Density of Gasses

August 13, 2010

by:  Tami O’Connor

Why do some objects float while others sink?  Archimedes discovered that an object is buoyed upward with a force equal to the weight of the fluid displaced.  An object will float in a fluid whenever its weight is less than the weight of the fluid displaced; otherwise it will sink…  So what does this mean in English??? An easier way to think about it is that an object that is less dense than the fluid it is in will rise to the top of the more dense fluid.

In demonstrations of liquids of varying densities, the liquid with the greatest density will sink to the bottom of the container while the less dense liquid will remain on the top.  There are wonderful demonstrations you can conduct with your class using immiscible liquids (liquids that do not mix) of different densities, and there are a number of high interest experiments your students can conduct using liquids of different densities.  If you find this topic interesting, please visit the blog we wrote on the W-Tube.

Gasses also have varying densities, but in the elementary and middle school classrooms, students don’t often have the same opportunity to work with gasses as they would liquids, or more often, liquids and solids.

I had an activity that was always a big hit in my classroom or during science assemblies that clearly demonstrated that helium is less dense than air.  It involved a small helium tank, a filled helium balloon (I always used Mylar to eliminate any issues with latex allergies), and a clear kitchen garbage bag.  I would bring out the helium balloon and ask my students why it floated.  It is usually easier for younger students to comprehend density when you relate floating and sinking to objects they recognize like balloons or boats…  Most students  answered that the helium is lighter (or less dense) than the air, and therefore, the balloon floats.  The next thing I did was to inflate the clear kitchen garbage bag with helium.  After it was about 3/4 full of helium and floating with the closed end facing upward, I would show that the bottom of the bag can be open yet no helium escapes.  I often received some perplexed looks from my students until I explained that the less dense gas that was inside the bag was trying to get out of the top (sealed end) of the bag, rather than the bottom.

Then, while holding the bottom of my floating kitchen garbage bag, I would ask the students to estimate how much helium vs. air was in the bag.  Since both gasses are clear, and the helium inside the bag made it float, this was not an easy question to answer.  I would then ask my students to come up with a plan to determine where the line between the helium and the air was.  There were always interesting ideas, but the easiest one I found was to simply use the helium-filled Mylar balloon to determine where the air ended and the helium began.  By releasing the Mylar balloon into the open end of the clear garbage bag, the balloon floated up until it hit the helium and remained floating (about 3/4 of the way up the bag).  Even the younger students were able to explain that the helium balloon was lighter than air so it floated above the air.  It didn’t float above the helium, however, since even though the balloon contained helium, the Mylar added weight keeping the balloon at the bottom of the helium layer.

The next lesson compared hot air to cooler air.  For this activity I needed a sunny but cool day, a Solar Tube complete with string, and… a lot of room.  By a lot of room, I mean a large field devoid of trees…  The best bag to use is found at Educational Innovations, because at 60 feet, it is the longest and creates the most impressive demonstrations!  The bag I am talking about is made from a very thin, black plastic.  I would tend to conduct this activity early in the day when the air was cool.  By unraveling the tube, holding open the end of it and jogging, so the air entered the opening, we were able to inflate the bag easily.  When the bag was filled with air, we tied the open end, attached the string and held the Solar Tube so it was fully exposed to the sunlight.

As we know, dark objects absorb heat.  In a short time, the Solar Tube became warm to the touch, and my students were clearly able to see the tube expanding because the plastic on the tube became taut.  Within about 5 minutes the Solar Tube floated high into the sky!  Because the air inside the tube became hotter, the molecules moved apart, taking up less space.  When there are fewer molecules in a given space, the substance becomes less dense than objects around it that have more molecules in the same volume. Few things are more impressive to an elementary or middle school child than to see a 60 foot long hot dog shaped balloon rise high into the sky, tethered only by a kite string…

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Posted by Tami O'Connor

Bring Some Magic Into Your Classroom!

July 10, 2010

by: Ted Beyer

One of my favorite authors, Arthur C. Clark, once said, “Any sufficiently advanced technology is indistinguishable from magic.” This has been quoted, misquoted and reused for years.  Of course, it’s perfectly true, and magicians have been using science as part of their acts for centuries. Things that we take for granted today were once bleeding edge technology. I remember in high school reading that sometime ‘soon’ (this was more than 30 years ago) there would be TVs that would be so thin that they would hang on the wall like pictures – impossible! A generation before, the concept of television itself was astonishing, and a generation before that, moving pictures of any kind were magical.

As I started to think about this, I suddenly realized that there are many products that we sell here at Educational Innovations that are used – currently – by magicians as ‘tricks’ in their act. Let’s take a look….

Perhaps one of the most used and simplest items would be our Sodium Polyacrylate, which magicians know as ‘slush powder’ among other names. This hydrophilic long chain polymer relies on hydrogen bonding to bind with almost any water based liquid and create a gel-like mass, the volume of which is about the same as the liquid that you introduce. In simple terms, when almost any liquid comes in contact with this stuff, it almost instantly becomes a near solid. This is a quick and easy way for magicians to ‘vanish’ liquids in any number of tricks.

Magicians pay dearly to acquire this material, and yet it is readily available and used in many industrial applications. Perhaps the most widespread use is in disposable diapers.

A close cousin of ‘regular’ Sodium Polyacrylate is what we know as ‘snow polymer’ or Instant Snow.  Actually this is chemically identical to ‘regular’ Sodium Polyacrylate, and shares its hydrophilic properties, but since it has a huge number of cross links compared to the original form, it expands massively, and quite quickly, producing a mass of fluffy white ‘flakes’ that resemble snow in look and feel. This material also finds its way into a variety of illusions.

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I recently discovered that there is a product sold to magicians that is nearly identical to the Amazing Ice Melting Blocks . In the trick, the magician tells the audience that both blocks are identical (of course they aren’t), and that he will heat one block by bombarding it with his ‘psychic waves’ and cause the ice to melt. Magicians LOVE the fact that is uses real ice – in fact they can get it from audience members thus proving that it is real. The ice is placed on the blocks, and he concentrates oh-so-hard, and low and behold, the designated block melts the ice, and the ‘control’ block’s ice stays nicely whole…Presto!

Well — Thermodynamics! Of course one block is an insulator, and the other a conductor (plastic and aluminum in our version) that look practically identical. Heat transfer in the aluminum block ‘magically’ melts the ice, and the insulator preserves it.

In the classroom, this can be taken a step further: Allow the students to handle the blocks, and predict which one will melt the ice more quickly. Of course the aluminum block feels colder than the plastic block (it’s drawing the heat away from their hand more quickly), and so more often than not they will predict that the ‘cold’ block will preserve the ice. The look of amazement on their faces as the ice vanishes into a puddle in seconds is priceless (be sure to watch the video for the product).

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Here’s something that I think is amazingly cool – a wire that, no matter how misshapen, when heated will instantly spring back to straight. It’s Nitinol Wire, a sophisticated alloy of Nickel and Titanium that falls into a special class of materials called shape memory alloys. These can be set into new shapes using a jig to hold the wire in a specific shape, and then heat-treating it. Magicians use pre-set shapes (often in the shapes of the suits on a deck of cards) which can be shown straight, and only went placed into warm water or exposed to the heat of a lighter, do they “magically” spring back into their pre-set heart, club, etc. incarnation.

You can make your own shapes, and of course you can do the reverse – bend the wire into the shape of a rival school’s mascot, or their letter, and obliterate it with a quick dip into a cup of warm tap water, leaving the wire straight once more.

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As anyone who knows me will tell you, one of my two favorite products at Educational Innovations is the large Eddy Current tubes (the other is our Chinese Spouting Bowl).  The magicians have grabbed on to this physics concept as well, with products such as “Newton’s Nightmare’ and others.

“Newton’s Nightmare” consists of an aluminum tube with a series of holes down one side, a magnet, and a non-magnetic mass identical in appearance to the magnet. Using slight of hand, the magician can control which mass the audience member drops down the tube, and by doing so he can ensure that his will always take several seconds, and the audience member’s mass will drop just as you would expect it to – instantly. How does that work? Amazing!

Well, amazing physics anyhow.

A complete understanding of how eddy current tubes work is fairly complex, but in the simplest terms: Lenz’s law states that a magnet passing through a coil will produce an electric current. An eddy current tube can be made from any non-ferrous, electrically conductive material that acts as the ‘coil’.  As a magnet passes through the tube it creates the aforementioned electric current, and that, in turn, creates an electromagnetic field. The interaction of that electromagnetic field and the field of the permanent magnet cause the magnet to drop through the tube at a greatly reduced speed as compared to the effect of gravity.

Four factors determine just HOW slow it’s going to go: The conductivity of the tube (more conductive is better!) the power of the magnet (again, more is better), the thickness of the walls of the tube (once again, more is better), and finally how closely the magnet fits into the tube (tighter is better).

In the Large Copper tube (ED-100), the walls are quite thick, which allows us to use a smaller magnet. The advantage to this is that when you look down the tube, you can see ‘daylight’ all the way around the magnet as it falls slowly through the tube. Use a tighter fitting, more powerful magnet like our M-165, and the drop time more than doubles. Educational Innovations also has Eddy Current tubes in aluminum, and small, thinner walled copper tubes, one of which has a slit in it so you can see the magnet fall. There is even a monster one meter copper tube that comes with magnets yielding a drop time of over 25 seconds!

Hey, I understand how the thing works, but it STILL seems like magic to me!

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Posted by Tami O'Connor

Concentrating Sunlight – It’s Easy!

July 9, 2010

by: Martin Sagendorf

On a Bright Day:

A great deal of energy falls on the Earth’s surface – roughly 1 kW per square meter.  This is about 0.6 Watt per square inch.  This doesn’t sound like much energy, but suppose we collect and concentrate 63 square inches of this sunlight?  These 63 square inches would collect about 38 Watts of energy.  This doesn’t sound like much, but…

Suppose We Could Then:

Concentrate these 38 Watts into an area of only 1/8 of a square inch?  This is exactly what we can do with an inexpensive plastic Fresnel lens.  We’ll focus the sunlight into an area 3/8” in diameter – this is the equivalent of 300 Watts per square inch!  With this energy level, we can easily ignite a piece of wood, boil some water, and even melt a penny.

A Suitable Device:

Is described in the book, Physics Demonstration Apparatus and in the blog The Sun’s Energy.

Now we’re going to describe how to build a much simpler version that works just as well – one that uses a very inexpensive Fresnel lens and is very easy to construct.

Much Simpler:

In fact, this version so simple and inexpensive that many setups can be made for little more than the cost of the Fresnel lenses.  For each setup:

A lens board:

  • A plastic Fresnel lens (7-1/8” x 10-1/4”)
  • Two pieces of 11” x 14” corrugated cardboard
  • Some masking tape

For the demonstrations shown:

  • Four spring-type clothes pins
  • A large paper clip
  • A penny (minted after 1981)
  • A (large-tipped) permanent black marker
  • A ½” copper pipe cap
  • Detergent
  • Water (to fill the pipe cap)

The Lens Construction:

It is absolutely necessary that the Fresnel lens be held ‘plane’ so that it may achieve a good focus.  To achieve this, two pieces of corrugated cardboard are used to mount the lens – their corrugations are placed at right angles.

  • Ensuring that the hole outline in each piece is correctly oriented, cut an opening in each which is ½” smaller than the lens – use a single-edge razor blade or a hobby knife.  When laminated, this pair will be much stiffer (and planer) than a single piece of cardboard or two pieces with parallel corrugations.
  • Place the two cardboard pieces together and, using masking tape, tape the hole edges and the outer edges together
  • Place the lens over the opening and tape it over the opening with the grooved face downward (to protect the face from scratches)
  • A NOTE:  It’s a strange property of corrugated cardboard: it always seems to warp – to counteract this I store these lenses under some heavy books

Of Absorbance:

The ‘blacker’ the absorbing surface – the greater the amount of energy absorbed.

Of Thermal Conductivity:

For our purposes, less is better – the object to be heated should be supported such that there is minimal energy loss via the supporting device – hence the use of ‘low-contact-area’ paperclips for holding a penny or a piece of wood supporting a pipe cap.

Igniting Wood:

It really isn’t necessary to make a black spot on wood – it will quickly ignite.  For even faster ignition, use a black marker to make a 3/8” diameter ‘spot’ at the focus location.

A Simple ‘Water Holder’:

We’ll use a ½” copper pipe cap (sometimes called a ‘tube cap’).  However, because the copper is highly reflective, we need to make its surface much darker – the darker the better.

Clean the pipe cap with strong detergent (to remove any residual oil).  Dry it well.  Use the black marker to ‘color’ both the inside and outside surfaces.

Remember to mention to the students that so long as the pipe cap contains water, its temperature cannot exceed the boiling point of water.  But, if the water is boiled off, the cap’s temperature will rise to that which the concentrated sunlight can produce – allowing this to occur may ‘burn’ the cap’s coating – to reuse the cap simply blacken it again.

A Simple ‘Penny Holder’:

Using the black marker, make a (very black) ‘spot’ about 3/8” in diameter in the center of the penny’s face.  Bend (using pliers) the paperclip as shown – note the small ‘hook’ at the end of the inner part of the paperclip, this prevents the penny from ‘squeezing’ out.

In Use:

Good results require a bright & clear sky – even a slight haze will dramatically decrease the sun’s available energy.  Interestingly enough, even the sunlight on a very clear day in the winter will provide sufficient energy for good demonstrations – however, the noonday ‘higher sun’ in the summer months is far better.

Place the object to be heated on the ground.  Two students holding opposite sides of the lens board can position the board for direction and focus.  The ‘altitude’ of the sun will determine the positioning (height and angle) of the lens and the positioning of the object to be heated.  A support for the bottom edge of the lens board will enhance stability (e.g. a box or a stack of books).  It is necessary to focus and maintain the focus for a continuous period (sometimes up to 60 seconds) to achieve the necessary heating time.

The lens must be orientated perpendicular to the sun’s rays, with the object to be heated located directly in-line behind the lens.  A typical Fresnel lens of this type has a focal length of 10” to 11”.  The ‘ruled’ side of the lens must face the sun.

Some Examples:

Focus the sunlight into the smallest spot possible (1/4” – 3/8” dia.).

The sunlight need only be focused such that the whole spot area is smaller than the black surfaces of pipe cap.

Construct the ‘penny holder’ such that the penny is held perpendicular to the light beam.  Focus the sunlight into the smallest spot possible (1/4” – 3/8” dia.).

Some Obvious Cautions:

This lens will produce temperatures in excess of 600 degrees F (300 degrees C)!  Caution students that they must not place their hands (or anything else) within the concentrated sunlight.  It is highly recommended that everyone wear (U-V resistant) sunglasses if they are likely to look directly at the object being irradiated (there can be a great deal of reflected U-V energy).   Advise students not to handle any object that has been heated until it is cooled sufficiently.  For a penny, this can take several minutes.

NEVER LOOK AT THE SUN THROUGH THE LENS ! ! !

These Are:

Great demonstrations for any General Science, Earth Science, Physical Science, or Physics class.  They truly do illustrate the energy in everyday sunlight – energy that we all know is ever-present, but little appreciate its magnitude until we actually witness its concentrated power.  Or:

Truly memorable Solar Energy Labs can be created wherein each group of students is supplied with:

  • A lens board (pre-made)
  • Several clothespins
  • Several paperclips
  • A piece of wood (like, 4” x 4” square)
  • A ½” copper pipe cap (previously cleaned)
  • A U. S. penny (1982 or after)
  • A large-tipped permanent black marker
  • A few mL of water
  • A pair of pliers (for bending paperclips)
  • Sunglasses or darkened glass/plastic (U-V rated)

The assignments:

  1. Discover how quickly how some materials – like wood – can be quickly ignited (time required?)
  2. Construct a support for a penny and melt a penny (time required?)
  3. Boil a small quantity of water (time required?)

Some follow-up questions:

  • “Why does the wood ignite so quickly?”
  • “Why does it take a longer time for the penny to melt (than for the wood to ignite)?”
  • “Why does the water boil so quickly?”
  • “Why is it (sometimes) necessary to have a black surface on the material to be heated by the sunlight?”

A Pre-Lab:

Might include discussions about:

  • Energy (the ability to do work)
  • Solar energy
  • Energy conversions (light to thermal)
  • Absorption/Reflectance
  • Specific Heat
  • Lenses (common and Fresnel)

And possibly:

  • Each group constructs their own lens board

Notes:

  • Remember, the actual exercises must be done on a day when the sunlight is ‘really bright’ – anything less simply will not provide the energy necessary.
  • Always store a Fresnel lens away from direct sunlight – under the correct circumstances it can start a fire.
  • ‘Dusting-off’ the Fresnel must be done with compressed air – wiping with paper or cloth will scrape material into the grooves and diminish its optical quality.

Guidelines for building the Atomic Penny Vaporizer are detailed in 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.

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Posted by Tami O'Connor

Great Balls and Fire!

June 13, 2010

by:  Tami O’Connor

When two 1-pound, 2-inch diameter, chrome steel spheres are smashed together, enough heat is generated at the point of contact to burn a hole in ordinary paper!  This dramatic demonstration has been a favorite of students in every grade for as long as I have been teaching!

There are a few considerations when allowing students (especially younger ones) to conduct this activity on their own…  First, the spheres are pretty heavy, so if they were either dropped on a foot or onto a nice tile floor, the result would not be good.  Also, be sure that the only thing between the spheres is paper or aluminum foil.  Fingers caught between the colliding spheres would not  be happy.  Finally, all participants should wear safety glasses, as it is not unusual for a small piece of paper to fly off after the spheres collide.

The Procedure:

Have an assistant hold the top edge of a piece of regular white paper vertically.  Hold one sphere in each hand on either side of the paper.  Quickly move the spheres together until they collide against the paper.  If they do not burn a hole in the paper the first time, try again and move the spheres together more quickly.  Examine the hole in the paper.  You will see that the areas around the edges of the hole are actually singed, and you will smell the burning paper!

Repeat the activity; however, this time use aluminum foil in place of the paper.  You will observe concentric circles radiating outward from the impact point.  This is a clear way to visualize shock waves!

Explanation:

This demonstration graphically illustrates how kinetic energy is transformed to heat energy.  Though some sound energy is produced, the force centered at the small where the spheres collide generates enough heat energy to burn the paper.  According to Newton, F=MA.  The amount of force between the two spheres is a function of the mass (which is constant) and the acceleration (which is controlled by the person moving the steel spheres).  The faster one smashes the spheres together, the greater the force.

A note from Ron Perkins:

Some time around 1996, the Smashing Steel Spheres demonstration was presented to a group of teachers in Dr. Larry Peck’s, AP summer program at Texas A&M, taught by Kristen Jones and Lisa McCaw.  One enterprising teacher tried the demonstration later that evening with some old spheres that he had around the house.  Imagine his surprise when he obtained sparks after colliding the rusty spheres together with a piece of aluminum foil held in between.  He had rediscovered the thermite reaction: Fe3O4+Al ->Fe+Al2O3+Heat and Sparks. (the numbers in the equation should be subscript, but there is no way to do this in the program we use for the blog…)

Since then, there has been a frantic search for rusty spheres.  It is possible to rust the Educational Innovations’ spheres, but it is usually a very slow process.  Dr. David Shaw, MATC in Madison, Wisconsin, has reported that a few months in the presence of fumes from the chemical storage closet works well…

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Posted by Tami O'Connor

Seeing a Magnetic Field in 3-D

June 12, 2010

by: Martin Sagendorf

This is actually quite easy to do.  This clearly illustrates that magnetic fields are not flat (as too frequently demonstrated in the classroom).

This easy-to-make construction requires only four components:

  1. A clear plastic bottle (about 1-3/4” in one dimension) – the one illustrated below is a 12.6 fl oz ultra concentrated Joy ® dishwashing soap bottle – Note that any bottle originally containing soap or detergent will require repeated rinses to completely remove all of its original contents.
  2. Six 17 mm x 3 mm Neodymium ring magnets
  3. A very small quantity of fine iron filings
  4. 2 lengths of ¾” x 3” clear tape (clear ‘packaging tape’ works better than the usual roll-type transparent tape)

The low cost of these materials and their reusability makes this an ideal class-wide experience.  Further student explorations are readily accomplished by using a variety of magnet types, numbers, locations, polarities, and shapes.  Applicable magnet types (also available from Educational Innovations) are

Ceramic Bar Magnets (0.875” x 1.875”)

Ceramic Ring Magnets (1.25” O.D.)

Neodymium Ring Magnet (0.75” O.D.)

Neodymium Large disk (1” O.D.)

Similar demonstrations can utilize larger magnets and larger bottles.  This one uses a 2-1/2 inch diameter bottle and two large (2” O.D.) ceramic ‘donut’ magnets removed from the magnetron in a discarded microwave oven.

And this one illustrates the magnetic field created by a 2” O.D. ceramic ‘donut’ magnet placed under a 2-1/2” diameter bottle.

CAUTION: Rare earth magnets are very strong and very brittle.  They will attract each other quite unexpectedly.  There always exists the possibility of fractures and flying pieces. Everyone MUST wear safety glasses/goggles when working with these magnets!

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.

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Posted by Tami O'Connor

Iron Filings Exploration

June 11, 2010

by:  Michelle Bertke

Simple iron filings can be used for a variety of interesting experiments and demonstrations.  Magnetism is a mysterious concept that can be difficult for students to grasp.  Magnetic fields are the forces surrounding a magnet that are identified by how they interact with adjacent magnets and other metal objects.  While magnetic fields are ‘invisible’ they can be observed by sprinkling iron filings on a white paper with magnets beneath. 

By lightly coating the surface of the paper, the magnetic field will appear as filings align themselves with the field.  Different magnets, depending on their strength and shape will create varying patterns in the iron filings.  A bar magnet with a distinct north and south will show characteristic lines of a magnetic field.  Circular magnets may show multiple lines indicating multiple magnet fields.  The stronger neodymium magnets will cause the iron filings to pile up in spikes due to the increased strength. This demonstration can lead to a discussion about magnetic fields: What they are, Where they can be found, and How they are used in the world around us.

A simple magnetic fluid can be prepared by mixing iron filings with vegetable oil.  This fluid will flow when free from a magnetic field and stop solid when next to a magnet.  The mixture can be put in a glass or plastic vile so students can observe how the fluid flows.  When the fluid is held over the magnet, it will pile up (as long as it is in the range of the magnetic field).  When the magnet is removed, the tower will collapse into a puddle.  Students can then experiment with magnets of varying strengths to observe how the strength of a magnet will affect the height of the tower that is able to be formed.

Iron filings can also be mixed with sand to illustrate how different substances interact with magnets.  A pouch can be made out of freezer bags.  (You will want to hot glue the openings to prevent spills, as seen in the picture.)  When filled with a mixture of sand and iron, the bags can be made to appear homogeneous.  You can ask your students what they think is in the bag.  (Some perceptive students may come up with the correct answer depending on prior knowledge and the previous activities.)  You can also ask if they think they would ever be able to separate the two substances.  When you drag a magnet along the surface of the bag, the iron filings come to the surface, separating from the sand.  This simple illustration can lead to discussion of what is magnetic and why. 

This is also a great activity to relate the idea of magnetism to a household toy that they should all be familiar with: the etch-a-sketch.  This toy relies on a similar set up to make designs and, like our sand pouches, the designs disappear when the toy is shaken.  A discussion of other applications of magnetism in the home will likely follow.

Michelle is a graduate student at the University of Notre Dame.  She and another graduate student, Melanie Bunda, run a program called Science and Stories.  This program focuses on children from ages 6 to 10 and allows the participants to explore science though books.  They use a number of Super! Wow! Neat! science supplies from Educational Innovations.

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Posted by Tami O'Connor

Our Magnetic Field

June 1, 2010

by:  Martin Sagendorf

We recognize heat & cold, dry & damp, light & dark, and sound & silence.  However… I find it absolutely fascinating to consider that we also live within something that we can’t see, hear, touch, or taste.

We all Know:

Our planet has a giant magnet near its core and that its field extends over the whole of the Earth’s surface.  But, do we ever really think about this field that passes through soil, rocks, buildings… and us? Granted, relatively speaking this ‘field’ isn’t particularly strong.  In fact, it’s a rather weak field when compared to those of a horseshoe magnet or, particularly, a modern Rare Earth magnet.

A Great Demo:

Uses a few very strong Rare Earth magnets to illustrate that this field really is everywhere – even in the classroom.  What we want to make is a really graphic and dynamic demonstration.

It’s Easy to do:

Just hang a magnet from the ceiling – Any bar or ‘donut’ magnet will align itself to the Earth’s magnetic field.  Here’s a very effective construction:

Materials:

  • A new pencil with eraser (7-1/2” long)
  • 6 rare earth (Neodymium) ring magnets (E.I. Neodymium Ring Magnets #M-185)
  • 5 feet of light-weight string or heavy thread (or very light fishing line)
  • A ‘flag’ indicating N (cardstock: 3/4” x 2-1/2”)
  • A 24” length of 3/4” or 1” wide masking tape
  • A few inches of clear tape or Glue (Duco®)
  • A very small ball bearing ‘fishing swivel’

Construction:

  • Sharpen the pencil
  • At 3-1/4” from the top of the eraser, tie one end of the cord around the pencil (look on the Internet to see how to tie an ‘Improved Clinch Knot’ – use two turns – this knot really works quite well)
  • Using a single Ring Magnet as a measure, wrap masking tape around the pencil on both sides of where the string is tied to the pencil – wrap-on sufficient lengths to make a snug fit for the magnet (this will be about 12” per side)
  • Make up two magnet stacks with three magnets in each
  • Use a magnetic compass to determine the ‘polarity’ of the stacks – the stack ends that attract the south-pointing end of the needle are the ‘north ends’ of the magnets – you’ll want these ends to be towards the sharpened end of the pencil
  • Slip the stacks of magnets over the ends of the pencil – the stacks will be aligned so they are attractive – carefully move the stacks together (on) each side of the cord
  • Hang the assembly from an overhead support
  • Place the N flag over the sharpened end of the pencil – centered about 1-1/2” from the pencil point
  • The pencil should hang nearly horizontally
  • To adjust any deviation from horizontal, add some masking tape on the appropriate side of the pencil
  • Once level, use clear tape or glue to fasten the flag
  • Splice the fishing swivel into the cord – about 12” above the pencil – use knots as above

In Use:

Suspend the unit from the classroom ceiling such that it is just accessible for ‘spinning’ by students entering the classroom.  This will become an everyday occurrence.

This Simple Device Illustrates:

  • That the Earth’s magnetic field is everywhere
  • A force we can’t see or feel (directly)
  • ‘Force-at-a-Distance’
  • That the very small magnetic field (force) from a nearby hand-held magnet is sufficient to cause a disturbance in the ‘local’ magnetic field (even if a magnet is totally grasped within one’s hand – the magnetic field simply goes right through one’s hand)

Notes:

  • These little magnets are very strong – be very careful when handling them
  • When separating the magnets, slide them apart one-by-one
  • When placing magnets together – do so with caution – they will very suddenly attract each other
  • These magnets will fracture if subjected to an impact force – wear safety glasses
  • Keep the magnets well away from credit cards and other magnetic storage devices
  • ‘Spinning’ may result in ‘wind-up’ (a residual twist) in the cord – especially so when using a ‘stiffer’ cord (e.g. light-weight fishing line) – the pencil may ‘come short’ of pointing at North.  The fishing swivel (the smallest one you can find) will minimize this.  But, even with the swivel, there will be times that it cannot completely compensate for the wind-up – if this happens, simply lift the pencil very slightly to release the twist in the cord.  As an aside, fishing swivels usually come several to a package – choose the one with the smoothest action and place a drop of very light oil inside it.

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 » | Earth Science, Elementary Level, Experiments, High School Level, Magnetism, Middle School Level, Physics | Tagged: , , , , | Permalink
Posted by Tami O'Connor

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!


2 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.

1 Comment | 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

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