A magnetic field exists wherever there is force on a magnetic pole. As we saw with electric and gravitational fields, a magnetic field is a field of force.
You can make a magnetic field in two ways: using a permanent magnet, or using the movement of electric charges, usually by having an electric current. You should be familiar with the magnetic field patterns of bar magnets (Figure 24.2). These can be shown using iron filings or plotting compasses.

Figure 24.2: Magnetic field patterns: a for a bar magnet; b for two attracting bar magnets and c for two repelling bar magnets.

We represent magnetic field patterns by drawing magnetic field lines.
- The magnetic field lines come out of north poles and go into south poles.
- The direction of a field line at any point in the field shows the direction of the force that a ‘free’ magnetic north pole would experience at that point.
- The field is strongest where the field lines are closest together.
An electromagnet makes use of the magnetic field created by an electric current (Figure 24.3a). A coil is used because this concentrates the magnetic field. One end becomes a north pole (field lines emerging), while the other end is the south pole. Another name for a coil like this is a solenoid.
The field pattern for the solenoid looks very similar to that of a bar magnet (see Figure 24.2a), with field lines emerging from a north pole at one end and returning to a south pole at the other. The strength of the magnetic field of a solenoid can be greatly increased by adding a core made of a ferrous (iron-rich) material. For example, an iron rod placed inside the solenoid can act as a core; when the current flows through the solenoid, the iron core itself becomes magnetised and this produces a much stronger field. A flat coil (Figure 24.3b) has a similar field to that of a solenoid.

Figure 24.3: Magnetic field patterns for a a solenoid, and b a flat circular coil.

If we unravel an electromagnet, we get a weaker field. This, too, can be investigated using iron filings or compasses. The magnetic field pattern for a long current-carrying wire is very different from that of a solenoid. The magnetic field lines shown in Figure 24.4 are circular, centred on the long current-carrying wire. Further away from the wire, the field lines are drawn further apart, representing the weaker field at this distance. Reversing the current reverses the direction of the field.

All magnetic fields are created by moving charges. (In the case of a wire, the moving charges are free electrons.) This is even true for a permanent bar magnet. In a permanent magnet, the magnetic field is produced by the movement of electrons within the atoms of the magnet. Each electron represents a tiny current as it circulates around within its atom, and this current sets up a magnetic field. In a ferrous material, such as iron, the weak fields due to all the electrons combine together to make a strong field, which spreads out into the space beyond the magnet. In non-magnetic materials, the fields produced by the electrons cancel each other out.

Field direction

The idea that magnetic field lines emerge from north poles and go into south poles is simply a convention.
Figure 24.5 shows some useful rules for remembering the direction of the magnetic field produced by a current.

Figure 24.5: Two rules for determining the direction of a magnetic field, a inside a solenoid and b around a current-carrying wire.

The right-hand grip rule gives the direction of magnetic field lines in an electromagnet. Grip the coil so that your fingers go around it following the direction of the current. Your thumb now points in the direction of the field lines inside the coil; that is, it points towards the electromagnet’s north pole.
Another way to identify the poles of an electromagnet is to look at it end on, and decide which way round the current is flowing. Figure 24.5a show how you can remember that clockwise is a south pole, anticlockwise is a north pole.
The circular field around a wire carrying a current does not have magnetic poles. To find the direction of the magnetic field you need to use another rule, the right-hand rule. Grip the wire with your right hand, pointing your thumb in the direction of the current. Your fingers curl around in the direction of the magnetic field.
Note that these two rules are slightly different. The right-hand grip rule applies to a solenoid; the fingers are curled in the direction of the current and the thumb then gives the direction of the field. The righthand rule applies to a current in a straight wire; the thumb is pointed in the direction of the current and the fingers then give the direction of the field lines.