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2. Atomic Structure

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MIT 5.111 Principles of Chemical Science, Fall 2014
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Instructor: Catherine Drennan

The backscattering experiment of Rutherford is recreated in the classroom setting. Ping pong balls are used to represent alpha particles and Styrofoam balls connected to a series of strings represent nuclei in a piece of gold foil.

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  1. Your error rate will render the results meaningless… you should have instructed the radioactive students to actually ensure that the ping pong balls cross hit somewhere within the area of the wooden frame…. tsk, tsk, tsk…..

  2. I feel like when I was in high school and college, my brain wasn't developed enough yet to appreciate or be interested in this stuff. I saw these sorts of courses as "work" to suffer through. But now that I'm in my 30's, I finally have the attention span and the interest to learn these things. Unfortunately, I am no longer in college, and have a boring 9-5 job. Thank you MIT and YouTube for making this material available so I can at least enjoy it on nights and weekends!

  3. If a cathode ray is by definition a stream of electrons, then how could the cathode ray have a positive charge? And why should only protons stream out instead of a linear combination of protons and electrons, so that the "net charge" could be assumed to be equal for both electrons stream's deflection and proton stream's? You apply more voltage difference, so there is definitely more energy in the ionized H_2 molecules' movement, and I suppose this could justify the additional presence of positive ions, but then, once they're out to reach the linear path, shouldn't they cancel out, if both are exiting at the same time? I mean ok, thermal noise is a random process, so even with a theoretical (averaged) voltage difference of 0 we can still have random fluctuations, both positive and negative, thus leading to, theoretically, a positive or negative net current – further justifying the presence of protons despite cathode rays being the main thing there. But this introduces another problem, that is even the amplitude of the random process is indeed a random variable. And since dV/dt is proportional to the current, which itself is dQ/dt, then the "amount" of charge Q itself, for a given time interval in which the experiment is observed, should be a random process as well. So… Since the ray is a "stream" of (electrons?), which I interpret as a varying and moving set of elementary particles, then the net charge at a given time should be a random variable. I must be missing something, so I wonder; how could the dude be so sure that whatever he saw had the same charge as the electrons, in absolute value? Like, from my pov, one could have a stream of positively charged particles that are fewer with respect to the negative ones previously found in the ray. Where's the proof they are equal? I don't think this experiment is as easy and trivial as it was presented. Could someone clarify, please?

    Also, it is unclear whether the "increased" voltage difference was applied on the plates or on the illustrated battery – that would change the whole thing even more, but still make no sense to me.

    Edit: after re-watching, I see that the voltage difference is meant to be Delta V, which in the picture is the voltage drop between the two plates. Okay, so this could further change the problem; if the battery voltage is kept constant and the plates' is increased, then no stuff about ionization, at the first stage, should change – same amount of electrons exiting from the cathode, same number of protons, if any, exiting as well (at least on average). Now, since I still can't explain myself the flow of protons towards the low potential zone, I'm just assuming it's due to either the random nature of particles or by the fact that an increased V would produce a negative variation of the magnetic flux, to the point where a huge instantaneous variation can occur, giving rise to, after a very small time, a very negative magnetic field which is now the factor that dominates the Lorentz force (even though if this is the case, then simply waiting for longer time would give the same result even with lower voltage difference), thus producing a force parallel to the electric field, but in the opposite direction, making protons come out instead. This would justify how the increment in potential difference between the plates could lead to a "leak" of protons stream. But still, that would still be influenced by both thermal noise, thus variable impedance, and the Lorentz force's electric field factor. I don't really see how can a visual result imply such confidence in the "equal charge" statement. To me, it sounds like an equivalent statement to claiming that the charge at a given point of a circuit A is the same as the charge at the same point in a circuit B, completely identical to A, just because we see current in both circuits and because the charge of an electron is always the same. I mean, circuit A could be powered up by a 12V battery and circuit B by a 5V one, or circuit A could be exposed to higher temperatures than circuit B, leading to a different impedance and thus different currents. Ok, current != charge, fine, but from what I see in this experiment, one point of interest is chosen and then, for a given time interval, the flow is observed in that point. If charge builds up in that point, then this is equal to integrating the current, giving in fact the amount of charge which is dependent on the interval length, a further reason to add uncertainty in the statement that charges are equal. I am aware that thermal noise may be negligible since we're talking about 3 orders of magnitude in difference, but I still don't think that, visually, one can deduce whether the charge is approximately the same or not, especially when dealing with light, whose intensity after a threshold is not as influential/noticeable in luminosity as when you're within a middle interval of intensities.

    Dunno, maybe I'm misinterpreting the experiment in general. Also, I don't understand whether there is a common ground between plates and the cathode, and if the battery powers up both the cathode/anode H_2 circuit and the capacitor, or if they're two separate circuits.

  4. thank you for these! i was watching the older version (2009), when dr drennan and dr vogel-taylor were co-teaching. i reaaaaaally love it. now that it is 2021 i wonder, does it matter that these are so outdated? my guess is moooostly, no – but if anyone has information they want to share about any of these question, i send huge appreciation!:
    are there any/significant differences in the content taught in the 2009 version and this version?
    and/or has there been any science research which has rendered this version or the 2009 content obsolete/outdated?
    and/or does anyone know if there is/will be soon, an MIT OCW update available? (or non-MIT alternatives that are as amazingly taught?)


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