Yesterday, my research group and I had the privilege of teaching the Happy Hollow Elementary Science Club about magnetism. Well, not everything about magnetism, but a few fundamental principles: the non-linear relationship between magnetic field strength and distance, and the effect of increasing a system's magnetic moment.
In English, the first part means that the force you feel from a magnet increases very quickly, actually, stupendously quickly as you get closer to the magnet (and vice-versa). In mathematics, this type of relationship has a name: exponential. The second part means that if you change the number of magnetic particles in a system, you affect the field strength of that system. To stretch this paragraph out a bit longer: if you take a magnet and break it exactly into two, you will get two magnets with half the total field strength in each.
My role in the demonstration was to deliver the opening monologue, which I did, along with the following visual aide.
Not to be confused with a Torii, this simple device consisted of a pair of (in actuality, we had six) super-strong magnets taped to the center of a suspended, wooden dowel. A pair of steel washers were attached to both the base of the contraption and a pair of plastic cups, which were suspended wirelessly (you get the idea) under the magnets.
The only difference between the two washers was their respective distance from the magnets. The jar on left of the first picture contains plastic beads, which were used as units of mass.
The demonstration was to show that even a teeny, tiny difference in distance (about 1 cm) affects the attractive force felt by the washer immensely; the closer washer being 'pulled' more strongly than the further away washer. This discrepancy was made quite clear by the difference in the number of beads each washer/cup assembly was able to support. The cup on the right fell after ~20 beads, but the cup on the left, a mere centimeter closer, was able to hold a cupful of beads and probably more! I was able to milk the tension by taking bets at the beginning of each trial and counting slowly as I incremented the number of beads in each cup. (My demonstration was followed by two hands-on activities, which let the kids explore the concepts more deeply.)
Overall, I was very impressed by the acuity of the kids. In fact, quite a few of them were already familiar with the concept of magnetism and asked a number of questions which were fundamentally interesting: "Can you get a magnet with only one pole?", "What happens if you break a magnet into two?", "Is the magnetic field blocked by tape?", "Why are some materials magnetic and others not?"... All valid, scientific questions; some yet to be answered!
For the lay person, I would highly recommend: Physics 2000, a wonderful presentation of physics concepts in the style of the Socratic Dialogue. If you manage your way through all the applets there, I also recommend PhET and Physics 8.02 @ MIT.
For the more advanced reader, I can't express what a revelation Prof. Lewin's online lecture videos were for me: 8.02 Electricity and Magnetism.