(a) magnetic fields are due to moving charges or permanent magnets

The idea

A magnetic field is a region where a magnetic force acts on magnetic materials or moving charges. This statement identifies two independent origins. First, moving charges: any current in a wire is a flow of charge, so a current-carrying conductor is surrounded by a magnetic field; a single charge moving through space also produces one. Second, permanent magnets: in iron, cobalt and nickel, atomic magnetic moments align to give a net field without any external current. The two origins are linked, because magnetism in materials arises from moving charges at atomic scale. A useful test is to ask whether a current or a magnetised material is present. Field strength falls with distance from the source, and the field direction depends on current direction or on the orientation of the magnet's poles.

What earns the marks

• Moving charges produce a magnetic field; a steady current is a flow of charge and so creates a field around the conductor.
• A single charge moving with velocity v produces a magnetic field; a stationary charge does not.
• Permanent magnets produce a magnetic field because atomic magnetic moments within domains are aligned.
• Magnetic materials include iron, cobalt and nickel; alignment of domains gives a net magnetic field.
• The field direction depends on the direction of conventional current or on the orientation of the magnet.
• Field strength decreases with distance from the current or magnet.

Common mistakes and how to put them right

• Thinking a stationary charge produces a magnetic field.

  Put it right: The charge must be moving, so a stationary charge has an electric field but no magnetic field.

• Believing permanent magnets contain no moving charge.

  Put it right: Magnetism in materials arises from atomic-scale moving charges whose moments are aligned.

• Assuming the field around a current is uniform.

  Put it right: It is strongest close to the conductor and weakens with distance.

• Confusing magnetic poles with electric charges.

  Put it right: Isolated magnetic poles have not been observed, whereas positive and negative charges can exist separately.

Exam tips

• Read the stem carefully to identify whether the source is a current, a moving charge or a permanent magnet before choosing an option.
• Eliminate options that describe a stationary charge producing a magnetic field, since that is impossible.
• Check the direction wording: conventional current direction is opposite to electron flow, so field direction follows conventional current.
• Where an option mentions field strength, ask whether it correctly states that the field weakens with distance.