The formula for the magnetic field of a single moving charge, or short current segment, is expected to be taught in AP Physics C: E&M. However, there are very few experiments that demonstrate this. In this article, I provide details of how a modern lab of this type could be carried out and provide data that demonstrate effectively how such a magnetic field will decrease as 1/r2. This is usually referred to as the Biot–Savart law, although the nomenclature overlaps with Ampere’s law.1,2 In a previous article, we saw that the Phyphox app could be used to get data from a smartphone’s built-in magnetometer.3 Such a ubiquitous and sensitive probe is in almost every student’s pocket and is thus readily available to measure the inverse r2 dependence of a short current segment.

Phillips and Sanny4 have already shown that a long U-shaped current-carrying...

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