WE CAN'T GO TO SAN FRANCISCO, BUT JOIN US 4-6PM JUNE 10, 2002 IN THE LARGE LECTURE HALL (0111) IN THE SCIENCE BUILDING FOR A MINI-PHYSICS EXPLORATORIUM: PHYSICS PHENOMENA AND WHY THEY HAPPEN. I. There will be 3 hands-on labs:, which can be done wih varying degrees of math precision. More information about these can be found on http://www.phyast.pitt.edu/~jth/safrica/labs waves and light scattering. collisions resistors and current flow: parallel and series circuits. II) Of more interest to the general audience might be various demonstrations which will be set up. Some examples follow, together with suggested zero-order motivating comments to relate these phenomena to others students may have seen or might see, or to give some heuristic framework in which to think about them. These explanations might be termed "novice explanations", as opposed to the "advanced" explanations also available in this directory for the different experiments. A. optics: 1. Holograms: when light scatters from an object it makes more complicated patterns than the shadows we normally see from sunlight blocked by ourselves. a hologram is a special kind of "3 dimensional picture" on film, which captures all the information about an image. looking through it you can look at the image in the same way you would look at a real object.... LOOK AT ONE! and talk with the student presenters about why it happens. 2. Crossed polarizers (rotate middle one, etc): light travels as a wave, with oscillations (movement back and forth) perpendicular to the direction of travel. objects called "polarizers" are able to "pick out" only one component of the oscillation (the part parallel to the transmission axis of the polarizer). normally if two polarizers are "crossed" (that is, the second polarizer has its axis of transmission perpendicular to the first), then no light can pass the pair. But what happens when a third polarizer (or other transparent object) is inserted between the pair? 3. Virtual images with diverging lenses: real images (such as focussing sunlight on materials for a fire) are formed when different rays physically come together. In virtual images, however, the rays extrapolate back to a common point, which tricks our eyes. but when you put your hand where the object would be, you find your hand in "thin air". why? 4. Parabolic focussing mirros (sound, light, heat.. match, etc). in this case the light (or other waves) really do come together in a real image. see how the focussed energy can really light a match. how does it work? 5. uv light (to show fluoresence). Ultraviolet light is "beyond violet" in the spectrum. Ultraviolet light has the highest energy. When an object absorbs ultraviolet light, it can later give off light at lower energies (longer wavelengths, in the visible spectrum). This re-emission or "giving back" the light occurs over a longer time period with a "decay time" of order seconds. B. electricity: 1. generator to show motion -> electricity if you move a wire in a magnetic field, positive and negative charges are separated. this separation of charges is the basis of useful electricity. see the generator light a light bulb? 2. van de graaf generator in van de graaf generator electrons are moved onto a conducting sphere by a belt. as the electrons pile up they provide an electrical energy source ("voltage"). see the more detailed writeup for a description of what experiments might be done. 3. cathode ray tube The basis of pictures in a TV or waveform measurements on an oscilloscope is the cathode ray tube: a beam of electrons, deflected by vertical and horizontal electric fields, impinging on a phosphorescent screen. When the electrons strike the screen, they excite the molecules in the screen to higher energy levels. As the screen molecules lapse back to their ground state, photons are emitted, giving dots of light on the screen. All together, the dots of light represent the voltages guiding the electrons. If color is required, beams of different energies are used to excite the atoms in the screen to different energy levels, therefore giving different photons (of different energies and therefore different colors) as the molecules decay. C. mechanics: 1. gyroscopes (gravitational precession, etc. think of a spinning top: as long as it is spinning fast, it will not fall to the ground, but if it is slightly oriented away from the vertical, it will begin to "precess" (the top of the top will make a small circle about the vertical axis). finally, as the top slows down, it falls to the ground. "precession" is the basis for MRI medical scans and other applications. navigational gyroscopes depend on keeping the "top" spinning very fast so that it always maintains its direction. the gyroscope or "spin" effects need some advanced physics to describe them well... hope one day you'll be doing the explaining! 2. angular momenta (spinning stool, bicycle wheel, etc) here are some other spin effects. all of them depend on an idea that the total amount of "spinning" is "conserved", or adds up to the same total, unless there are special twists on an object. You'll be able to see how an ice skater feels when she or he extends her/his hands or brings them in to change the spin speed. Also try holding a spinning bicycle wheel on a special rotatable platform. How does your spin change as you change the bicycle wheel orienation? Why? WHAT happens you can say today. WHY needs some physics background to really explain. (See the "advanced" explanation, if you want to). 3. large bowling ball pendulum Did you ever wonder how to make a "grandfather" clock? This clock is based on a pendulum. Check sometime about how long it takes a swing to go back and forth. (a swing is a kind of a pendulum). Is it the length of the swing or the size of the person sitting in it which changes the period of the swing (how long it takes to swing back and forth. There are a lot of neat things about pendulums. But one is the idea that it has a period (T) and a cycle. If you start it from rest, at your nose, it will swing across to the other side of its arc, and then come back and just barely miss hitting your nose. The reason is that the height to which the ball travels gives it a kind of energy, just as its speed gives it a kind of energy. As the pendulum goes back and forth, its energy changes from "height" energy (or "potential" energy) and "motion" energy (kinetic energy). Unless it gets more energy by someone pushing it, it can't go any higher than it started before stopping and turning around. So... how much do you believe? Want to try it out? Don't let your classmates stand on the other side and give the pendulum a push! 4. coupled springs/pendulums/tuning forks another funny thing is that springs or pendulums or even tuning forks which are identical can vibrate or make their cycles moving exactly together. you can even start one moving and the other will couple to it through vibrations in the surroundings. almost seems magic when you see a tuning fork which has not been struck start to vibrate just because its nearby twin is vibrating! 5."visiting see saw"... levers From early days you know that on a see-saw (or teeter-totter), a small person can be "small but mighty" as he or she raises a parent or older kid, if the heavier person can only be persuaded to sit close to the fulcrum and let the little person sit far away. And levers are used in practical applications to allow a small force to move a large object. In this demonstration, with pennies and a meter stick, you can test this out for yourself. It turns out that the amount of "twist" you can exert depends not only on the force but also on the "lever arm" (the distance, in perpendicular projection, from the point of application of the force to the axis about which the twist is occurring). When the see-saw is not moving, we saw it is at the balanced condition. For this condition to obtain, the "torques" (or "twists) in different directions cancel out. For the simple case of the seesaw, the force is the weight of the pennies (mg), and the lever arm at balance is just the distance of the objects from the axis. Try it out! D. other: 1. Magdeburg hemispheres... how does air pressure work? it is said that even a team of horses could not pull apart these evacuated hemisphers... the air pushes in too strongly. 2. sound wave/water wave/light wave interference waves from a boat spread out behind the boat, with crests and troughs travelling away from the boat. if another boat is in the vicinity, it will also make a wave. when the two waves meet they "interfere", to make a new pattern. when the new pattern is higher than either of the two individual ones, they "constructively interfere", adding up the individual disturbances. when the new pattern is nearer zero than either one individually, the two waves "destructively" interfere. light waves are shown in the scattering hands on activity. check here to see a similar happening with water waves, or sound waves (made by vibrations of the air, which then causes your eardrum to vibrate).