Magnetic Materials and Field Patterns (Paper 2)

Explained

Hard and soft magnets, and drawing the field

A magnetic field is a region where a magnetic material, a magnet or a current carrying wire experiences a force. It is invisible, so we represent it with field lines, and the rules for drawing them are marked quite strictly.

Hard and soft magnetic materials

Magnetically hard materials, such as steel, are difficult to magnetise but keep their magnetism afterwards. That makes them right for permanent magnets.

Magnetically soft materials, such as iron, magnetise easily and lose it just as easily. That makes them right for electromagnet cores and transformer cores, where the magnetism has to switch on and off with the current.

Hard and soft here have nothing to do with physical hardness. They describe how firmly the material holds onto its magnetism.

Drawing field patterns

Four rules apply to every diagram. Field lines always run from north to south outside the magnet. They never cross. They are closer together where the field is stronger. And every line needs an arrow.

Around a bar magnet the lines curve from the north pole round to the south, closest together at the poles where the field is strongest.

Between two opposite pole faces facing each other, the field is uniform: straight, parallel, equally spaced lines running from the north face to the south face.

Around a straight current carrying wire the lines are concentric circles, closer together near the wire. Their direction is given by the right hand grip rule, with the thumb along the current and the fingers curling the way the field goes.

Around a solenoid the field looks like that of a bar magnet outside, and is strong and uniform inside. Its strength increases with more turns, a larger current, or an iron core.

Plotting a field for real

Place the magnet on paper and move a small plotting compass around it. The compass needle lines up with the field at each point, so marking the ends of the needle repeatedly and joining the dots traces a field line.

Iron filings sprinkled on paper over a magnet show the shape immediately, but they give no direction, because a filing has no north end. Combining the two, filings for the shape and a compass for the direction, is the complete method.

Why the earth has one

A compass needle points north because the Earth itself has a magnetic field, produced by movement in its molten iron core. Its geographic north is a magnetic south pole, which is why the north seeking end of a compass is attracted to it.