Pearson Edexcel International GCSE Science Single Award 15: Physics: Magnetism and electromagnetism

Topic 6 links field patterns to the force on a current and its use in motors and loudspeakers. These notes follow Pearson 4SS0 Issue 4. Magnetic materials, electromagnet construction, induction, generators and transformers belong to broader routes and are excluded.

Magnetic field lines

A magnetic field is a region where a magnetic material, magnet or current can experience a magnetic effect. A field line represents field direction and relative strength. Outside a bar magnet, arrows run from north to south. Lines are closer where the field is stronger and never cross, because the field at one point cannot have two directions.

Iron filings reveal the shape of a field but do not show direction. A small plotting compass can map direction point by point. In an investigation, place the magnet under paper, keep it fixed, mark the compass needle direction at many positions and join the marks smoothly.

Between unlike poles facing each other, field lines connect across the gap. Two broad, parallel pole faces placed close together can produce an approximately uniform magnetic field in the central region. Uniform means the field has the same direction and strength, represented by straight, parallel, equally spaced lines.

Between like poles facing each other, lines bend away from the central region and do not connect directly. Distinguish this from a field-free region: the resultant may be small at a particular point, but nearby fields still exist.

Magnetic field around a current

An electric current in a conductor produces a magnetic field around it. For a straight wire, field lines form concentric circles. Reversing current reverses field direction. Increasing current strengthens the field.

This current-produced field can interact with an external magnetic field. The interaction creates a force on the current-carrying conductor when it has a component perpendicular to the field.

The motor effect

A current-carrying wire in a magnetic field experiences a force because its magnetic field interacts with the external field. The force becomes larger when current increases or magnetic field strength increases. Reversing either current or field reverses force direction; reversing both leaves force direction unchanged.

Use Fleming's left-hand rule:

• first finger points from north to south, the magnetic field;
• second finger points in conventional current direction;
• thumb points in the force or motion direction.

The three directions are mutually perpendicular. Electron movement is opposite to conventional current, so do not point the second finger with electron flow.

Simple d.c. motor

A rectangular current-carrying coil sits in a magnetic field. Opposite sides of the coil carry currents in opposite directions, so they experience forces in opposite directions. This pair of forces turns the coil.