Students consider how changing the volume of a system changes the internal energy of the system. Students use plastic graph models to explore these functions.
Orient Yourself to the Physical System & the Graph: The surface represents measurements of internal energy on a kilogram of water vapor in a piston (a graduated cylinder with a moveable top). The purple surface graph is \(U(S,V)\).
Make Predictions: Starting at the red star, as you increase the volume of the system, what happens to the internal energy of the water vapor?
Answer: The internal energy could increase, decrease, or stay the same. It depends on how the volume is increasing (and what is happening to the entropy).
Student Ideas: Most students will interpret “increasing volume” as “hold the entropy fixed”. This is, of course, a possibility, but it is not the only possible way to increase the volume. From their math classes, many students will have the idea that for a partial derivative, everthing else must be held constant.
Discussion: Alternative Graph Could you use the other surface? If so, how?
Discussion: Does it matter where you start? Does your answer depend on the initial state of the system
Helping Students Understand the Graphs: Students may need guidance to recognize the meaning of the contour lines, and the fact that they can use either surface to answer this question (although one is much easier that the other).
Consider a Special Case: Is it possible to change the volume without changing the internal energy? (Is your answer consistent with your answer to the previous prompt?)
Answer: Yes, it is possible - you move along a level curve of the internal energy. Along this level curve, both volume and entropy change.
Discussion: Checking Consistency Some students will happily answer “internal energy increases” to the first prompt and “yes it is possible to change the volume and not change the internal energy” to the second prompt without resolving the inconsistency between the two answers.
Signs of Partial Derivatives: Using the purple surface graph, determine if the following derivatives are positive, negative, or zero.
\[\left(\frac{\partial U}{\partial V}\right)_S \quad \quad \quad \quad \left(\frac{\partial U}{\partial V}\right)_T \quad \quad \quad \quad \left(\frac{\partial U}{\partial V}\right)_p \]
SUMMARY PAGEGoals
- For functions of two independent variables, you need to specify changes in both independent variables to determine how the function changes.
- Can lead into a lecture about partial derivatives and specifying a path.
Time Estimate: 15 minutes
Tools/Equipment
- A Purple \(U(S,V)\) Plastic Surface for each group
- Dry-erase markers
Intro
- Students need to be oriented to the surfaces and what they represent.
Whole Class Discussion:
- The main take-away is that for functions of two variables, you need to specify a change in both the independent variables to know the change in the function.
- You can ask student groups to share an example of a path where the volume increases and the internal energy: increases, decreases, and is constant.
- This activity leads into ideas about (1) how the number of independent variable you have in a thermal system corresponds to the number of ways of getting energy into or out of a system, (2) you need to specify the path for a partial derivative, and (3) for thermal system, you actually CANNOT “hold all the other variables constant”.
group Small Group Activity
30 min.
group Small Group Activity
30 min.
group Small Group Activity
30 min.
group Small Group Activity
30 min.
group Small Group Activity
30 min.
assignment Homework
Consider one mole of an ideal monatomic gas at 300K and 1 atm. First, let the gas expand isothermally and reversibly to twice the initial volume; second, let this be followed by an isentropic expansion from twice to four times the original volume.
How much heat (in joules) is added to the gas in each of these two processes?
What is the temperature at the end of the second process?
Suppose the first process is replaced by an irreversible expansion into a vacuum, to a total volume twice the initial volume. What is the increase of entropy in the irreversible expansion, in J/K?
group Small Group Activity
60 min.
Mechanics Gravitational Potential Energy Zero of Potential Introductory Physics
Students examine a plastic “surface” graph of the gravitational potential energy of an Earth-satellite system to explore the properties of gravitational potential energy for a spherically symmetric system.group Small Group Activity
30 min.
group Small Group Activity
30 min.
Mechanics Gravitational Force Gravitational Potential Energy Derivatives Introductory Physics
Students examine a plastic "surface" graph of the gravitational potential energy of a Earth-satellite system to make connections between gravitational force and gravitational potential energy.assignment Homework
A diesel engine requires no spark plug. Rather, the air in the cylinder is compressed so highly that the fuel ignites spontaneously when sprayed into the cylinder.
In this problem, you may treat air as an ideal gas, which satisfies the equation \(pV = Nk_BT\). You may also use the property of an ideal gas that the internal energy depends only on the temperature \(T\), i.e. the internal energy does not change for an isothermal process. For air at the relevant range of temperatures the heat capacity at fixed volume is given by \(C_V=\frac52Nk_B\), which means the internal energy is given by \(U=\frac52Nk_BT\).
Note: in this problem you are expected to use only the equations given and fundamental physics laws. Looking up the formula in a textbook is not considered a solution at this level.
If the air is initially at room temperature (taken as \(20^{o}C\)) and is then compressed adiabatically to \(\frac1{15}\) of the original volume, what final temperature is attained (before fuel injection)?