Pearson Science Double Award Physics 6: Magnetism and Electromagnetism

Magnetic fields explain attraction, repulsion, forces on currents and induced voltage. The 4SD0 boundary includes fields, simple motors, loudspeakers and generators. Detailed electromagnets at points 6.9 to 6.11 and transformer treatment at points 6.17 to 6.20 are separate-Physics content.

Magnets and fields

Like magnetic poles repel and unlike poles attract. Magnets also attract magnetic materials such as iron, even when the material is not already a permanent magnet. The magnet induces magnetism by aligning domains.

A magnetic field is a region where a magnetic material, magnet or current-carrying conductor experiences force. Field lines show the direction a north test pole would move. Outside a bar magnet they run from north to south, never cross, and are closer where the field is stronger.

Magnetically hard materials retain magnetisation and suit permanent magnets. Magnetically soft materials magnetise and demagnetise readily and suit temporary magnetic roles. “Hard” and “soft” describe magnetic behaviour, not mechanical hardness.

Map fields using a plotting compass at many positions or iron filings for a broad pattern. A compass gives direction; filings show alignment but not arrow direction. Two unlike poles placed facing each other with close, flat pole faces produce an approximately uniform field, represented by parallel equally spaced lines.

Fields from currents and motor effect

An electric current in a conductor produces a magnetic field around it. Around a straight wire the field lines are concentric circles, with direction reversed when current reverses.

A current-carrying wire placed in an external magnetic field experiences a force because its field interacts with the external field. Use Fleming's left-hand rule: first finger field, second finger conventional current and thumb force. The force is greatest when wire and field are perpendicular.

Increasing current or magnetic field strength increases the force. Reversing either current or field reverses force; reversing both keeps the force direction unchanged.

In a simple direct-current motor, opposite sides of a current-carrying coil experience opposite forces, producing a turning effect. A split-ring arrangement reverses current every half-turn so rotation continues in one direction. In a loudspeaker, changing current in a coil changes force, moving a cone and creating sound waves.

Electromagnetic induction

A voltage is induced when a conductor moves through a magnetic field or when the magnetic field through a coil changes. Relative change is essential: a stationary magnet and coil with an unchanging field produce no induced voltage.

Increase induced voltage by moving faster, using a stronger field, increasing coil turns or increasing the effective rate of field change. Reversing motion or field reverses induced polarity. A complete circuit is required for an induced current, but voltage can be detected across an open coil.