Quick answer: The first law of thermodynamics states ΔU = Q + W: the internal energy of a gas changes by the heat added plus the work done on it. This AP Physics 2 lab on first law thermodynamics gives the aim, the equipment, the safety points, a step-by-step method, sample results and 7 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
When a gas is heated at constant pressure it expands and does work. In this lab you heat a trapped sample of air in a water bath, record its volume at six temperatures and use the results to find the work done, the amount of air and the fraction of the heat that becomes work. It links a graph to the first law.
Level: AP Physics 2 (algebra-based) · Curriculum: AP P2 Unit 9
Aim
Heat a fixed amount of air at constant pressure in a water bath, measure how its volume changes with temperature, and use the results to find the work done by the gas.
Equipment
- 50 mL gas syringe with a free-moving, lightly lubricated plunger
- rubber stopper or cap to seal the nozzle
- beaker of water, hot plate and stirrer
- thermometer
- clamp stand
Safety
Hot water scalds. Use heat-proof gloves when moving the beaker and never seal a syringe that is clamped rigidly. Wear eye protection.
Method
- Trap 50 mL of air in the syringe and seal the nozzle. Place it in a water bath at 20 °C with the air column below the water surface.
- Stir and wait 2 minutes, then read the volume.
- Raise the bath to 30 °C, 40 °C, 50 °C, 60 °C and 70 °C, waiting 2 minutes before each reading. Tap the plunger gently to avoid sticking.
- Record the room pressure as 1.01 × 10⁵ Pa.
Results
Sample measurements:
| θ (°C) | T (K) | V (mL) |
|---|---|---|
| 20 | 293 | 50.0 |
| 30 | 303 | 51.6 |
| 40 | 313 | 53.5 |
| 50 | 323 | 55.0 |
| 60 | 333 | 56.9 |
| 70 | 343 | 58.4 |
Analysis questions
Question 1 Constant pressure
Explain why the pressure of the trapped air stays constant, and state the name of this process.Question 2 The graph
Plot V against T in kelvin and draw the best-fit line. Describe the shape.Question 3 Gradient
Calculate the gradient of the line in mL/K, using the first and last points.Question 4 Absolute zero
Extrapolate the line to V = 0 using your gradient. Estimate the temperature at which V would reach zero.Question 5 Amount of air
Calculate the amount of air from the first reading using n = pV/(RT).Question 6 Work done
Calculate the work done by the gas between 20 °C and 70 °C.Question 7 Heat needed
Air is diatomic, so Q = (7/2)nRΔT at constant pressure. Calculate Q and the fraction of Q that becomes work.
