IGCSE CP11 – Magnetic Fields | Edexcel International GCSE Physics

Theory — Magnetic Fields

A magnetic field is a region where a magnetic force is experienced. Field lines show the direction and strength of the field — closer lines mean a stronger field.

Magnetic Field Lines

Field lines run from North to South outside the magnet (inside they run S→N)
Lines never cross — each point has only one field direction
Closer lines = stronger field (higher flux density)
A plotting compass needle always points in the direction of the field line at that point

Bar Magnet

The field emerges from the North pole, curves around, and enters the South pole. The field is strongest at the poles (lines densest) and weaker further away. At the midpoint between two poles of equal magnets, the field can cancel to give a neutral point.

Current-Carrying Conductor

A current through a straight wire creates a circular magnetic field around it. The direction follows the right-hand rule: point your thumb in the direction of conventional current — fingers curl in the direction of the field.

Current out of page (⊙) → field is anticlockwise
Current into page (⊗) → field is clockwise
Field strength ∝ current, and decreases with distance from wire

Solenoid

A solenoid (coil of wire) produces a field similar to a bar magnet outside — with a North and South pole — and a uniform field inside (parallel lines). The polarity is determined by current direction.

Look at one end: if current flows anticlockwise → North pole; clockwise → South pole
Adding an iron core greatly increases the field strength
The field inside a solenoid is uniform — this is used in MRI machines and particle accelerators

Procedure

Equipment

Bar magnet / current-carrying wire / solenoid · Sheet of paper · Iron filings in a sprinkler · Plotting compass · Pencil

1

Place the source under the paper

Place the magnet (or set up the wire/solenoid carrying current) flat on the bench. Lay a sheet of paper over the top, ensuring it lies flat.

2

Sprinkle iron filings

Gently sprinkle iron filings from a sprinkler onto the paper, covering the area above the source. The filings will lie randomly at first.

💡 Sprinkle evenly and not too densely — too many filings obscure the pattern.

3

Tap the paper gently

Tap the edges of the paper gently several times. The vibration allows the iron filings to rotate freely and align with the magnetic field. The field pattern becomes visible.

💡 Tap gently — too hard and the filings jump and lose the pattern.

4

Use a plotting compass to find directions

Place a plotting compass at a point on the paper. Mark a dot at the tip of the needle, move the compass so its tail is at that dot, mark another dot. Repeat to trace a field line. The compass always points from S to N along the field line direction.

5

Draw and annotate the field pattern

Draw smooth curves through the dots to show field lines. Add arrows showing the direction (N→S outside magnet). Label N and S poles.

🧲 Select a source, then follow the stages: 1 Sprinkle filings → 2 Tap paper → 3 Place compass → 4 Show field lines.

Source

Current Direction

Stages

How to use Select a source above, then work through the stages in order. In Stage 3, click anywhere on the canvas to place a plotting compass.

Questions

Question 1

A student places a plotting compass at different points around a bar magnet. Describe what happens to the compass needle (a) near the North pole, (b) near the South pole, and (c) midway between the poles on the side (equatorial position). What do the compass readings tell us about the direction of the magnetic field?

(a) Near the North pole: the compass needle points away from the North pole — field lines emerge from N, so the compass (which points in the field direction) points outward, away from the magnet's N pole. (b) Near the South pole: the compass needle points toward the South pole — field lines enter at S, so the needle points inward toward the magnet's S end. (c) At the equatorial position (midway, to the side): the compass needle points parallel to the length of the magnet, from N toward S — the field lines at this position run roughly parallel to the magnet's axis. The compass readings show the direction of the magnetic field at each point, because the north-seeking pole of the compass always aligns with the field direction (from N to S outside the magnet).

Question 2

A vertical wire carries a conventional current directed upward (out of the page when viewed from above). (a) In which direction do the magnetic field lines circle the wire? (b) If the current is doubled, what happens to the field pattern shape and the field strength?

(a) Using the right-hand rule: point the right thumb upward (direction of conventional current out of the page). The fingers curl anticlockwise when viewed from above — so the field lines circle the wire anticlockwise. (b) The shape of the field pattern (concentric circles) stays exactly the same. However, the field strength at every point doubles — the circles become "denser" in the sense that the magnetic flux density B is proportional to the current I. The spacing between field lines (representing equal flux intervals) would halve, showing the field is twice as strong at every distance from the wire.

Question 3

Compare the magnetic field produced by a solenoid with that of a bar magnet. In what way is the field inside the solenoid special, and how is this useful in practice?

Outside the solenoid, the field is identical in shape to that of a bar magnet — field lines emerge from the North end, curve around, and re-enter at the South end. The solenoid therefore has a North and South pole, and attracts/repels other magnets accordingly. Inside the solenoid, the field is uniform — the field lines are parallel and equally spaced, meaning the magnetic flux density is the same at every point inside. This is very different from a bar magnet, where the internal field is not easily controlled. The uniform internal field is extremely useful: it is used in MRI (magnetic resonance imaging) scanners to produce a consistent field across the patient, in loudspeaker voice coils, in particle accelerators to steer beams, and in electromagnetic relays and motors where a consistent force is needed.