Thermodynamics: Energy in Action

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Period:

Topic: Temperature vs. Internal Energy & The First Law

Part 1: Defining the Terms

1. In your own words, what is the difference between Temperature and Internal Energy?

2. The "Iceberg vs. Coffee" Problem:

A small cup of boiling coffee is sitting next to a massive glacier. Which has a higher temperature? Which has a higher internal energy? Explain why.

3. The Sparkler Question:

A spark from a sparkler is 1200°C, but it usually won't hurt if it lands on your skin. A 200°C oven will burn you instantly. Use the concept of total molecules and energy transfer to explain why.

Part 2: The First Law of Thermodynamics

ΔU = Q + W

The Cylinder & Piston Model

Imagine a sealed cylinder with a movable piston on top. The gas inside has an internal energy (U).

  • Heat (Q): Energy transferred into the gas (+Q) or out of the gas (-Q).
  • Work (W): Energy transferred by moving the piston. If an external force pushes the piston down to compress the gas, work is done ON the gas (+W). If the gas pushes the piston up to expand, work is done BY the gas (-W).
  • ΔU: Change in Internal Energy
  • +Q: Heat added to cylinder
  • +W: Work done ON piston (Compression)

Calculate the change in internal energy (ΔU) for each scenario:

A.

You add 500 J of heat to a gas in a sealed, rigid container (the container cannot expand, so no work is done).

  • Q = ______ J
  • W = ______ J

ΔU = J

B.

A cylinder of gas is compressed abruptly. You do 300 J of work ON the gas, and 50 J of heat escapes from the cylinder to the room.

  • Q = ______ J (Hint: Pay attention to the sign!)
  • W = ______ J

ΔU = J

C.

A system absorbs 400 J of heat from its surroundings. Because it gets hotter, it expands and pushes a piston outward, doing 150 J of work BY the system.

  • Q = ______ J
  • W = ______ J (Is work done BY the system positive or negative W?)

ΔU = J