IGCSE Physics 0625 – Simple Phenomena of Magnetism
4.1 Simple Phenomena of Magnetism
Learning Objective
Describe how to plot magnetic field lines using a compass or iron filings and explain how a compass can be used to determine the direction of the magnetic field.
Key Concepts
A magnetic field is a region around a magnet where magnetic forces can be observed.
Field lines are a visual representation of the direction and relative strength of the magnetic field.
Field lines emerge from the north pole and enter the south pole of a magnet.
The density of field lines indicates the strength of the field – closer lines mean a stronger field.
Plotting Magnetic Field Lines with a Compass
Materials needed:
Bar magnet (or any permanent magnet)
Small compass (preferably a magnetic needle on a low‑friction pivot)
Sheet of white paper
Pencil to mark the compass direction
Procedure:
Place the magnet on the centre of the paper and secure it so it does not move.
Position the compass at a point some distance away from the magnet.
Allow the compass needle to come to rest. The needle points along the direction of the magnetic field at that point.
Using a pencil, draw a short line segment in the direction indicated by the needle. This line represents a small segment of a field line.
Move the compass a short distance along the drawn line and repeat steps 2‑4.
Continue the process, gradually tracing a continuous curve that starts at the north pole and ends at the south pole.
Repeat the whole process from several starting points around the magnet to obtain a complete set of field lines.
Important notes:
Keep the compass at the same height above the paper to avoid distortion.
Field lines never cross each other.
The spacing between lines gives a visual cue of field strength.
Plotting Magnetic Field Lines with Iron Filings
Materials needed:
Bar magnet
Fine iron filings (e.g., steel wool shavings)
Transparent sheet of paper or a shallow tray
Optional: a piece of cardboard to protect the work surface
Procedure:
Place the magnet on the centre of the paper or tray.
Gently sprinkle a thin, even layer of iron filings over the paper, covering the area around the magnet.
Tap the paper lightly or blow gently to help the filings settle.
The filings will align themselves along the magnetic field lines, forming a pattern that can be observed directly.
Observations:
Filings form continuous curves that start at the north pole and end at the south pole.
Where the field is strongest (near the poles) the filings are densely packed.
The pattern is symmetrical for a bar magnet.
Suggested diagram: Sketch of iron filings forming magnetic field lines around a bar magnet.
Using a Compass to Determine the Direction of the Magnetic Field
The compass needle aligns itself with the local magnetic field direction. The following steps allow a student to state the field direction at any point:
Place the compass at the point of interest near the magnet.
Allow the needle to come to rest; the north‑seeking end points in the direction of the magnetic field vector B at that location.
Record the orientation of the needle relative to a fixed reference (e.g., a drawn north‑south line on the paper).
Repeat at several positions to map the field direction throughout the region.
Mathematical representation:
\$\vec{B} = B\,\hat{u}\$
where \$\hat{u}\$ is a unit vector pointing in the direction indicated by the compass needle.
Comparison of the Two Methods
Aspect
Compass Method
Iron Filings Method
Equipment required
Compass, paper, pencil
Iron filings, paper or tray, magnet
Result type
Discrete line segments that can be traced
Continuous pattern formed automatically
Accuracy of direction
High – each measurement is taken individually
Qualitative – shows overall pattern
Visualising field strength
Requires measuring spacing of traced lines
Density of filings directly shows strength
Ease of set‑up
More time‑consuming
Quick and simple
Common Misconceptions
Field lines are physical objects. They are a visual aid; the magnetic field exists everywhere, not only where lines are drawn.
The compass needle points to the north pole of the magnet. It points in the direction of the magnetic field, which at a point near a magnet is tangent to the field line.
Field lines can intersect. They never cross because a single point in space can have only one direction for the magnetic field.
Summary Checklist
Identify north and south poles of a magnet.
Use a compass to trace field lines: place, wait for rest, draw direction, repeat.
Use iron filings to obtain a quick visual of the field pattern.
Remember that the compass needle aligns with the direction of \$\vec{B}\$ at its location.
Interpret the density of lines or filings as an indication of field strength.
Exam Practice Question
Question: A student places a compass at three positions A, B and C around a bar magnet as shown in the diagram. At A the needle points east, at B it points north, and at C it points west. Explain what these observations tell you about the direction of the magnetic field at each position.
Answer outline:
At each position the north‑seeking end of the needle aligns with the local \$\vec{B}\$ direction.
Therefore, \$\vec{B}\$ at A is directed east, at B north, and at C west.
These directions are consistent with the field lines emerging from the north pole, curving around the magnet, and entering the south pole.
Further Investigation
Students can compare the field patterns of different magnet shapes (e.g., horseshoe magnet vs. bar magnet) using both methods to see how geometry influences the field.