Demonstrations

Summer Solstice in Wooster Hall

Eric Myers Astronomical, Buildings, Demonstrations 0 Comment
Summer Solstice in Wooster Hall

Wooster Hall at SUNY New Paltz has a neat feature.   The main staircase is exactly aligned along a north-south line, and skylight windows in the ceiling were placed so that light from those windows lines up at the bottom of the staircase at solar noon on the equinoxes.    In the summer the sun is higher, and so the light from the skylights lines up with the top stairs of the staircase.     It’s become an event on campus to come watch the lights slowly crawl over until they line up with the staircase.

The first time I watched this, last spring, I was inspired to create a time-lapse video; but without preparing ahead of time I ended up standing up against a wall for an hour, taking pictures every minute, and then later writing a Python script to assemble the frames into an animated GIF. The results can be found here, and the technical details are here.

For the subsequent Summer Solstice I was ready with both an iPhone set to time-lapse mode and a Raspberry Pi programmed to take pictures every 5 seconds. The result from the Raspberry Pi is now on YouTube (watch the stripes of sunlight on the top stairs, not the people):

Technical details of how the Raspberry Pi was configured may be shared later. Instead of trying to assemble the time-lapse video on the Raspberry Pi itself, this video was assembled using iMovie on an iMac.   (I tried to use software called TLDF, but it requires frame sizes of at least 800 pixels, and the frames captured for this video were 640×480.) The result was an mp4 video file instead of an animated GIF. Perhaps I’ll get to try TLDF at the fall equinox….

Van de Graaaaafffff!

Eric Myers Demonstrations 0 Comment

My colleague and friend Glenn Geher from the New Paltz Psychology department was visiting our new building, and so I showed him around our teaching lab space.    He was especially excited about the Van de Graaff generator, so I gave him a little demo:

 

What’s happening? As I explained at the beginning, the device uses an electric motor to drive a rubber belt which carries electric charges (electrons) off of the metal sphere, leaving behind a net positive electric charge.   Here’s a good diagram from Wikipedia:

Figure 1: Schematic image showing the operation of a Van de Graaff generator.

Eventually, the electric charge on the sphere becomes so great that the electric field it creates is strong enough to break through the nearby air (which is called “di-electric break-down”) and a spark of charge jumps from the small electrode to the sphere.  This is a lot like what happens when a bolt of lightning jumps from the ground to a cloud, or vice-versa.  That’s the clicking sound you hear, even if you can’t see every spark.

Then I put the metal pie plates on top of the metal sphere.   The electric charge now builds up on the pie plates as well.   It’s a fundamental property of electric charges that like charges repel, while opposites attract.   Each pie plate is then, in turn, repelled by the metal sphere and the plates below it, and goes sailing up into the air.

In the video I say that the belt carries electric charges up and onto the sphere, which sounds backwards from what I described above.   But it’s really the same thing.   The belt actually takes electrons from the metal sphere, and electrons have negative electric charge, so this leaves behind extra positive electric charge.   You can just as well imagine that the belt brings extra positive charges up to the metal sphere;  the effect is the same either way.   In fact, being able to tell which kind of charge is actually moving  is a bit of a challenge.   Benjamin Franklin originally came up with the names “positive” and “negative” charge, based on his own idea of which kind was flowing where, and as it turns out he got it backwards.

 

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