Physics 12.4 - 12.6 - Field maps and compasses

The field-line model

Magnetic field lines are a model, not physical threads. At any point, the arrow gives the direction in which the north-seeking end of a small compass would point.

Around a bar magnet:

• the lines curve from one pole to the other;
• outside the magnet, arrows point from N to S;
• the lines never cross, because the field cannot have two directions at one point;
• the field is stronger where the lines are closer together, so it is strongest near the poles.

A uniform magnetic field has the same strength and direction throughout the region shown. Its field lines are straight, parallel and equally spaced, with all arrows pointing the same way.

[DIAGRAM: asset_slug: phy_g_bar_and_uniform_fields; description: External bar-magnet lines point N to S; uniform-field lines are straight, parallel and equally spaced.]

The diagram uses more lines near a bar magnet's poles to represent a stronger field. The exact number of drawn lines is not a measured quantity; compare their concentration within the same diagram.

Mapping with a plotting compass

A plotting compass is a small magnetised needle that can rotate freely. Its north-seeking end aligns with the field at its position, so several readings can reveal both the shape and direction of a field.

To trace the field around a bar magnet:

1. Place the magnet under a sheet of paper, draw its outline and label N and S.
2. Put the plotting compass close to N. Mark the positions of both needle ends and identify the north-seeking end from its N mark or the end that points roughly north when tested away from the magnet.
3. Move the compass so its south end is on the previous mark for the north end. Mark the new needle direction.
4. Repeat until the trace reaches S. Join the marks with a smooth curve and add an arrow from N to S.
5. Start at several different positions near N and repeat to build up the field pattern.