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GCSE level Physics exam revision notes:
Electromagnetism
10.4
Uses of solenoid coils and electromagnets - magnet, relay switch, bell,
loudspeaker, microphone, Maglev train, MRI scanner
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INDEX for physics notes on
electromagnetism and its uses
10.4
Examples of uses of solenoid coils and electromagnets
What do we use
solenoid coils for? Electromagnets are a good example?
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.
e.g.
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.
(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)
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.
(e)
Loudspeakers and microphones
Loudspeakers and microphones
use an oscillating electromagnet system running off alternating
current (a.c.).
For more details
of (e) see
Motor effect of an electric
current including the loudspeaker
Generator effect - applications
- including the microphone
(f)
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.
INDEX for physics notes on
electromagnetism and its uses
Key points about electromagnetism
-
Information
sources for Doc Brown's key points: IGCSE-GCSE physics are based on
textbooks & syllabus-specifications for students taking the UK AQA, Edexcel,
OCR 21st Century Science, OCR Gateway science suite, WJEC, CCEA and CIE GCSE
physics 9-1 level science examinations
Here's a comprehensive set of
summary revision notes on solenoid coils, tailored
to the GCSE/IGCSE Physics specifications across major UK
exam boards (WJEC, CCEA, CIE, AQA, Edexcel, OCR):
REMINDERS
What is a Solenoid?
A solenoid is a
long
coil of insulated wire wound in a helical shape.
When an electric current passes through it, it produces a
magnetic
field similar to that of a bar magnet.
- Magnetic field inside:
strong, uniform, and parallel
- Magnetic field outside:
weaker, similar to bar magnet field lines
- Polarity:
- Current flows anticlockwise
→ North pole
- Current flows clockwise
→ South pole
How to Increase Magnetic Field Strength
- Increase the
current
- Increase the
number of turns per unit length
- Insert a soft iron core
(creates an electromagnet)
- Use
tightly packed coils
Applications of
Solenoids
| Application |
Description |
Why Solenoids Are
Used |
| Electromagnets |
Solenoid with an iron core |
Magnetic field can be switched on/off |
| Electric bells |
Attracts armature to strike bell |
Repeated switching creates ringing |
| Relays |
Switches circuits using electromagnet |
Allows small current to control large
one |
| Scrapyard cranes |
Lifts magnetic materials |
Strong, controllable magnetic field |
| MRI machines |
Generates strong magnetic fields |
Uniform field needed for imaging |
| Solenoid valves |
Controls fluid/gas flow |
Converts electrical signal to
mechanical action |
| Speakers and microphones |
Converts electrical signals to sound
(and vice versa) |
Uses motor/generator effect with
solenoids |
| Railway signal systems |
Solenoids can switch tracks and
operate signals |
|
Typical Exam Board Specification Content
|
Key Focus Areas |
| Magnetic fields in solenoids,
electromagnets, applications like bells and relays |
| Magnetic field patterns, solenoid
structure, practical uses |
| Magnetic field strength, solenoid
versus bar magnet, electromagnetism |
| Solenoid field shape, increasing
strength, uses in devices |
| Electromagnets, solenoid field lines,
practical applications |
| Right-hand grip rule, solenoid
polarity, induced magnetism, applications |
Student Tips for Exams
- Draw field lines:
Show parallel lines inside solenoid, curved lines outside
- Use the right-hand grip rule:
Thumb = field direction (North), fingers = current
- Explain clearly:
Use terms like “uniform field,” “induced magnet,” “electromagnet”
- Link to real-world devices:
Bells, cranes, relays, speakers
- Practice polarity questions:
Clockwise versus anticlockwise current
- Remember control:
Solenoids allow magnetic fields to be turned on/off
- Link to electromagnetism:
Explain how current creates a magnetic field that moves a plunger or
armature.
- Mention energy conversion:
Electrical → Magnetic → Mechanical.
- Use diagrams: Especially
for electric bells, relays, or valves.
Keywords, phrases and learning objectives
on electromagnetism
Be able to describe and explain the uses of solenoid coils
in electromagnets e.g. magnetic crane
or magnetic recycling separator, how it functions in a relay switch
or an electric bell,
the working of a loudspeaker coil, the solenoid in a microphone.
Know that electromagnets are used in the Maglev train
and an MRI scanner.
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