Students are placed into small groups and asked to create an experimental setup they can use to measure the partial derivative they are given, in which entropy changes.

Students are shown the contour graph of a function of two variables and asked to find the derivative. They discover that, without a function to differentiate, they must instead think of the derivative as a ratio of small changes. This requires them to pick two nearby points. Which two?

Consider a hanging rectangular rubber sheet.
We will consider there to be two ways to get energy into or out of
this sheet: you can either stretch it vertically or horizontally.
The distance of vertical stretch we will call \(y\), and the distance
of horizontal stretch we will call \(x\).

If I pull the bottom down by a small distance \(\Delta y\), with no
horizontal force, what is the resulting change in width \(\Delta x\)?
Express your answer in terms of partial derivatives of the potential
energy \(U(x,y)\).

This short small group activity introduces students to the Leibniz notation used for partial derivatives in thermodynamics; unlike standard Leibniz notation, this notation explicitly specifies constant variables. Students are guided in linking the variables from a contextless Leibniz-notation partial derivative to their proper variable categories.