In this activity, students will explore how to calculate a derivative from measured data. Students should have prior exposure to differential calculus. At the start of the activity, orient the students to the contour plot - it's busy.
Goals
- To calculate derivatives on a contour map.
- Many different derivatives can be determined at a point.
- Think about derivatives as ratios of small changes.
- Reinforces the idea that a derivative is a ratio of small changes along a path
- In thermo, derivatives are often invertible?
- The sign of a derivative does not depend on the direction you're traveling along the path
Time Estimate: 30 minutes Tools/Equipment
- Contour graph with P,V,S,T,U contours
- Student handout for each student
- A personal or shared writing space for each student to write/draw/sketch.
Intro
- Orient the students to the contour plot - it's busy.
- Exposure to differential calculus
Whole Class Discussion / Wrap Up
- How students report out their answers to Explore.
- Many students will not be willing to call the ratio of changes a derivative until you've taken the limit that the changes become infinitesimal. Talk about linear approximation and/or tables of data.
- Most students will have heard in a math class that derivatives are not invertible. In thermo, they often are as long as you don't change the path.
- Many students will think that if you reverse the path, the derivative will change sign. Discuss how the signs of both the numerator and denominator of the ratio of small changes swaps, leaving the derivative unchanged.
Orient: The plot shows various thermodynamic quantities for water vapor in an insulated piston (cylindrical thermos with a movable top) at different states. From state (point) \(A\) to state (point) \(B\), estimate the following quantities:
Verbal Description | Symbol | Estimate (with Units) |
Change in volume: | \(\Delta V_{A\rightarrow B}\) | |
Change in entropy: | \(\Delta S_{A\rightarrow B}\) | |
Change in temperature: | \(\Delta T_{A\rightarrow B}\) | |
Change in pressure: | \(\Delta P_{A\rightarrow B}\) | |
Change in internal energy: | \(\Delta U_{A\rightarrow B}\) | |
At what stage would you be willing to label these quantities with `d's?
Discussion: Small Some students say “small” but not what it is small compared to. The instructor can talk about linearity in the WCD.
Discussion: Infinitesimals “At what stage would you be willing to label these quantities with `d's?”
Explore: Pick two of the variables in the table and determine the rate of change of one with respect to the other from \(A\) to \(B\). What experiment could you do to measure this rate?
Discussion: Derivatives: “Under what circumstances would you be willing to call this rate a derivative?” Have students plot graphs of their functions to see how linear they are. Many students will not be willing to call this a derivative because we aren't taking a limit - talk about linear approximation.
Discussion: Where? If a student is willing to call this rate a derivative, at which point is it the derivative? It is the best approximation to the derivative somewhere between the two points.
Discussion: Slope Is their derivative a slope? Of what?
Follow-up: How would you represent this derivative symbolically? Some students write \(\Delta U / \Delta T\) vs. \(dU/dT\); some students write subscript \(S\) vs. \(A \rightarrow B\). A good opportunity to talk about how the derivative ought to depend on the value of \(S\).
Interpret: Determine the reciprocal of the rate you calculated. How would you physically interpret this number?
Discussion: Experiment What experiment would you do to measure this quantity?
Discussion: Inversion The derivative can be flipped as long as the path is unchanged.
Follow-up: How would you represent this rate it symbolically?
Reinterpret: How does the rate you previously calculated change if instead you went from state \(B\) to state \(A\)?
Answer: The rate (and therefore the derivative is the same.
Discussion: Sign Many students will initially say that the derivative changes sign.