In Demo Veritas

Gas laws

Today, the physics professor was demonstrating centripetal force, by swinging a bucket around and not spilling the water within. Then, to up the ante, he put a wineglass full of faux wine on a small board made like a swing.  He got it going, and whipped it around four or five times before the wineglass became unstable and flew off.  It flew in a straight line, right into the wall.  The only piece of glass bigger than a dime was the stem.   (Faux wine is six drops of red food coloring and one drop of blue in a wineglass of water.)

The evening school is doing gas laws. PV = nRT.   or PV = Nk_BT.    Both sides of the equation are energy.  Yow!

Our golf ball atmosphere is loud and goofy, a perfect example of what we used to do before computer simulations got so good.  A turntable with paddles whacks plastic practice golf balls around a glass box, with a wire screen lid that is counterweighted to about twenty grams force down.  All that random bouncing pushes the screen to the top of the glass box.  Did I say loud?  What?

A shaking jig for bb’s shows some more physical analogy of thermal motion.   A fifteen centimeter window covered by a petri dish, shaken by three speaker drivers, pushed by a massive bipolar power supply/amplifier, driven by a Pasco signal generator.   All on an overhead projector, and projected on the front screen.  Gotta be three or four kilobucks of equipment there, and no substitutions work.  I love that Kepco amp.  Sigh.

Balloons and boiling water, balloons and liquid nitrogen.  Like a dangerous birthday party trick, but really all about the T for temperature term in the equation.  Helium balloon collapses, carbon dioxide balloon shrivels up completely.  Boiling water is only four thirds as hot as room temperature, so a balloon fitted to the beaker will swell up, but just enough to make the point.  Liquid nitrogen is one quarter of room temperature, enough to make the helium balloon sink in air, until it warms up and floats away.

Fifty five gallon drum on a vacuum pump, no surprises there.  Marshmallows in a bell jar swell up three fold, and then relax to normal size while the vacuum pump keeps working.  Let the air back in and the marshmallows turn into an hourglass shaped raisin-like object.   Working the P for pressure angle of the gas law equation.

Three plastic soda bottles with 100 ml of vinegar stand with balloons on their necks.  In the balloons are five, ten and twenty grams of sodium bicarbonate, ready to be dropped into the vinegar by raising the balloons.   That speaks to the n of the equation, or number of moles of gas produced.

An old wooden organ pipe is blown with air, helium and sulfur hexafluoride.  Speed of sound in each of the gases affects the pitch of a pipe, said pitch being the standing wave that fits the pipe.

A tub of water with a cylinder is there to catch gas by displacement of water, just as a lab demo.

A plastic water bottle with a fine mesh screen on the bottom holds a liter of water just fine.  In fact, you have to shake the bottle to get any water out.  Or you could open the screw top and let air in, splash, mwahahaha.

That is a lot of demos.  I am tired.  I hope the night crew is willing to put all these things away, because I do not want to wait until 9:30 to go home.

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Hey, where did the categories go?

Must have been the upgrade to wordpress.  I liked the categories, I guess I will put them back in.  I can go over my ouevre and re-assign.  I haven’t hit 100 posts yet, so its not that laborious.

Physics demos today.

A rigid pair of verticals and a horizontal clamped up to make a suspension for a 1.00 m pendulum, with a 30 mm brass ball as the bob.  The bob can move in two directions, making it a circular pendulum.

A 65# wrecking ball is suspended from the ceiling skyhook with wire rope.  The physics victim takes hold of the ball, and walks backwards to the blackboard, drawing the ball up to his chin.  With head against the blackboard, the victim releases the ball.  It swings out as a pendulum, and then swings back in.  Conservation of momentum predicts the wrecking ball will not hit the chin.  The owner of the chin can be forgiven for flinching.  The ball returns to within a centimeter of its release point.  It is a viscerally impressive lesson in physics.

How much equipment does it take to measure little g, by dropping an apple from the ceiling of the lecture hall?  A lot.  I’ll post photos on Thursday when we do it again for another physics class.

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Classes begin tomorrow? Where did the summer go?

I could use another week of Atlantic City, NJ.  The surf off of the Chelsea Ave. beach was stellar, and I want more.

The torrential rains of two days filled the area between the boardwalk and dunes with brackish pools.  The wind whipped waves moving from the left of a walkway found only a one foot gap to get by, and on the sheltered side on the right, formed an enormous single slit water wave experiment.  Beautiful.  Alas, I was running and had no camera.

At least there are demonstrations to do tomorrow.  For Life and Physical Sciences A, at 9 am:

Green balloon with helium.  Blue balloon with hydrogen.  Candle on end of stick, matches to light.

Plus liquid N2, soda bottle and trash can.

Also, some elements : Sulfur, Copper wire and copper powder and copper bar, Zinc, halfnium, germanium. And some cupric chloride.  Is there any way I could test the conducitivity of these materials?

Off to make the demos.

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Liquid nitrogen diamagnetic, liquid oxygen paramagnetic.

Demonstration of the inadequacies of Lewis dot structures and valence bond theory.  Both oxygen and nitrogen look to have their unbonded electrons as pairs.  :N:::N: and ::O::O::.  Pairs of electrons have magnetic spins which effectively cancel, making it a reasonable prediction that nitrogen and oxygen are both diamagnetic.

The experiment uses liquid nitrogen and liquid oxygen, because the effect is hard to see with gases, and easy with liquids.

A big magnet with a small gap is handy, so I use an old  synchrotron magnet, with steel washers to narrow the gap even further.  Takes a bit of doing to get those washers in place.  To take these pictures, we used a lightbox to illuminate.  For the demonstration in class, the magnet is placed on an overhead projector, with a plastic sheet protecting the glass.  The gap is shadow projected on a screen.

Liquid nitrogen goes in both Dewars, pre-cooling the small one so we don’t waste oxygen.  An oxygen cylinder is connected to a copper tube, which enters the big Dewar, coils six or seven times, and then exits up and over and down into the small Dewar.   With the oxygen flowing, the coil is immersed in the colorless LN2, and begins condensing liquid oxygen.   Pale blue LO2 squirts into the small Dewar.

With about 100 ml. of LO2, I remove the coil from the LN2  and close off the oxygen tank.  Time to pour and compare. LN2 first, because we have lots of it and the pole pieces need cooling, and (spoiler alert) it is diamagnetic and won’t stick around.

Then I pour LO2 into the gap.

The LO2 sticks in the gap.  Attracted to the strong magnetic field is paramagnetic, and paramagnetic means unpaired electrons.

So two bonding schemes give way to molecular orbital theory, and this simple experiment is conclusive proof that the oxygen molecule has unpaired electrons.

Last picture is a flash shot close-up.  The LO2 is boiling away, spitting out blobs of liquid, and cold vapor that hits the humid air and forms a fog.  The washers are white from frost.  As the liquid boils off, the oxygen gas still prefers the high field, and forms a torus of cold gas, visible by condensed water vapor at its periphery (not visible in this picture, the effect shows up clearly with the shadow projection.)

Making liquid oxygen gives the professor the chance to do another demonstration.  On a fire-proof surface is an 800 ml pyrex beaker.  I char a cardboard tube from a roll of paper towels with a torch, blow out the flame, and put it smoldering end down in the beaker.

A quick pour of LO2 into the beaker turns the tube into a flaming dart.  The effect is like an upside down rocket engine, with the beaker full of orange fire, and a tongue of fire shooting from the top of the tube.  The tube is entirely consumed, leaving only a bit of ash and a cracked beaker.

Ansari X Prize winner, Scaled Composites SpaceShipOne, uses a rocket engine with liquid oxidizer and solid fuel, like the demo, but way more sophisticated.

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All about the demos…

Summer school is all upon us.  Chemistry demonstrations are boiling on the bench.  Today is all physical chemistry.

Heat capacity of copper is measured by dropping 64 grams of metal at 100°C into 100 grams of water in a styrofoam cup at room temperature, and measuring the temperature rise of the whole.

Speed of sound in gas is demonstrated by blowing helium, air, and sulfur hexafluoride through an old wooden organ pipe.  The pipe length supports a standing wave, and the frequency of the standing wave is proportional to the speed of sound in the pipe.  Fill the pipe with helium and the tone goes way up, with sulfur hexafluoride and the tone goes way down.

No clue what the bucket of ice requested is for.  Sometimes that is the way it goes.

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