound waves
vibrates the diaphragm which induces an oscillation in the electrical
circuit - the electrical signal.
The electrical signal could in turn
be used to re-generate sound in a
loudspeaker.
A microphone behaves like a loudspeaker in reverse -
The right-hand side of the diagram was borrowed from my loudspeaker
diagram!
The coil of wire surrounds
one pole of a permanent magnet, which is itself surrounded by
the other pole of the magnet - look for the
N and
S
on the diagram above.
The coil, in which the current
is induced, is connected to the flexible diaphragm cone cover made of
thin plastic or metal.
If the coil moves in the
magnetic field, a p.d. is induced in the coil.
This is no different in principle
to the working of a generator - a coil moves through a magnetic
field,
but there is no rotation, it is a
'to and fro' vibration effect.
The initial movement comes
from the sound waves hitting the flexible diaphragm.
The sound wave vibrations cause
the diaphragm to vibrate and move in resonance with the sound wave.
In turn, the coil
automatically moves at the same frequencies as the sound wave
inducing a varying current in the coil.
Also, the louder the
sound, the bigger displacement of the coil.
Therefore the movement of the
coil generates (induces) an electrical current signal that can be used to reproduce
both the frequencies and relative volumes (loudness) of the sound
waves in a loudspeaker system or a recording device
like a tape recorder.
See also
Sound waves nature and properties explained
Transformers are dealt
with on another page
TOP OF PAGE
and sub-index
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
ALL My GCSE
Physics Revision Notes
OR use the [GOGGLE
SEARCH BOX]
