1:13 AM
@EliahKagan that is what I meant, sorry

No problem. That stuff was the stuff I mentioned there that I was hoping to get around to saying, anyways.
Regarding how the meaning of validity is typically clear in actual use: an argument is valid iff you can reason deductively from its premises to its conclusion (without making mistakes). A sentence is valid iff you can satisfy yourself of its truth by reasoning deductively, even with no premises. A schema is valid iff all the sentences it matches (i.e., that take its form) are valid.
Anyway, since you're exceptionally busy from yesterday or so until a ways into February, it occurs to me that you might not want to do much formal symbolic stuff during this time. (But perhaps you do; if so, you can let me know.) Do you want to consider some Hilbert's-hotel style problems informally instead?

4 hours later…
5:06 AM
Story about distinguishing between physics and chemistry:

When I was doing my physics-for-teaching course at the University of East London, we had one module on nuclear physics. I remember the teacher talking about how nothing you could physically do to a radioactive substance would have any effect on its radioactivity. He said something like:

You can heat it to 1000 degrees, melt it down, burn it up, turn it into a gas, smash it into a million pieces, react it with anything you like, and it will make no difference, because all you're doing is messing with the electrons. The electrons are

5:45 AM
@Zanna On the other hand, one could move two samples closer to each other.
Such as two masses of plutonium-239, each of which is by itself subcritical.

O.O
:)
That was the only course about physics I did, which is why I don't know much XD
@EliahKagan hmm I suspect that, during this time, if I take a break for longer than 2 minutes, it will consist of anything but sitting at a desk in front of a screen reading or trying to work something out...

6:21 AM
:)

6:48 AM
one thing that makes me sad while marking exams, is that students expect technology to be mysterious and opaque in its workings. This question has a diagram showing how this raindrop detector works, and the task is to explain how this detector causes the windscreen wipers to turn on. But the students write "there is a detector that automatically detects the rain and it turns the windscreen wipers on". The picture is right there. But they assume hidden magic nonetheless.

9 hours later…
4:11 PM
@Zanna Do students believe that to be considered a correct answer? Or is it a way of writing something, rather than nothing, on questions where they don't know the answer or lack sufficient remaining time to write a correct answer?
@Zanna Hmm. My impression is that you know a quite substantial amount of physics...

@EliahKagan I have simplified things a bit, but, certainly it is better to write something than nothing. Last session, on the paper I was team-leader for, there was a question that asked how an analog signal could be converted to a digital signal. At the pre-standardisation meeting, we decided to amend the mark scheme to allow one mark (of three available) for "use an AD converter" if nothing else in the response was creditable
@EliahKagan maybe enough for my purposes and enough to enjoy. I loved that course! We had a super gruff and grouchy and strict teacher for the energy module. Everyone disliked him except me I think. He gave the best problems. Used to have dreams about those problems. Used to wake up laughing with the answers after struggling all evening. I loved that course. I had finally become a good student, by which I mean a dedicated one.