How fields fall with distance

EMFs from any source usually get less as the distance from the source increases. Quite often, this can be approximated as one of three basic types of fall off with distance:

Inverse first power of distance
(also referred to as one over the distance or reciprocal of distance)

• at double the distance the field is reduced to a half
• at three times the distance the field is reduced to a third and so on

an example is the magnetic field from a net current in a distribution circuit

Inverse square of distance
(also referred to as one over the distance squared or inverse second power of distance)

• at double the distance the field is reduced to a quarter
• at three times the distance the field is reduced to a ninth and so on

an example is the magnetic field from some transmission lines (either with a single circuit or two circuits but untransposed phasing)

Inverse cube of distance
(also referred to as one over the distance cubed or inverse third power of distance)

• at double the distance the field is reduced to an eighth
• at three time the distance the field is reduced to a twenty-seventh and so on

an example is a transmission line with transposed phasing, or a domestic appliance

In practice, fields rarely follow these power laws exactly, departing from them particularly at very small distances or very large distances. Nonetheless, there is usually a good range of distances where these are good approximations.

See here for more detail on the power laws followed by transmission lines in particular showing the range of distances they are good approximations and where they break down.

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