Magnetic fields

Theoretical ideas have practical applications

Wrong Track: The magnetic field is just the iron filings around the magnet.

Wrong Track: The magnetic field is those lines.

Wrong Track: The magnetic field is what makes the magnet work.

Right Lines: The magnetic field fills the space around a magnet, establishing a region within which the magnet will attract or repel another magnet.

Grounding theoretical ideas

Thinking about the teaching

What is a magnetic field? From a scientific point of view it's a theoretical concept that allows us to account for the action–at–a–distance working of magnets and magnetic materials; in other words, how it is that magnets can attract and repel one another without being in contact. After working with magnets in both primary and secondary school, students may have different understandings about magnetic fields.

In some ways it's odd that the magnetic field concept is taught at all within the 11–16 age range (and sometimes to even younger children!). Magnetism is a familiar topic for study in lower secondary school science and historically it's well established in the curriculum. However, comparable treatments of electric and gravitational fields (addressing matters such as field patterns) are reserved for post-16 physics courses. So why does magnetism appear lower down the school? Maybe it's because students can sprinkle iron filings around a magnet to enable them to see a magnetic field. Electric and gravitational fields are not open to the same kind of practical investigation and possibly on this basis have not been considered for inclusion within the 11–16 curriculum.

With these thoughts in mind, using magnetic field ideas to explain the working of devices such as the electric motor demands careful teaching.

Important ideas to establish with students are as follows:

• The magnetic field fills the space around a magnet, indicating where this magnet will attract or repel another magnet.
• The magnetic field fills the space around a magnet: it is three-dimensional.

The magnetic field is a theoretical concept: it's not tangible (although Michael Faraday, in developing the magnetic field concept, found it useful to think of the field lines as if they were real objects).

The field lines around a magnet show where the field is located and the paths along which the magnet exerts a force on magnetic materials.

When two magnets are placed close to each other, they attract or repel each other, such that the magnetic field lines are shortened or straightened by the action of the force.