Today, I had just finished my Physic paper. At first, I was very happy cause the paper is quite easy and I can do all the question( not sure correct or not lah ). But after gearing from Joseph, Wai Ming and Tim, the question 4(a) actually is more complicated than I think.
This is a standard thermodynamic question with a piston and cylinder of different cross-sectional area, bigger in the upper part and smaller in the lower part. Then they add a weight on the piston to increase the pressure adiabatically and the question ask for final pressure and temperature. I just calculate the pressure, P1 and then the temprerature T1 using formula. According to them, when pressure increases to P1, the volume V1 is actually smaller than the V of the smaller part. Meaning, the piston actually 'stucked' in the upper part. The V will be the volume of the smaller part. And the pressure, P1 is wrong cause the piston is supported by the wall of the lower part. Then, we have to use the V of lower part to find new P2 and T2.
Summary, for a 6 marks question, first use the extra weight to find P1, V1 and T1. Then, V1 is smaller than V so it is illogical. We had to use the V of lower part as final volume. This make the P1 and T1 wrong. Using the V, we find another P2 and T2 as answer.WTH!
Ok, if you are confused, sorry. Just ranting. :)
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