The First Law

Prerequisites

For this section, students should have:

  • A working ability to perform total differentials.
  • Basic understanding of what a partial differential represents in a physical process.

In-class Content

QUIZ

Activity: Comparing the System and Surroundings

Link to Comparing the System and Surroundings Activity

Activity Highlights

  1. This integrated laboratory activity is designed to help students understand heat, heat capacity and entropy.
  2. Students use a water proof resistor to heat ice water while measuring the temperature.
  3. In the lab write-up, students analyze their data to find heat capacity, latent heat, and changes in entropy.

Lecture: First Law of Thermodynamics (10 minutes)

  • This lecture is an introduction to the first law of thermodynamics
  • Some students may have heard it mentioned before, but many may have never heard it
  • The fact that energy is conserved is an idea that will come naturally to most students, but it may also lead to other inferences that are incorrect
    • For example, many students incorrectly assume that entropy is also conserved
Lecture notes from Dr. Roundy's 2014 course website:

The first law of thermodynamics simply states that energy is conserved. But it is useful to look at those two non-state variables work and heat. Both are changes in energy of a system, so we can write the first law as $$\Delta U=Q+W$$ where $U$ is the internal energy of the system, $Q$ is the energy added to the system by heating, and $W$ is the work done by the system (or the energy removed from the system by working). It is more convenient mathematically, however, to have a framework for talking about infinitesimal changes in the total energy… $$dU=đQ+đW$$ If I stretch a rubber band and snap it shut repeatedly, what will happen to its temperature? If students look puzzled, suggest they assume that as long as you do this reasonably quickly, the air doesn't get heated up much (nor the fingers). [SWBQ]

Lecture: Snapping a Rubber Band

  • SWBQ: If you stretch and then let a rubber band snap, what will happen to the internal energy and the temperature of the rubber band?
  • This is an interesting example, as rubber bands may change temperature in either direction as it is being stretched, dependent on the type of rubber band used. However, because the process returns to its original state, you can use the First Law to show that the temperature must increase regardless.

Homework

  1. (CoffeeAndBagels) What goes here?

  2. (ExtensiveIntensiveChecking) What goes here?

  3. (ExtensiveInternalEnergy) What goes here?


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