Field strength factors

Magnetic field strength describes how strong the magnetic effect is at a particular point. It is not necessarily the same everywhere around a current-carrying conductor.

How current affects field strength

At a fixed distance from the conductor, a larger current produces a stronger magnetic field. A smaller current produces a weaker field.

This comparison must be made at the same distance and with the conductor arranged in the same way. Otherwise, more than one factor has changed and the effect of current alone cannot be identified.

How distance affects field strength

For a fixed current, the magnetic field is strongest close to the conductor. Its strength decreases as the distance from the conductor increases. The magnetic effect therefore becomes weaker as you move outwards from the wire.

Field diagrams can represent this by drawing field lines more closely spaced where the field is stronger and farther apart where it is weaker. The lines are a model: they show the pattern, direction and relative strength of the field, not physical objects surrounding the wire.

Separating strength from direction

Current has both a size and a direction. Increasing the size of the current strengthens the field. Reversing the current reverses the direction of the circular magnetic field, but if the size of the current stays the same, this does not by itself increase its strength.

Distance also affects strength, not the circular sense of the field. Moving farther from the wire makes the field weaker, while the field still circles the conductor in the direction set by the current.

Making valid comparisons

To describe these relationships clearly, change one factor at a time:

• compare different currents at one fixed distance from the conductor;
• compare different distances while keeping the current fixed.

This leads to the two essential conclusions: magnetic field strength increases when current increases, and decreases when distance from the conductor increases.