Science Never Sucks

May 27, 2010

by: Tami O’Connor

One of my all time favorite air pressure activities is an oldie and a goodie!  It involves getting an egg into a classic, hard-to-find milk bottle, like the ones delivered to grandma’s door.  Unfortunately, some students (and some teachers) still think an egg can actually be sucked into a bottle.  As you probably know because the air pressure is greater outside of the bottle than inside, the better explanation is that the egg is literally pushed into the milk bottle.

Here is the explanation… The demo begins by placing two or three burning matches or a burning strip of paper into the empty bottle.  Then a shelled, moistened hard-boiled egg is placed on the mouth of the bottle.  The egg is clearly larger than the opening in the bottle.  The air inside the bottle begins to heat up and subsequently expands.  It is easy to notice the egg dancing around a bit as the air inside the bottle escapes around it.

Shortly after the flame inside the bottle extinguishes, the egg enters the bottle with a noticeable pop!  There is no doubt that the kids just love this demonstration.  I’ve done it at a number of science demonstrations and assemblies, as well as in my own classroom, and the response is always a surprised gasp followed by applause!

The only thing the kids enjoy more than watching the egg enter the bottle is watching me trying to get the egg back out of the bottle…  In my younger days I learned that I could only do that demo once since I had to wait to get the kids out of my class before I would attempt to blow the egg out of the bottle.  I’ve had egg on my face more than once…  The trick is, after blowing into the mouth of the inverted bottle, moving your face away quickly… well, very quickly.

As I got more creative (and spoke with more experienced teachers), I realized that there are other ways of getting the egg out of the bottle besides blowing into the mouth.  Another way to accomplish the same results (while remaining clean!) is to pour hot water on the outside of the bottle while the egg is seated the neck of the inverted bottle.  I usually rinse the inside of the bottle out with cold water first.  The process of running hot water over the outside of the bottle filled with cool air serves to warm the bottle, thus warming the air inside the bottle and causing the air to expand once again, forcing the egg out of the bottle.  Blowing a hair dryer over the outside of the bottle achieves the same results.

If you want to merge two lessons into one, you can also use an Alka Seltzer tablet, or vinegar and baking soda, to generate Carbon Dioxide gas inside the bottle, and force the egg out by increasing the amount of gas inside the bottle.

Another common, but erroneous, explanation  can be found on the web and even in some books. In fact, about half of the explanations on the web seem to use this explanation: that the burning material removes oxygen, thus lowering the pressure inside the bottle.  This ignores the fact that, for each molecule of oxygen removed, a molecule of carbon dioxide or two molecules of carbon monoxide are formed.

Some students have argued that it’s gravity that pulls the egg inside the bottle… That’s questionable given that the egg is much larger than the mouth of the bottle, but one easy way to combat that question is actually my favorite way to demonstrate the concept of air pressure.  This idea was shared with us by science teacher, Jeff Feidler of Ursuline Academy in Wilmington, DE.

Cut a small piece from the large end of the egg so that it stands easily.  Place a birthday candle in the narrow part of the egg and ignite the candle.  Lower the bottle onto the egg so that bottle touches the surface of the egg.  As the candle extinguishes, air pressure should be sufficient to allow the bottle to be lifted while the egg is hanging on.  Due to the lower air pressure inside the bottle, the egg will remain in the opening of the bottle.  Hold the bottle steady. The egg will eventually be pushed upward into the bottle.  This version of the demonstration will take a little longer than the traditional method detailed above, but is a great way to celebrate birthdays in your classroom…..and to show that gravity is not the explanation!

Educational Innovations sells these hard-to-find milk bottles for an additional activity that utilizes a  one way mesh screen.  Water can be poured into a bottle covered with a screen, and when the bottle is inverted, the water doesn’t come out!

Procedure:
1. With an elastic band attach a double layer of nylon net screen to the top of a milk bottle.
2. Show students that water can easily be poured into the bottle through the screen.
3. Place a small piece of card stock (ca. 7 x 7 cm; 3 x 3″) on top of the
screen, hold it in place with your hand, and invert the bottle over a sink or bowl.
4. Slowly slide the card out.
5. Ask students why you can you pour water into the bottle, but when inverted the water does not flow out?
6. Tip the inverted bottle slightly and then bring back to the upside down position. The water will begin to flow out of the bottle while it’s tilted and then will stop flowing when the bottle is back in the starting position.

Why does this happen? The force of flowing water allows the water to enter the bottle through the screen. Water in motion tends to remain in motion.  When the bottle is inverted, the water stays in the bottle because the molecules of water have a greater attraction to themselves than to the screen. The water is said to exhibit surface tension. In addition, when the bottle is inverted, a small amount of water is lost from the bottle, the air which remains at the top of bottle slightly expands, and the pressure of the air inside the bottle is slightly less than the outside atmospheric pressure. The combination of the water’s surface tension and the greater outside atmospheric pressure explains why the water tends to remain in the bottle. When the bottle is tipped slightly and then returned to the upright position, outside air enters the bottle and water runs out until the forces return to static equilibrium.

Whether you have your own bottle or choose to purchase one from Educational Innovations, you can have tons of science learning fun with your students in almost every grade level!


3 Comments | Chemistry, Elementary Level, Experiments, High School Level, Middle School Level, Physics | Tagged: , , , | Permalink
Posted by Tami O'Connor

Chladni Plates

May 21, 2010

by:  Martin Sagendorf

An Odd Name: They’re named for the German physicist Ernest Chladni who popularized them in the mid-1700s.  His name is pronounced: kläd’nêz.

They are: Thin plates (sprinkled with fine particles) vibrated perpendicular to their plane.

How? – Then and Now: Long ago Chladni used a cello bow to excite the edge of a thin metal or wooden plate.  Today, we can use an oscillator, amplifier, and an electro-mechanical oscillator.  We have a great advantage, we can easily vary the frequency of excitation thereby providing a whole vista of experimentation.

A 17 in. x 14 in. guitar shape at 200 Hz

The same piece at 235 Hz.  There are many more resonances at higher frequencies


What the Plates do:

Vibrate (in multiple modes) as functions of:

  • plane dimensions
  • mass per area of the planes
  • excitation frequencies
  • locations of excitation

Why do This?:

To study the resonance conditions of the (usually) wooden parts of stringed instruments; e.g. violins, oboes and guitars – although similar studies are applied to pianos, drums, cymbals, and bells.

In Practice:

  • The plate under study is (often) vibrated (and supported) at its center of gravity
  • Salt is sprinkled on the plate’s surface
  • Starting with the vibration at a low frequency (e.g. 100 Hz), slowly increase the frequency until a first resonance is obtained – adjust the amplitude of vibration as necessary to achieve salt migration – you should be able to hear the sound – too much amplitude will cause excessive motion of the salt (and poor patterns)
  • Successive resonances are observed with the salt moving from pattern to pattern
  • ‘Rock’ the frequency very slowly around a resonance point to achieve exactly the resonance frequency (sharp salt lines)

12” Square at 258 Hz.

  • Increasing the driving frequency causes the salt to move into the next higher resonance patterns

At 495 Hz

At 870 Hz

At 1259 Hz

  • All plate shapes will exhibit multiple resonance conditions
  • Some salt will vibrate off the plate.  Use a large shaker to add salt as necessary.

Why the Patterns?:

When the plates achieve a resonance condition, ‘standing waves’ are created.  This is, in fact, analogous to the similar effect in a vibrating string – except this is in two dimensions.

At resonance, the plate’s anti-nodes will be oscillating up-and-down energizing the salt – the salt will (naturally) move towards a lower energy level.  The lower level is a node.  That’s where the salt will collect (and remain), creating the lines we see.  These are the lower energy (non-vibrating) zones.

The ‘Exciter’:

Any commercially available electro-mechanical unit will work well for this demonstration.  However, these units are expensive (>$200).  An alternative is to build-your-own as illustrated in the book Physics Demonstration Apparatus .  Its cost is a (discarded) mid-range audio speaker, a wooden box and a construction, coupling the speaker’s cone to a vertical rod.  Building the unit, as shown in the book, does require some machined metal parts and a little ingenuity can simplify the unit’s construction (wooden pieces in place of aluminum pieces).  However, be mindful that the air’s varying humidity will affect the ‘fits’ of wooden components – that’s why the book’s design utilizes aluminum for the top plate and the rod guide.

The Home-Made Oscillator:

The Plates:

Although wood and cardboard will work, both are susceptible to warping.  For this reason, I make plates of very thin steel and aluminum – typically about 1/64” (0.0156”) thick.  My sources are the (discarded) side panels of tower computers and the covers of (discarded) microwave ovens.  Don’t attempt to use sheet metal shears to cut plates from these.  Instead, use a very fine (at least 24 teeth-per-inch) band saw blade or a similarly fine-tooth saber saw blade.  These methods yield a flat surface at the periphery of the plate.  The demonstrations do require a very flat surface to produce acceptable resonance patterns.  Be sure to file the edges free of burrs.

Plates in the range of 12 inches (square/round) work quite well.

Drill a hole at the plate’s center-of-gravity.  Use a banana plug to connect the plate to the vibrating rod.

Some Additional Patterns:

Two resonance patterns of a 12 in. diameter round disk:

At 175 Hz

At 240 Hz

Two resonance patterns of a 12 in. square with rounded corners:

At 180 Hz

At 290 Hz

In the Classroom:

This is a wonderful real-time demonstration.  And, even better, the plates can be photographed at their resonance frequencies, to be compiled into labs, reports, science projects, as either hard-copy or as PowerPoint presentations.

Endless possible plate shapes provide a great variety of investigations – different sizes of square, rectangular, round, and musical instrument shapes – enough explorations to keep several groups of students truly engaged in fascinating exercises.

Marty Sagendorf is the author of the book Physics Demonstration Apparatus. This amazing book is available through Educational Innovations and includes ideas and construction details, including all equipment necessary, for the creation and use of a wide spectrum of awe inspiring physics demonstrations and laboratory equipment.  Included are 48 detailed sections describing hands-on apparatus illustrating mechanical, electrical, acoustical, thermal, optical, gravitational, and magnetic topics.  This book also includes sections on tips and hints, materials sources, and reproducible labels.

5 Comments | College Level, Experiments, High School Level, Middle School Level, Physics | Tagged: , , , , , , , , | Permalink
Posted by Tami O'Connor

Making Optics Demos Easier

May 20, 2010

by:  Martin Sagendorf

We’ve all likely encountered the time-consuming effort required to set up an optics demonstration; all the necessary components are on hand, but they don’t easily work together.  The difficulty is obvious: the various components are either ‘loose’ or mounted at differing heights.  Thus: wasted and frustrating time ‘shimming’ with books and pads to match the heights of the components.

The solution is simple: choose a height (above bench top) and mount every optical component at the same (optical centerline) height.  But, how does one choose a height?  Simple: first, determine the optical component with the highest centerline then second, build supports for all the other components – matching this centerline height.

I began with a 100 Watt clear light bulb mounted upon a wooden base – the center of the filament was 4-3/4” above the bench top.  I then ensured that everything else I had, or planned to incorporate in demos, could be centered at this height.

The supports shown in the following illustrations are of ¾” pine – either screwed or glued together.  Where required, various combinations of rubber feet and jackscrews provide support and positioning capability.  When applicable, stacks of steel washers are incorporated to add stability.

Mirrors (three or four) are frequently required for light and laser reflection demos.  Standard mirror material can be used, but first-surface mirrors are better. Two jackscrews are incorporated to provide positioning of the light’s reflected beam.

Both right angle and equilateral prisms are much easier to use when mounted.  Three screws provide fine adjustment of the exit light path.

Mounted lenses are clamped, or attached to, a vertical board extending from the base.  When the lens centerline is accurately located there is no need for adjusting screws.

Unmounted lenses are held with lens clips attached to a base.  The adjusting screws are very useful with this construction.

An adjustable-width slit is easily made with two single-edge razor blades.

Laser pointers and LED flashlights are wonderful, and inexpensive, light sources.  Both are very convenient to use once suitably mounted.

Many demonstrations require a projection screen.  Two of the smaller size are often required – especially for reflection/refraction demos.

Similar optical component mountings are easily designed and fabricated to mount any optical component – e.g. filters, gratings, and special prism types.

Additional information and many applications of these optical mounting devices are detailed and illustrated in the book Physics Demonstration Apparatus.

Marty Sagendorf is the author of the book Physics Demonstration Apparatus. This amazing book is available through Educational Innovations and includes ideas and construction details, including all equipment necessary, for the creation and use of a wide spectrum of awe inspiring physics demonstrations and laboratory equipment.  Included are 48 detailed sections describing hands-on apparatus illustrating mechanical, electrical, acoustical, thermal, optical, gravitational, and magnetic topics.  This book also includes sections on tips and hints, materials sources, and reproducible labels.

Leave a Comment » | College Level, Experiments, High School Level, Physics | Tagged: , , , , , , , | Permalink
Posted by Tami O'Connor

Meteorites

May 14, 2010

by:  Ted Beyer

Ever since I was a kid, I have been fascinated with space. I would look up at the stars, and I just knew that other people were up there somewhere, looking back at our little point of light, and thinking the same kind of thoughts. When I was 7 years old, Neil and Buzz landed on the moon, and I was sure that somehow, when I grew up, I would get there, too. (incidentally, that’s me in the red on the right in the picture–and on the left? Well, that’s Buzz Aldrin!)

Dreams can be dashed by reality and time, but the desire can still live on. I became a collector of all things space…and NASA…and then I found that collecting meteorites was not only possible, but also fascinating. Here are objects that spent millions of years wandering through space, only to endure a fiery entry through our atmosphere to end up, astonishingly, in my hands.

The more I researched these space travelers, the more I became fascinated with their vast variety and appearance. When most people think of meteorites, they tend to think of the Nickel – Iron type (or at least I did). Heavy metal, often pockmarked, objects, dull black or grey. Wasn’t I surprised to discover that the Irons make up only about 6% of FOUND meteorites, by number, and 11.3% by weight. In collections, they make up 27.7%. The Chondrites (one form of the stony meteorites) make up 75% by weight and 85% by number found!

In spite of this, my small collection still only has one stony. My one and only stony (so far); pictured to the right, is a slice of the Ghubra meteorite, which was found in Oman in 1954.  See the white spot on the bottom left side of the specimen?  I am assured by experts that the spot, called a chondrule, is older than the planet Earth by as much as 500 million years!

The pride and joy of my collection is one of the rarest types of Irons. A type of Stony – Iron meteorite called a pallasite. Pallasites are made up of Iron – Nickel, but have crystallized almost gem-like grains of olivine, a silicate that varies in color from a brownish yellow to an olive green, embedded in the iron-nickel matrix which can rage up to 1 cm or more in size. These are easily the most beautiful of all meteorite types. As I said, they are also quite rare – making up less than 1% of specimens found. As you might imagine, they fetch a fairly high price (I saved many pennies to get mine).

The first meteorite I acquired was one of the most common and best known. The Campo del Cielo (Field of Heaven) meteorite find is truly vast. Known by natives for uncounted years, it was first found and named by Spanish explorers in 1576 in the Chaco province of northern Argentina. The two largest known masses are 37 and 18 tons and are considered national treasures. Tens of thousands of smaller masses ranging from just few grams to hundreds of kilos have been found, and are often found for sale to collectors.

I have been lucky enough to visit the Tucson Gem and Mineral show several times. This is the largest mineral show on the planet, and is held every year in February. These visits have enabled me to see many meteorites (and even buy a few!), and meet several of world’s leading collectors and dealers. On a recent visit, we were able to acquire some Campo del Cielos that were in somewhat distressed condition (being iron, they were somewhat rusty, having not been properly cared for). I spent a fair amount of time figuring how to stabilize them, and more time to actually do the work and remove most of the rust, so at least for now, they are in pretty good shape. More could be done, but I thought it might be interesting for you to get in on the activity as well.  You can purchase these meteorites that range in size from 160g to 395g from Educational Innovations.

On the same trip, I also found a truly marvelous book on meteorites, somewhat predictably called “Meteorites”. Written by Alain Carion, one of the leading collectors, it is aimed at people who don’t know much about meteorites, but are interested in them, and I read it cover to cover on one leg of my trip home. I learned more in those several hours than I had managed to absorb in the prior few years. I highly recommend it.

I think that people with even a slight interest in space and the unknown will be fascinated by the simple act of holding a meteorite in their hand. An object that has spent the vast majority of its life wandering between the planets – going places that most of us can only dream of going — holds its own magic.

3 Comments | Earth Science, Elementary Level, High School Level, Middle School Level | Tagged: , , , , , , , , | Permalink
Posted by Tami O'Connor

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