Solution errata are in dropdowns so you don’t have to see it.
I’ve provided very rough cutoffs for 2024-2026. Don’t take them too seriously; they’re just there for reference and I don’t actually know the real cutoffs as scores are not released.
In general, this year’s problems are way too long but otherwise physically simple, similar to recent APhOs (so this is a good thing if you are practicing for APhO, but bad if you like doing physics).
P9b: I'm pretty sure P9b is wrong but I'm not completely certain. Drop me an email if you have any comments about my analysis.
There is a pressure gradient in the liquid as the bubble is not necessarily in mechanical equilibrium, so the pressure $P_\text{ext}$ refers to the pressure of the liquid very far from the bubble.
I think the problem is actually asking for the availability function of the water-vapour system $A$, rather than the Gibbs function $G$, which is slightly different. Consider a system with internal energy $U$, entropy $S$ in an environment with external pressure $P_\text{ext}$ and external temperature $T$. The availability is then defined as
$$ A = U + P_\text{ext}V - TS $$
The problem is asking you to find the change in availability of the water-vapour system $\Delta A$. Use this for the next part too.
I have not checked this year very thoroughly as they do not return your working and I do not intend on going through this again. There was also an error in P9 and a few typos around but they have been fixed in the latest version.
Note: P1 is probably inspired by the 2018 All-Russian Olympiad and P4B is copied from the 2019 All-Russian Olympiad. P6B seems loosely inspired by APhO 2017 T1.
P1: Part (a) is phrased quite badly; just don’t do it. Assign the one mark to P7(c) or something.
P7A: Ignore the light ray reflecting off the top of the lens.
In reality lenses are usually thick enough that this isn’t an issue as the maxima would probably be blurred by imperfections, but because the question specifically states that it is a “thin lens” interference, they are pretty much shooting themselves in the foot.
P7B: Part (c) is pointless and has loads of issues. Skip part (c) and do part (d) directly. In part (d), you have to assume that $\delta \lambda/\lambda \ll \lambda r /d^2$, and find $I(\theta,t)$ in the limit of $\theta \ll 1$.
P7B
i was too lazy to check P2 and P9’s answer because P2’s answer was too big and i don’t feel like doing P9.
Note: multiple questions have minor deviations between the answer key and the question. It seems like the question/answer may have been modified last minute.
Note: P8 is copied from the 2016 All-Russian Olympiad.