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

The Sun’s Energy

March 25, 2010

by: Martin Sagendorf

Imagine students’ amazement when they actually see sunlight melt a penny!  This demonstration clearly illustrates the vast amount of energy illuminating the Earth’s surface.  In rough numbers: 70% of the Sun’s incident energy on our outer atmosphere is reflected back into space – only about 30% actually gets to the Earth’s surface.  But, as we experience, this is still a substantial quantity of energy.

Fortunately, this energy (I. R. – Visible – U. V.) is rather uniformly distributed over the Earth’s surface -  thus its overall intensity is such that we have a habitable environment.  However, as we all know, we can concentrate some ‘area’ of this energy to increase the ‘energy per area’ (a measure of this is the temperature of the area of concentrated energy).  A common magnifying lens (2-4 in. diameter) will concentrate sufficient energy to burn paper or other objects with a low flash point.

To achieve an even higher energy concentration it is only necessary to use a device that increases the ‘capture area’ of the Sun’s energy.  Fortunately, this is easily accomplished with an inexpensive ($3.50) Fresnel lens.  The Fresnel lens (64 sq. in.) used here will ‘collect’ nine times the energy of a 3 in. diameter magnifying lens –creating a ¼” diameter ‘spot’ of energy having a temperature of over 315 degrees C (600 degrees F).  This is sufficient energy to melt zinc.

All that’s required are a Fresnel lens, bright sunlight, and a means of holding a penny.  Actually, it is a bit more involved: the Fresnel lens must be positioned exactly perpendicular to both the sunlight and the penny; the penny must be of 1982 or later; and the penny must be supported by a thermally minimally-conductive means.

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.

And, as a footnote, it is not illegal to ‘vaporize’ a penny or any other United States currency.  By law, it is only illegal to alter U. S. currency if the intent is to defraud – melting a penny with the Sun’s energy is simply a wonderful example of energy and its effects – perfectly legal.

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Posted by Tami O'Connor

The Amazing Drinking Bird

December 1, 2009

by Tami O’Connor

Invented in 1945 by Miles Sullivan, the “drinking bird” has been a favorite of science teachers in every classroom from kindergarten through college. This amazing device is made of two glass bulbs (one representing the head and the other representing the body) joined by a glass tube (representing the neck).  Between the two bulbs, attached to the glass tube, is a metal fulcrum upon which the bird pivots.  The air has been removed from this closed device, and the bottom ball is filled with a colored liquid that has a high vapor pressure (methylene chloride). The rest of the bird’s body and head is filled with the vapor form of methylene chloride.

The demonstration is set up such that a glass, filled to the top with water, is placed in front of the drinking bird. The glass should be the same height as the pivot point of the bird. The bird’s head should be moistened and then the bird should be given a gentle push to begin it oscillating along the pivot point.  Eventually, the bird appears to drink repeatedly, on its own.  So, how does that happen??

The top bulb (head) of the drinking bird is covered with felt. After the felt is moistened with water and the water begins to evaporate, the temperature in the head decreases. This drop in temperature causes some of the vapor inside the head to condense, causing the pressure inside the birds head to decrease. The decrease in pressure in the top bulb causes the liquid from the bottom to be forced upward from the base. As the liquid flows into the top bulb, the bird’s center of gravity moves upward causing the bird to tip forward, dipping its beak into the glass of water.

After the bird tips over and is horizontal, the bottom portion of the glass tube is no longer in the liquid. The glass tube, now 90 degrees to the surface allows the vapor from the bottom to travel to the top until the pressure is equalized.  At the same time, liquid in the column flows back to the bottom bulb. The weight of the bird is now primarily below the pivot point, so the bird returns to a vertical position.

The liquid in the bottom bulb is now exposed to the temperature of the ambient air, which is slightly higher than that of the bird’s head. This cycle continues as long as there is enough water in the glass to moisten the felt on the bird’s head. This cycle gives the appearance of a bird drinking!

SUGGESTED CLASSROOM ACTIVITIES

I) Classroom Discussion

Q. Is this an example of perpetual motion?

A. No. The cycle repeats itself only as long as the water evaporates from the head

Q. What is needed in order for the Drinking Bird to work?

A. A difference in temperature between the head and body.

II) Student Challenges

  1. Observe the operation of the Drinking Bird and explain how it works.
  2. Discover a way to make the Drinking Bird cycle faster.
  3. Predict what will happen if a fan blows air toward the Drinking Bird. Does it make a difference which direction the air blows?
  4. Predict the result of using warmer or cooler water in the glass.
  5. How long will the bird cycle without needing a refill of the water in the open container? Can you find a way of causing the bird to cycle longer?
  6. Is there a difference in the cycle rate on a humid day vs. a dry day? Can the bird be used to determine the relative humidity in the air?
  7. Predict the result of placing a small inverted aquarium over the bird. Does this cause the bird to cycle more or less? (Note: as soon as the water in this closed system reaches its vapor pressure, water from the felt can no longer evaporate and the bird stops.)
  8. Can you attach a thread to the bird so that it does useful work, e.g. lifting a small paper clip?

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Posted by Tami O'Connor

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