Introduction

Extra (ancillary) information in math from the physics

Given the following function, where K is a known constant,

what is

When this problem is given to physicists, they almost always give the answer:

In choosing this, they are typically using the interpretation of x and y as the coordinates of a point in a plane, and of r and θ as the polar coordinates of that point. Thus, the bring in the extra (ancillary) information defining polar coordinates:

A mathematician would typically not use the ancillary information and say: "The function A(.,.) is K times the sum of the squares of the first variable and the second variable. Therefore the answer should be:

The physicist would respond,"That's awful! You can't add r² and θ²; they have different units."

The mathematician would reply, "If you mean only r², you have to give it a different name, since the functional dependence is different. Something like:"

The physicist says, "I don't want to use a different symbol. The A reminds me that I am calculating a vector potential. If I used B, I might forget and interpret it later as a magnetic field instead. In any case, I know what I mean even it it isn't mathematically precise about the functional dependence."

The moral here is that in physics, you can't just look at what the math equation is saying. We often load physical meaning on a mathematical symbol in a way that changes the way we interpret the math.

This problem was shown to me by Corinne Manogue of Oregon State University.

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