Explain that magnetic forces are due to interactions between magnetic fields

The idea

A magnet does not push or pull another magnet or a current-carrying wire through empty space; each object is surrounded by a magnetic field, and the force arises where those fields overlap and interact. Picture two bar magnets: each has field lines running from its north pole round to its south pole, and when the magnets are brought together the two field patterns combine. Where the combined pattern is squeezed or stretched, the magnets experience a force. The same idea explains why a wire carrying a current feels a force in a magnetic field: the current makes its own field, and that field interacts with the field of the magnet. To answer a multiple-choice question, look for the option that names two fields interacting, and reject options that say a magnet acts directly on the other object with no field involved.

What earns the marks

• Identifies that each magnet or current-carrying conductor is surrounded by its own magnetic field.
• States that the force arises from the interaction between two magnetic fields, not from direct action at a distance.
• Recognises that a current-carrying wire produces a magnetic field that interacts with the field of a nearby magnet.
• Rejects the idea that magnetic forces act without any field being present.

Common mistakes and how to put them right

• Describes the force as one magnet acting directly on another with no field involved.

  Put it right by naming the two interacting fields.

• Assumes only permanent magnets have fields.

  Put it right: A current-carrying conductor also has a magnetic field.

• Treats the field lines as the force itself.

  Put it right: Explain that the force comes from the interaction of the fields the lines represent.

Exam tips

• Read every option and cross out any that mention direct action without a field.
• Look for the word 'interaction' or 'interact' linking two fields.
• If an option mentions only one field, check whether it explains where the second field comes from.