Isobaric Process Input Form
Pressure–Volume Plot
An isobaric path appears as a horizontal line on a pressure–volume diagram because the pressure stays fixed while the volume changes.
Example Data Table
| Scenario | Pressure | Initial Volume | Final Volume | Initial Temperature | Moles | Expected Trend |
|---|---|---|---|---|---|---|
| Constant-pressure heating | 101.325 kPa | 1.00 m³ | 2.00 m³ | 300 K | 40.624 mol | Temperature, work, and heat increase |
| Moderate expansion | 150 kPa | 0.40 m³ | 0.55 m³ | 320 K | 22.553 mol | Positive work and positive heat input |
| Compression at fixed pressure | 90 kPa | 0.80 m³ | 0.60 m³ | 360 K | 24.055 mol | Negative work and cooling behavior |
Formula Used
For an ideal-gas isobaric process, pressure remains constant throughout the path.
Ideal gas relation: PV = nRT
At constant pressure: T₂ / T₁ = V₂ / V₁
Boundary work: W = P(V₂ − V₁)
Internal energy change: ΔU = nCv(T₂ − T₁)
Enthalpy change: ΔH = nCp(T₂ − T₁)
Heat transfer: Q = ΔU + W = nCp(T₂ − T₁)
The calculator converts entered units to SI units first, performs the thermodynamic calculations, and then presents the final values clearly for interpretation.
How to Use This Calculator
- Enter the constant pressure value and choose the correct pressure unit.
- Provide initial and final volume values using a shared volume unit.
- Enter the initial gas temperature and select Kelvin, Celsius, or Fahrenheit.
- Supply the amount of gas in moles.
- Select a gas model, or choose custom heat capacities.
- Click the calculate button to view results above the form.
- Review work, heat, enthalpy, internal energy, and graph output.
- Use the CSV or PDF button to export your results.
Frequently Asked Questions
1. What is an isobaric process?
An isobaric process is a thermodynamic change that happens at constant pressure. Volume and temperature may change, but the pressure remains fixed throughout the process path.
2. Why is work easy to compute here?
Because pressure is constant, boundary work is simply pressure multiplied by the change in volume. On a P–V graph, that work equals the rectangular area under the horizontal process line.
3. Does heat always equal work in an isobaric process?
No. Heat equals work only in special cases. Generally, heat input covers both the rise in internal energy and the boundary work done during expansion.
4. Why does temperature change with volume?
For an ideal gas at constant pressure, temperature is directly proportional to volume. If volume doubles, absolute temperature also doubles, provided the amount of gas stays unchanged.
5. What does the gas model selection change?
The selected gas model changes Cv and Cp. Those heat capacities control internal-energy change, enthalpy change, and total heat transfer for the same temperature difference.
6. Can I use Celsius or Fahrenheit safely?
Yes. The calculator converts entered temperatures to Kelvin internally. That keeps thermodynamic ratios and energy equations consistent before showing output in your chosen temperature scale.
7. What does the ideal-gas check error mean?
It compares your entered pressure with the pressure implied by nRT/V at the initial state. A nonzero difference suggests rounding, inconsistent inputs, or a nonideal assumption.
8. When would compression give negative work?
If final volume is smaller than initial volume, the system is compressed. The gas then has negative boundary work because the surroundings do work on the system.