Clay Logan, of Balclutha Primary School, asks :-

How does a magnetic field go through things?

John Campbell, a physicist at the University of Canterbury, responded.

A field is just a force that a charged particle observes when put at that position. For example, unbalanced charges near a test charge will cause a force of an electrical nature on the test charge, and the charge will try to move. We are quite happy that electric fields go through things where the charges cannot move freely. This includes most materials except metals.

Every material is made of atoms and each atom has between one and 92 electrons strongly bound to it and therefore cannot normally leave it. Hence an electric field, for example in a light wave, can travel through a non-metallic material, for example air, water or glass. If we wish to shield from electric waves then we surround what we are shielding with a good conductor of electricity. Metals are unusual in that for their atoms an outer electron or two are free to leave the atom and move throughout the metal. At the surface of the metal the applied electric force generates an electrical current which counteracts the applied electric field so there is no electric field inside a thick metal.

If you look at the door of a microwave oven you will see that it is covered with a metal grill. The tiny holes let light through, so you can see what is happening inside, but the metal stops the much longer wavelength microwave electric fields getting out and cooking your eyeball.

Magnetic fields are more tricky in that they are generated only by moving charges, and the force is at right angles to both the direction of travel of the moving charge and to the magnetic field. You can pass an electric current through a coil of wire and generat a magnetic field to attract, for example, a steel nail.

All atoms have electrons orbiting the nuclei so all atoms generate magnetic fields. Mostly the electrons pair off so that the effect of one orbital electron is cancelled by its pair. These do have a weak magnetism but only when an external magnetic field is applied to it.

In strong magnets each atom has not only one or a few electrons unpaired but they interact with the neighbouring atoms so each produces a magnetic field in the same direction. A U shaped magnet will produce a magnetic field all around it but this outside field can be reduced by placing a magnetic material, for example a steel keeper, to join the two poles. The Earth has a magnetic field, produced by electrical currents in the molten material below the surface. Monitoring the magnetic field at the Earth's surface is one of the few ways we can study the interior of the Earth.

Magnetic fields will go through most materials which are non-magnetic so penetrate more materials than electric fields do. We can shield from magnetic materials by surrounding what we want to shield by a strongly magnetic material. In one sensitive apparatus I have I surround it with superconducting metal and that prevents stray electric and magnetic fields penetrating into the apparatus.