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GCSE level Physics exam revision notes:
Electromagnetism
10.1
Introduction to electromagnetism
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electromagnetism and its uses
10.1
Introduction to electromagnetism - how is it caused by an electric current?
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).
Diagram 1.
You can show the magnetic field patterns by sprinkling fine iron filings
on a sheet of paper at 90o to the copper wire carrying a d.c. electric
current.
Diagram 1.
demonstrate the presence of the magnetic field using iron filings and thickish wire carrying a relatively
high d.c. current.
Diagrams 2. and 3.
For the above diagrams
2. and 3. - 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)
(a) If you reverse the direction
of current flow, you also reverse the direction of the magnetic field.
(b) 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!)
 (c)
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.
It is a non-linear graph.
INDEX for physics notes on
electromagnetism and its uses
Key points about electromagnetism
-
introduction - what is it?
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 Electromagnetism tailored to the
GCSE/IGCSE Physics specifications across major UK exam
boards (AQA, Edexcel, OCR, WJEC, CCEA, CIE):
Electromagnetism – Summary Revision Notes
What is Electromagnetism?
- Electromagnetism
is the interaction between electric currents and
magnetic fields.
- When a current flows through a
conductor, it produces a magnetic field around it.
- This principle is used in
electromagnets, electric motors,
generators, transformers, and more.
| Quantity |
Symbol |
Unit |
| Magnetic flux density |
( B ) |
Tesla (T) |
| Current |
( I ) |
Ampere (A) |
| Length of conductor |
( L) |
Metre (m) |
| Force on conductor |
( F ) |
Newton (N) |
Formula (Force
of Motor Effect):
F = B x I x L
(Only when the conductor is perpendicular to the magnetic
field)
See Part
11.3
Calculating the
size of the force produced by the motor effect
Right-Hand Rule (Grip Rule)
Used to determine the direction of the
magnetic field around a current-carrying wire:
- Thumb:
Direction of current
- Fingers:
Direction of magnetic field lines (circular around the
wire)
For
solenoids:
- Curl fingers in direction of current
through coils
- Thumb points to the north pole
of the electromagnet
Factors Affecting Magnetic Field Strength
- Current
– Higher current = stronger field
- Distance
– Field strength decreases with distance from the wire
- Number of turns
(in a solenoid) – More turns = stronger field
- Core material
– Soft iron core increases strength (used in electromagnets)
-
See Part 10.3
How
to increase the magnetic field strength of a solenoid coil?
Required Practical Experiment Applications
- Plotting magnetic fields
using compasses or iron filings
- Investigating electromagnets:
number of turns, current, core material
- Demonstrating the motor effect
using a wire in a magnetic field
Typical Exam Board-Specific Content relating
to electromagnetism
|
Key Topics |
| Magnetic fields, solenoids,
electromagnets, motor effect, Fleming’s Left-Hand Rule (HT),
transformers (HT), National Grid |
| Magnetic fields, electromagnets, motor
effect, electromagnetic induction, transformers, generator effect |
| Permanent and induced magnets,
magnetic fields, electromagnets, motor effect, transformers, National
Grid |
| Magnetic fields, electromagnets, motor
effect, electromagnetic induction, transformers, National Grid |
| Magnetic fields, electromagnets,
Fleming’s Left-Hand Rule, electric motors, transformers, generator
effect |
| Magnetic fields, electromagnets, motor
effect, electromagnetic induction, transformers, right-hand rule,
generator effect |
Student Tips for Success
- Draw field lines:
Around bar magnets, wires, and solenoids
- Use mnemonics:
e.g. FBI for Fleming’s Left-Hand Rule (Force, B-field, Current)
- Practice diagrams:
Label poles, field directions, and current
- Link theory to real-world:
e.g. MRI machines, scrapyard cranes, electric bells
- Master equations:
Especially ( F = BIL ) and transformer equations (HT)
- Know the difference:
Between permanent magnets and electromagnets
Keywords, phrases and learning objectives
on electromagnetism
Know that electromagnetism is when a magnetic field is
created around a
wire carrying an electric current.
Know that from the direction of convention current, you can
predict the magnetic field direction from north to south
with the right-hand thumb rule.
Be able to describe how to show or plot the magnetic field
around an electromagnet using iron filings or a plotting
compass.
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