Isobaric Processes
For a quasistatic isobaric ideal-gas process at constant external pressure, W_by = PΔV = nRΔT and Q = nC_PΔT, assuming only pressure-volume boundary work.
Why this shows up in the exam
Gas under a constant piston load · Horizontal legs of a P-V cycle · Comparing work and heat fractions at constant pressure
Learn the idea
At constant pressure, heat supplies both internal-energy rise and expansion work. A freely moving loaded piston keeps pressure fixed while heating increases both temperature and volume.
🧠 Memory hook: Constant P: heat covers U plus the RΔT expansion bill.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- W_by = P(V_f - V_i) = nRΔT — work by an ideal gas during a constant-pressure process
- Q_P = n C_P ΔT — heat transfer when molar C_P is constant
How to approach it
- 1Verify pressure is constant
- 2Use W = PΔV with signed ΔV
- 3Apply Q = ΔU + W and cross-check with C_P
Common slip-ups that cost marks
- •Using PΔV when pressure is not constant
- •Equating Q with ΔU in an isobaric expansion
- •Using gauge pressure where absolute pressure is required
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Original chapter practice
Original questions for this chapter, not past-paper questions or an exact mapping to this individual concept.
A gas absorbs 500 J of heat and does 200 J of work. What is the change in its internal energy?
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