Latent Heat, Phase Change, and Heating Curves
At a first-order phase transition under fixed pressure, heat Q = mL changes phase at the transition temperature; a complete thermal path is divided into sensible-heat and latent-heat segments.
Why this shows up in the exam
Ice-water-steam mixtures · Reading heating and cooling curves · Estimating melting or vaporization times
Learn the idea
Temperature changes within a phase, but ideal phase transitions absorb or release latent heat at constant temperature. A heating curve is an energy itinerary: sloped parts warm one phase and plateaus rearrange molecular structure without changing temperature.
🧠 Memory hook: Slopes heat phases; plateaus change phases.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- Q_phase = m L — latent heat for mass m changing phase at the stated transition
- Q_total = sum(m c Delta T) + sum(m L) — piecewise energy across warming and phase changes
- Delta t_plateau = mL/P — plateau duration under constant net heating power
How to approach it
- 1Draw the initial and possible final states
- 2Compute energy segment by segment
- 3Stop when available energy is exhausted and report phase amounts
Common slip-ups that cost marks
- •Raising temperature during an ideal plateau
- •Using the entire mass as changed phase without an energy check
- •Skipping intermediate warming segments
🌟 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 wire 1 m long and cross-sectional area 2 mm^2 extends by 1 mm under a 200 N load. Find Young modulus.
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