Isobaric Heating: Expansion at Constant Pressure
1 · Predict
A gas is heated at constant pressure, so it expands (isobaric). If you let it expand to a larger final volume, does the gas do more work on its surroundings?
- A larger final volume means more work done by the gas.
- A larger final volume means less work done by the gas.
- Work done doesn't depend on the final volume.
2 · Set Up
- Open the isobaric-heating preset and press Reset. 1 mol of gas starts at 0.0249 m³, 300 K, at constant pressure.
- Enable the final-temperature readout on the chamber.
- Set the target (final) volume for each trial, and record the final temperature and the work done.
3 · Collect Data
| Final volume V₂ (m³) | Work done by gas W_gas (J) | Final temperature T₂ (K) |
|---|---|---|
| 0.03242538 | ||
| 0.03990816 | ||
| 0.0498852 |
Plot final temperature T₂ (y-axis) against final volume V₂ (x-axis) for your three trials. Is the line straight through the origin?
4 · Analyze
- For one trial, compute W_gas = P₁(V₂ − V₁) using P₁ = nRT₁/V₁ = 100 kPa, then T₂ = T₁·V₂/V₁. Compare both to the table.
- Explain, using W_gas = P·ΔV, why a bigger volume change at the same constant pressure means more work done by the gas.
5 · Extend
- Your T₂-vs-V₂ line is Charles's Law: at constant pressure, volume and absolute temperature are directly proportional. Explain why this follows directly from PV = nRT when P is held fixed.
- Some of the heat you add during isobaric heating goes into work (expansion) and some into internal energy. Which one gets more of the added heat for a monatomic gas (f = 3): the work term or the internal-energy term?
The Physics Behind This Experiment
Isobaric Work
At constant pressure, the work done by an expanding gas is simply pressure times the volume change: W_gas = P·ΔV — the area of a rectangle under a horizontal line on a P-V diagram.