Plotting Magnetic Fields (Required Practical: Compass Method)

This lesson covers the AQA Required Practical for investigating the magnetic field pattern around a bar magnet using a compass. This maps to AQA GCSE Combined Science Trilogy (8464) specification section 6.7.1 and the associated Required Practical Activity.

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Required Practical: Investigating the Magnetic Field Pattern of a Bar Magnet

Aim

To investigate the magnetic field pattern around a bar magnet using a plotting compass.

Equipment

• Bar magnet
• Plotting compass (small compass)
• Large sheet of white paper
• Pencil
• Ruler (optional, for neat work)

Method

1. Place the bar magnet in the centre of the paper and draw around it, labelling the N and S poles.
2. Place the plotting compass near the north pole of the magnet.
3. Mark a dot at the position of the north-seeking end of the compass needle.
4. Move the compass so the south-seeking end of the needle sits on the dot you just drew.
5. Mark another dot at the new position of the north-seeking end.
6. Repeat steps 4–5 until you reach the south pole of the magnet (or the edge of the paper).
7. Join the dots to form a smooth curved line — this is one field line.
8. Draw an arrowhead on the line pointing from N to S.
9. Repeat from different starting positions around the N pole to build up the full field pattern.
10. You should aim for at least 5–6 field lines to show the complete pattern.

graph LR
    subgraph "Compass Plotting Method"
        A["Place compass near N pole"] --> B["Mark dot at N-seeking tip"]
        B --> C["Move compass — S-seeking end on dot"]
        C --> D["Mark new dot at N-seeking tip"]
        D --> E["Repeat until S pole reached"]
        E --> F["Join dots — smooth curve with arrows N→S"]
    end

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Expected Results

The field pattern around a bar magnet should show:

• Curved lines emerging from the N pole and entering the S pole.
• Lines that are closest together at the poles (strongest field).
• Lines that spread out further from the magnet (weaker field).
• Arrows pointing from north to south on every line.
• The field lines never cross each other.

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Sources of Error and Improvements

Source of Error	Improvement
The Earth's magnetic field affects the compass	Align the magnet so its field is much stronger than the Earth's field (use a strong magnet, keep compass close)
Dots are too far apart to draw smooth curves	Place dots closer together (move compass in smaller steps)
Iron or steel objects nearby distort the field	Remove all magnetic materials from the area
Compass needle is sluggish or sticks	Use a well-made plotting compass; tap it gently to help it settle
Only a few field lines plotted	Repeat from many different starting positions

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Alternative Method: Iron Filings

You can also visualise magnetic field patterns using iron filings:

1. Place the bar magnet under a sheet of paper or on top of it.
2. Sprinkle iron filings evenly over the paper.
3. Gently tap the paper to allow the filings to align with the field.
4. The filings show the shape of the field but not the direction (they don't show N to S).

Method	Shows Shape?	Shows Direction?	Permanent Record?
Plotting compass	Yes	Yes (arrows N→S)	Yes (dots and lines drawn)
Iron filings	Yes	No	No (filings move if disturbed)

Exam Tip: If asked about the iron filings method, always state that iron filings show the shape of the field but not the direction. You need a compass for direction.

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Drawing Field Patterns for Different Arrangements

Two Magnets — Unlike Poles Facing (N–S)

• Field lines run from the N pole of one magnet to the S pole of the other.
• In the gap between the magnets the field is roughly uniform (parallel, equally spaced lines).

Two Magnets — Like Poles Facing (N–N or S–S)

• Field lines curve away from the gap between the magnets.
• There is a neutral point between the magnets where the field is zero (the two fields cancel out).

graph LR
    subgraph "Like Poles Facing — Neutral Point"
        N1["N"] -.->|"Field curves away"| NP["Neutral Point (field = 0)"]
        N2["N"] -.->|"Field curves away"| NP
    end

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Worked Example

Q: A student uses a plotting compass to investigate the magnetic field around a bar magnet. She notices that the compass needle points in random directions when it is far from the magnet. Explain why.

A: Far from the bar magnet, the bar magnet's field is very weak. At this distance, the Earth's magnetic field is stronger than the bar magnet's field, so the compass aligns with the Earth's field instead (pointing roughly north). The compass does not point randomly — it aligns with the dominant field. If the question says "random", it may be that the student is at a point where the bar magnet's field and the Earth's field are comparable in strength and at angles to each other.

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Common Mistakes

Mistake	Correction
"Iron filings show the direction of the field"	Iron filings only show the shape, not the direction
Forgetting arrows on field lines	Always add arrows pointing from N to S
Drawing field lines that cross	Field lines never cross
Not mentioning the Earth's field as a source of error	The Earth's field can affect compass readings

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Summary

• The Required Practical uses a plotting compass to map field lines around a bar magnet.
• Mark dots at the compass needle tip, move the compass, repeat, then join the dots.
• Field lines go from N to S, never cross, and are closer together where the field is stronger.
• Iron filings show the shape of the field but not the direction.
• The Earth's magnetic field is a source of error in this experiment.
• You should be able to draw field patterns for a single bar magnet, unlike poles facing, and like poles facing.

Exam Tip (AQA 8464): Required Practicals are frequently examined. Be ready to describe the method step by step, identify sources of error, and explain how to improve the experiment. Always mention removing nearby magnetic materials and the effect of the Earth's field.

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Understanding Why the Compass Method Works

A plotting compass is essentially a very small, freely pivoting permanent magnet. Its north-seeking pole aligns itself with the local magnetic field. By marking the position of the north-seeking tip, moving the compass so the south-seeking tip lies at that mark, and repeating, you are effectively tracing the tangent of the field at each point — which, when joined together, gives the smooth curve of a field line.

The method therefore relies on three key physical facts: