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GCSE level physics revision notes:
Magnetism
9.2
Plotting magnetic fields with a compass and drawing
the field lines pattern
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INDEX for physics notes on
magnetism
9.2
Magnetic fields - plotting them, drawing
them
You can map out the magnet field around a bar magnet by
using a small plotting compass.
A magnetic field is the region around a magnetic
(magnetised) object where a force acts on another magnet or another
magnetisable object/material.
represent compass plotting positions on the diagram below (N=>S).
The magnetic field lines should indicate that the
'magnetic flux' runs from the north pole to the south pole.
Inside the plotting compass a tiny bar magnet (compass needle)
aligns itself so that its north pole is attracted to the south pole
of the magnet it is near.
The same thing happens with a navigation
compass, which aligns itself with the Earth's magnetic field.
Your plotting compass does the same thing when well away from the
magnet - which shows the Earth creates a magnetic field -
because the Earth has an iron core and this swirling mass of
mainly molten iron generates a magnetic field.
Plotting! On a
sheet of white paper, place a bar magnet in the middle and draw a
rectangle around it, so you can always make sure it is in the same
place when plotting the magnetic field.
You carefully select a starting point close to a
pole and mark on the paper a dot by the end of the compass needle
arrow closest to the compass (* →).
Then mark another dot at the other end of the
compass needle arrow (→ *).
Then, move the plotting compass around, making a dot adjacent to
each end of the compass needle as you move from one position to
another.
Gradually a line a force can be drawn by joining
up the points all the way round from one end to the other end of
the magnet.
You just
repeat the procedure from a different starting point from the
north pole.
How to plot the magnetic field of a bar magnet
The results of plotting a magnetic field around a permanent bar magnet.
By convention, the lines of force run
around from the north pole to the south pole.
The lines of force show which way a
force would act on a north pole if it was placed at that point in the
field - see how the compass needle aligns itself along the line of
force.
The closer the lines of force are
together, the stronger the magnetic field.
The lines of force are closer
together nearest the poles - this means the strength of the magnetic
field is strongest close to the poles.
The further you get away from a
magnet, the weaker the magnetic field.
The strength of a magnetic field is
called the magnetic flux density and measured in teslas (T).
For on magnetic field density see
Electromagnetism, solenoid coils, uses of electromagnets
B is the magnetic field lines diagram for the attraction of unlike poles
Diagrams A and B show the magnetic field
lines around a
permanent bar magnet.
How to plot this was described above with the lines of
force running from north to south.
You can clearly show this pattern
using iron filings.
You place a permanent bar magnet
under a sheet of paper or plastic.
You then sprinkle iron filings
randomly on top of the sheet.
Then, tap the sheet gently and
repeatedly until the pattern of the magnetic field emerges.
The permanent magnet induces
magnetism in the iron filings and converts them into temporary
magnets which all line up - N - S - N - S - etc. along the lines of
force of the magnetic flux.
Its quicker than doing the
compass plot method, but you don't end up with a plot unless you
photograph it!
Diagram C shows the magnetic field
lines when
unlike poles of two bar permanent bar magnets are
brought close together and attracted.
When a N-S or S-N attraction happens the lines of force
join up from one magnet to another in a uniform pattern and the lines of force still run from
a north pole to a south pole.
D and E show the magnetic field lines diagram for the repulsion
of like poles
Diagrams D and E show when like poles
of two permanent magnets are brought close
together and repulsion takes place.
In this case the lines of force do NOT join together
from one magnet to the other, but they still run from a north pole to a
south pole.
INDEX for physics notes on
magnetism
Key points about magnetism -
plotting the magnetic field of a permanent magnet
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 detailed and exam-board-aligned
summary of how to plot the magnetic field of a permanent magnet,
tailored for GCSE/IGCSE Physics students across WJEC, CCEA, CIE, AQA, Edexcel, and OCR.
Plotting the Magnetic Field of a Permanent
Magnet
What Is a Magnetic Field?
- A magnetic field is the
region around a magnet where magnetic forces act.
- Field lines show the direction
and strength of the magnetic field.
- Direction:
Always from the north pole to the south pole
outside the magnet.
- Strength:
Strongest at the poles (where lines are closest together).
Required Practical: Plotting Magnetic Field
Lines
Using a Plotting Compass
|
Step |
Description |
| 1 |
Place a bar magnet
in the centre of a sheet of paper. |
| 2 |
Mark a dot
near one end of the magnet. |
| 3 |
Place a plotting
compass at the dot. The needle aligns with the field. |
| 4 |
Mark a second dot at the
tip of the compass needle. |
| 5 |
Move the compass so its
tail is on the new dot. Repeat the process. |
| 6 |
Join the dots with a
smooth curve to form a field line. |
| 7 |
Repeat from different
starting points to map the full field pattern. |
Plotting compasses contain a tiny bar
magnet. The needle’s north pole points in the direction of the magnetic
field.
Alternative Method: Iron Filings
- Sprinkle iron filings on
a sheet of paper placed over the magnet.
- Tap gently to reveal the field
pattern.
- This method shows shape and
strength, but not direction.
- Not really a satisfactory method at this
academic level, but could be photographed for a permanent record.
Typical Exam Board Requirements
|
Key Focus Areas |
| Describe and draw magnetic field
lines; use of plotting compasses. |
| Plotting field lines using compasses;
field strength and direction. |
| Practical method for plotting fields;
field line rules. |
| Field line patterns; compass method;
strength and direction. |
| Use of compasses and iron filings;
field line diagrams. |
| Plotting field lines; direction from
north to south; field strength. |
Student Tips for Exams
- Draw arrows
on field lines: from north to south.
- Spacing matters:
Closer lines = stronger field.
- Never cross field lines:
They must be smooth and continuous.
- Label poles clearly:
N and S on the magnet.
- Explain the method:
Especially for 6-mark practical questions.
Keywords, phrases and learning objectives
on magnetism
Be able to do, and describe with diagrams, the
method of how to plot and draw a magnet field of a bar magnet
using a plotting compass.
Be able to describe and sketch magnetic fields when like poles
are brought close together (repulsion) and unlike poles are brought
close together (attraction).
All the field line diagrams should be clear and
dealing with the magnetic fields of permanent
bar magnets.
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how to plot magnetic fields with compass
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explaining the theory of plotting - how to plot magnetic fields with
compass, revision preparation for AQA GCSE physics,
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Phil Brown 2000+. All copyrights reserved on Doc Brown's physics revision notes, images,
quizzes, worksheets etc. Copying of website material is NOT
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are unofficial. These revision notes on plotting - how to
plot magnetic fields with compass are suitable for AQA gcse physics,
plotting - how to plot magnetic fields with compass revision notes for Edexcel gcse
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INDEX for physics notes on
magnetism
(excluding
electromagnetism)
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