modphys 32/180: Atomic Model

On a superficial level, atomic scattering of alpha particles is a simple process to understand. My students have usually been able to repeat back the main observations and conclusions: most alphas pass straight through but few are significantly deflected, so atoms are mostly empty space, with a small, hard nucleus. However, I have always suspected that my students didn’t really understand how the experiment worked theoretically, nor why this complex system was required.

Last year, I began to use a simulation in which I hide an object under a board and have students try to figure out what it is by shooting marbles at it.


I use blocks to hold up the edges or the board. It is good for these to stick out a bit so the students don’t conflate their effect with that from the hidden object. For objects, I have tried a number of things. A water balloon or barely-unflated rubber balloon provides a good elastic collision. A heavy round mass is good too, but produces a distinctive tink, which sort of ruins the mystery. One good way to get an inelastic collision is to increase the rolling friction in a region by putting down a piece of thin felt or newspaper. The former represents the Rutherford atom, while the latter is more of a plum pudding model.

It is hard to assess the effect of an activity like this, but it seems to me that my students have been able to talk more cogently about the Geiger-Marsden experiment when they do the hidden object activity first.

A problem we run into is the direction the marble flies out of our clumsy hands. I think I will try to make some launchers, similar to what you would find in a pinball machine. Such launchers could also be good for studying projectile motion.


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