Imagine a glass ball, palm-sized, fused with a glass stand so it won't roll. Inside, a small vertical pole. Upon this, a freely-spinnable assembly of vanes. Think weathervane, think windmill, think paddlewheel. Each arm of this assembly is painted on one side with white, the other with black. It's designed to demonstrate a physical property, so there's very little frills, just exactly what is needed to let it do its thing.
This is from one that came out of the 80s:
SPACE AGE SPHERE - POWERED BY THE SUN!
The vanes, or wings in the Radiometer are alternately dark and light in color.
When the light strikes these wings, it tranfers heat to each one -- but not to the same degree. The lighter wing reflects the rays, and the dark wing absorbs the rays.
Actually, the atoms absorb the photons and emit phonons. My teacher said so.
Now, when the freely moving particles of air inside the Radiometer strike the light reflective vanes, they take on very little energy and do not bounce off very fast.
However, when the atoms strike the dark vanes, they take on a great deal of energy... and "kick" away at terrific speed.
Maxwell would be proud.
Result: the vane begins to move, and continues to move as the bits of air continue to "kick" away from the dark-sided vane.
damn that rising entropy...
Naturally, the stronger the light, the more energy there is to "heat up" the dark wing... keeping the bits of air bouncing away at a faster and faster rate as the light gets brighter.
dF = IdR
I was seven. Downtown Lansing had a museum called Impression 5, and was geared for the younger crowd. The idea was to make science experiential, to bring physics and chemistry and many other disciplines to children so they could appreciate it more.
I was in heaven. Every time. One visit, we went to the little shop in the corner, and there were little plastic dinosaurs and books about the water cycle and videos about how paper is made. And, to my delight years later, boxes of Radiometers. There is no company name on the box, only a teasing "Made & Printed in U.S.A."
I remember watching it spin years ago, as if it were a dream. The glass bulb was a lot bigger in my hands, for a while I wasn't allowed to touch it. I'm still afraid I'll break it, and lose another piece of childhood.
Since then there has been Quantum Mechanics. There has been Electromagnetics. There's even been first love and last rites and many more strange and wonderful things. And while I'll never stop striving for a stronger me, a better tomorrow, but sometimes I wish I didn't know now the things I didn't know then.