Describe the plotting of magnetic field lines with a compass or iron filings and the use of a compass to determine the direction of the magnetic field

4.1 Simple Phenomena of Magnetism

Learning Objective

Students will be able to:

• Define a magnetic field and explain the convention for magnetic field lines.
• Plot magnetic field lines using a compass or iron filings.
• Use a compass to determine the direction of the magnetic field at any point.
• Describe the forces between magnetic poles, the distinction between magnetised and un‑magnetised material, and the principle of induced magnetism.
• Recall common uses of permanent magnets and recognise the link to later topics (solenoids, current‑carrying conductors).

4.1.1 Magnetic Poles, Forces & Materials

• Poles: Every permanent magnet has a north‑seeking pole (N) and a south‑seeking pole (S).
• Attraction: Opposite poles attract (N ↔ S).
• Repulsion: Like poles repel (N ↔ N, S ↔ S).
• Magnetised material: Atomic magnetic domains are aligned, giving a net magnetic field (e.g., a bar magnet).
• Un‑magnetised material: Domains are randomly oriented, so the material shows no overall magnetic effect.

Example experiment: Suspend two small bar magnets on strings. Bring the N‑pole of one close to the N‑pole of the other – the magnets push apart, demonstrating repulsion. Replace one N‑pole with an S‑pole – the magnets attract.

4.1.2 Induced Magnetism

Simple demonstration: Hold a steel nail near a strong bar magnet. The nail becomes magnetised and can pick up paper clips. Remove the nail from the field and it quickly loses its magnetism.

Induced magnetism is the reason why iron filings align themselves along magnetic field lines (see Section 4.1.4).

4.1.3 Uses of Permanent Magnets (Cambridge Syllabus Requirement)

• Refrigerator doors and cupboard closures.
• Magnetic compasses for navigation (Earth’s magnetic field).
• Simple electric motors and generators.
• Magnetic levitation (e.g., maglev trains).
• Holding metal objects in workshops and laboratories.

4.1.4 Plotting Magnetic Field Lines

Field‑Line Conventions (required by the syllabus)

• Field lines emerge from the north pole and enter the south pole.
• They never cross; a single point in space has only one magnetic field direction.
• The density of lines (or of iron filings) is proportional to the magnitude of the magnetic field B.
  

  Closer spacing ⇒ stronger field. Roughly, if the spacing is halved, the field strength doubles.

4.1.4.1 Using a Compass

Materials

• Bar magnet (or any permanent magnet)
• Small compass (needle on a low‑friction pivot)
• Sheet of white paper
• Pencil for marking

Procedure

1. Place the magnet in the centre of the paper and fix it so it cannot move.
2. Draw a faint north‑south reference line on the paper.
3. Position the compass at a chosen point away from the magnet, keeping the compass tip in contact with the paper.
4. Allow the needle to come to rest. The north‑seeking end points in the direction of the local magnetic field 𝛅 at that point.
5. Draw a short line segment in the direction indicated by the needle; this represents a tiny segment of a field line.
6. Move the compass a short distance along the drawn line and repeat steps 3‑5.
7. Continue tracing until the curve reaches the opposite pole of the magnet.
8. Repeat the whole process from several different starting points to obtain a complete set of field lines.

Important notes

• Maintain the same height of the compass above the paper throughout the experiment.
• Field lines never intersect – if two lines appear to meet, re‑check the compass orientation.
• The spacing between traced lines gives a visual indication of field strength.

4.1.4.2 Using Iron Filings

Materials

• Bar magnet
• Fine iron filings (steel‑wool shavings work well)
• Transparent sheet of paper, shallow tray, or a piece of cardboard to protect the work surface

Procedure

1. Place the magnet on the centre of the paper or tray.
2. Sprinkle a thin, even layer of iron filings over the area surrounding the magnet.
3. Tap the paper lightly or blow gently to help the filings settle into the field.
4. The filings align themselves along the magnetic field lines, producing a visible pattern.

Observations

• Filings form continuous curves that start at the N‑pole and end at the S‑pole.
• Near the poles the filings are densely packed, indicating a strong field.
• The pattern is symmetrical for a bar magnet but changes with magnet shape (e.g., horseshoe magnet).

4.1.5 Determining the Direction of the Magnetic Field with a Compass

The compass needle aligns itself with the local magnetic field vector 𝛅. To state the field direction at a point:

1. Place the compass at the point of interest.
2. Wait until the needle stops moving; the north‑seeking end points in the direction of 𝛅.
3. Record the needle’s orientation relative to the drawn north‑south reference line.
4. Repeat at several positions to map the field direction throughout the region.

Mathematical representation

\$\vec{B}=B\,\hat{u}\$

where 𝛅 is the magnetic flux density and 𝛆 is a unit vector pointing in the direction indicated by the compass needle.

Quick‑Check Question

Q: In a region of a magnetic field a compass needle points directly south. What is the direction of the magnetic field vector 𝛅 at that point?

A: The north‑seeking end of the needle points south, so the magnetic field direction is also south (the field points from north to south).

4.1.6 Comparison of the Two Plotting Methods

4.1.7 Common Misconceptions

• Field lines are physical objects. They are a visual representation; the magnetic field exists everywhere, not just 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 magnetic field direction.
• Denser lines always mean a stronger magnet. Density indicates local field strength; a weak magnet can still produce closely spaced lines very near its poles.

4.1.8 Safety Reminder

• Handle the compass gently – a strong magnet can snap the needle and damage the pivot.
• Avoid placing strong permanent magnets near electronic devices, credit cards, or pacemakers.
• When using iron filings, work on a protected surface and clean up filings carefully to avoid inhalation.

4.1.9 Summary Checklist

1. Identify the N‑ and S‑poles of a magnet.
2. State the field‑line conventions (emerge from N, enter S, never cross, density ∝ field strength).
3. Use a compass to trace field lines: place, wait, draw direction, repeat from several start points.
4. Use iron filings to obtain a quick visual of the overall field pattern.
5. Remember that the compass needle aligns with the direction of 𝛅 at its location.
6. Interpret the spacing of lines or the density of filings as an indication of field strength.
7. Recall that opposite poles attract, like poles repel, and that a non‑magnetised piece of iron can become temporarily magnetised (induced magnetism) when placed in a magnetic field.
8. Know at least three everyday uses of permanent magnets.

4.1.10 Exam‑style 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:

• The north‑seeking end of the compass aligns with the local magnetic field vector 𝛅.
• Therefore, 𝛅 at A is directed east, at B north, and at C west.
• These directions are consistent with field lines that emerge from the N‑pole, curve around the magnet, and enter the S‑pole.
• The varying directions also illustrate that field lines are denser (stronger) near the poles and more widely spaced (weaker) farther away.

4.1.11 Further Investigation & Extension

• Compare the field patterns of different magnet shapes (horseshoe, U‑shaped, disc) using both the compass and iron‑filings methods.
• Investigate induced magnetism by varying the distance between a steel nail and a bar magnet; record how the strength of attraction changes.
• Further reading: The magnetic field inside a solenoid is almost uniform and parallel to its axis – a key concept for later syllabus sections.
• Related concept: A straight current‑carrying wire produces concentric circular magnetic field lines (right‑hand rule). This links magnetism to electricity in subsequent topics.