2021 LCHL Physics Sample Solutions....

I've uploaded here a set of solutions for the 2021 paper. Anybody who wants to is free to use them as they please (as long as its not commercial) - or to adapt them to their own uses.

If anybody sees any errors in there, please let me know. Though I have checked them against the marking scheme and listened in on Diarmuid Hickey's very interesting presentation about how the exam was marked this summer and I think they're reasonably robust.

One interesting thing that came up was in question 11, about Chadwick's experiment where he

bombarded a Beryllium nucleus with alpha particles. When I was working out the masses before and after to look for the mass defect, I used the alpha particle mass from the tables. But when I checked I found the marking scheme made use of the value for the mass of a helium atom from the latter part of the tables.

At first, I thought I had to be correct. After all, the question had specifically said that alpha particles were used, not helium atoms. But I have learned enough over the years to doubt myself, even when I'm convinced I'm right, and sure enough Diarmuid very clearly explained during that zoom call why the marking scheme was right.

If I understand it correctly its this: Ideally we would be using the masses only of nuclei in carrying out these calculations. After all, this is nuclear physics. But if we look up the masses of, say, Beryllium and Carbon in the tables, we are getting atomic masses: the values given include the masses of the electron.

So when it comes to choosing to use either the mass of the alpha particle or that of the helium atom, we choose the latter, even though the question refers to the former. And we do that because if every value we use in the calculation includes the masses of electrons, the issue will sort itself out and yield a valid value for the mass defect. But if we mix in the alpha particle mass with that of various nuclei, we are including the electron mass on one side of the equation but not the other, and thus straying into error.

It's actually an issue that was always on the back of my mind to worry about, without ever quite making it onto my must-figure-out-now list. Happily , though, we know that students who used either figures were awarded full marks as long as they followed though to the end.

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