We Water Molecules Stick Together!

January 14, 2013

tamiby: Tami O’Connor

I am a believer that observing discrepant events burns concepts into students’ memories far longer than simply reading the facts of the lesson from a text book.  A few years ago I was designing a unit on surface tension.  Because so many awesome hands-on activities deal with this topic, my greatest problem was picking and choosing!  In this blog, I will describe one of my students’ favorites.  It teaches about surface tension and capillary action.

DSC_0277Materials (per student):

  • 2 – plastic cups (I prefer Solo brand)
  • Electrical tape
  • 18 inches of white yarn
  • Food coloring
  • Water

Procedure:

DSC_0278Cut 2 pieces of electrical tape (1 inch each).  Using the tape, affix the end of the yarn to the inside bottom of one of the cups.  With the other end of the yarn, repeat with the second cup.  Put as much yarn as will fit into one of the cups, and add water until the cup is about half full.  Holding the cups close together, pour the water from one cup to the other allowing the yarn to flow with the water.  When the yarn is thoroughly saturated you are ready to begin.

DSC_0279Hold the cup with the water directly over the empty cup and pull the yarn taunt.  Slowly pour the water from the top cup into the bottom.  You should notice that the water flows from the lip of the cup and follows the yarn into the lower cup.  Reverse the position of the cups so the full cup is now above the empty one.  Offset the top cup so it is about one inch to one side of the lower cup.  With the yarn stretched tight between the two cups and the yarn from the top cup stretching over the lip on the side of the cup closest to the bottom cup, pour the water so it flows along the yarn and into the lower cup.  If done correctly, you will notice that, even though the top cup is not directly above the bottom cup, the water does not fall straight down but rather flows diagonally along the yarn.

You will find that you can offset the cups by several inches, and, as long as the yarn is tight and along the side of the top cup that is on the same side of the lower cup, the water will continue to follow the yarn into the lower cup.

Why does this happen? 

Molecules of water form a cohesive force with one another.  This force holds the molecules of the water together, so, when the weight of the water pulls it downward because of gravity, it in turn holds onto the water around it.  Since the fibers of the yarn are saturated with water, the water leaving the cup follows the yarn downward into the lower cup.

Try This:  After students have successfully poured water from one cup to another at an angle greater than 10 degrees, have them attach dry yarn between two new cups the same way they did before, but this time, keeping the middle section of yarn dry when water is added to one of the cups. Have the students try to pour the water at a 10 degree angle again with the new cups and dry yarn.  I suggest keeping a lot of paper towels on hand!

Why does this happen?

Since the yarn does not have any water on it, the water’s weight due to gravity acts on it without the cohesive force of the additional water in the yarn; therefore it falls straight down rather than diagonally across the yarn.

Next Activity:

DSC_0282Find books, boxes, or other objects that will raise the height of one of the cups above the desk.  Using the cups with the wet yarn, place the cup containing water on top of the raised surface.  Move the empty cup at an angle lower than the top cup.  Move the cups away from each other so the yarn is pulled taunt.  Leave the cups for about 20 minutes and observe the level of the water in the two cups when you return.

Why does this happen?

Capillary action is ability of a liquid to flow in opposition to external forces like weight due to gravity. It is defined as the movement of water within the spaces of a porous material due to the forces of adhesion, cohesion, and surface tension.  Because of capillary action, paper towels absorb spills, trees and plants are able to carry water and nutrients from their roots up through the plant tissue, and forensic scientists can use chromatography to help solve crimes.

More:

Some students have difficulty believing that the water from the raised cup is, in opposition to gravity, actually traveling up the yarn to the lip of the cup and then downward along the diagonal of the yarn into the lower cup.  That’s where the food coloring comes in…  close to the surface of the water, but being careful not to get it in the water, place a drop of food coloring on the yarn inside the top cup.  As the water travels up from the cup and along the yarn it will carry the food coloring along with it.  The food coloring will travel down the yarn showing the speed at which the water is moving.  As the color leaves the lip of the cup, use a second color on the yarn just as you did the first color.  Repeat each time the previous color leaves the cup until you have a rainbow of colors traveling down the yarn!

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1 Comment | Elementary Level, Experiments, Middle School Level, Physics | Tagged: , , , | Permalink
Posted by Tami O'Connor

Who Knew They Could Be So Dense?

January 3, 2012

by:  Tami O’Connor

Density is not typically an easy concept for most middle school students and even more difficult for younger students, but it doesn’t need to be.  We all know that D=m/V, but the easiest way I found to explain it to my students was to have them visualize a common dilemma in my home immediately preceding a vacation.  For years, as a poor starving teacher, I only had one suitcase, and it was actually a hand-me-down from my mother.  It was a medium sized Samsonite, hard cased piece of luggage.  When approaching the topic of density in my classroom, down from the attic it came.

My explanation began with an imaginary weeklong summer vacation to a low-key resort.  The class and I would brainstorm the items I needed to pack for my trip.  Generally, the list included items such as a few bathing suits, shorts, t-shirts, a pair of flip flops, some PJs, underwear and a few toiletries.  It was obvious by looking at the size of my suitcase that in addition to my meager belongings, I could have probably also fit one of my students in my bag…  ok, perhaps one of the smaller kids.

I explained that when I closed the suitcase, it was hard to see, simply by looking at it, how heavy it was.  The lesson didn’t stop there.  We now planned my one-week ski vacation to Vermont during the February break.  Once again, my students and I made up my pack list.  The list included a couple of heavy sweaters, long johns, gloves, a hat, boots… as you can imagine, the list went on and on.  The question was, where to put it all.  Of course, since I had only one suitcase, the answer was easy.

I would explain that the night before my winter trips, I could usually be found sitting on top of my very over-stuffed suitcase trying to close the latches!  I was always a little concerned that if the latch broke, my suitcase would explode and my belongings would be everywhere!  That always elicited a round of giggles as my kids visualized that catastrophe!  The interesting point was that once my suitcase was closed and latched, there was no way to determine how heavy it was just by looking at it.  Here it is… density in a suitcase!

The volume or size of my hard-sided suitcase never changed; however, the mass, or all the stuff inside each suitcase I packed was dramatically different.  This is when we started explaining more of the science of density.  The more tightly packed the molecules in an object are, the denser it is.

My next prop included two identical cardboard boxes.  The first had about eight bricks inside and the second was empty.  Without telling my class what to expect, I would ask for a volunteer to lift the box with the bricks.  After a bit of a struggle, the box would slowly rise above the table.  The other students could clearly see that the box was obviously very heavy.  I would then ask the same student to lift the second box.  Without fail, this box was hoisted so high that it almost flew out of the student’s hands!  Once again, we had two objects with basically the same volume but with drastically different masses.

My next question was: suppose my boxes were waterproof?  If I dropped them into the ocean, what would happen?  School aged kids all understand the concept of floating and sinking, so the obvious answer was that the box with the bricks would sink while the empty box would float.  I would explain that the bricks are denser than water, and that is why they sink. The air that filled the lighter box was less dense than the water, however, and therefore it would float.

In the next demonstration, I found two stainless steel spheres from Educational Innovations.  One was small and solid and the other one was much larger and hollow.  I would pass these around the classroom and asked the students to tell me which one had more mass (or was heavier).  Another unanimous answer:  the small, solid sphere was heavier.

Next, I would find another volunteer and blindfold my victim… I mean, my student.  I would then take two identical baskets, paper plates, or small plastic bowls and put each sphere in so they didn’t roll around.  I would ask my student to hold out his or her hands and would then place one plate in each hand and ask which was heavier…  Since both spheres are basically the same mass, the answer did not come as quickly as it did before the mass was spread out along a greater distance.  This was a perfect segue into the next unit on pressure.  But that would have to wait a week or two…

Of course, the observers in class were chomping at the bit to try the blindfold test.  Talk about active learning and a discrepant event!  Now that the class believed that both spheres were the same mass, I pulled out the large glass bowl filled with water, and I would ask my students to predict, based on the fact that we know that both sphere have the same mass, what would happen when I placed each sphere into the water.

This was amazing because, had I asked prior to blindfolding them, every student would have accurately predicted that the small sphere would sink, and the large one would float.  Now, a heated discussion usually ensued.  At this point, the mathematical formula was revealed (D=m/V).  When the mass increases and the volume remains the same, like the example of my luggage or the boxes with the bricks, the density increases.  At the same time, when the mass remains the same but the volume increases (like the small sphere vs. the large one) the density decreases.  Depending on the grade level I was working with. I would substitute simple numbers in the equation to show how changing the mass and volume affected the density.

Now, back to the discussion of floating and sinking spheres… After the explanation of how volume and mass affect density, most, if not all of my students agreed that the small sphere, being very dense, would sink, while the larger, lighter sphere would float.  Another successful seventh grade lesson!

Leave a Comment » | Density, Elementary Level, Experiments, Middle School Level | Tagged: , , , | Permalink
Posted by Tami O'Connor

If you Want a Good Thermometer, Don’t Use Your Body

November 26, 2011

by: Martin Sagendorf

An Easy Question:  Which is warmer – which is cooler?

In the strictest sense, it’s a matter of energy.  And we use temperature as a measure of energy level.  As we all know, the greater the energy level, the higher the temperature… But, although this is absolutely true; sometimes it’s not exactly what we perceive in everyday life.  When asked, we all can testify that when we touch a piece of metal we’ll say it feels cold.  But is it really cold?  Is it or isn’t it ‘cold’?

The Answer Is…

… very simple.  If the piece of metal is at room (ambient) temperature it cannot be ‘cold’ – it must be at the same temperature as the temperature of the room.

But First:

Let’s discuss ‘perceived temperature’: this is what we ‘think’ the temperature is.  It isn’t always the actual temperature (of the object we touch).  Thus we enter a wonderful combination of both physics and biology.  Physics describes the absolutes.  Biology describes the biological reactions (interpretations) of our physical world.

It’s a matter of thermal conductivity and our nerves.  Some materials are good conductors of heat (energy) and some are not.  Our nerves sense only temperature – so if thermal energy is rapidly removed from the tissues surrounding our nerve endings (like at our finger tips), our nerves sense that the temperature ‘they feel’ is cooler – e.g. the material is removing thermal energy from the body tissue surrounding the nerve ends at a rate faster than our body can re-supply energy to the tissues – thus our nerves sense this as ‘cooler’.

Now:

A truly illustrative and memorable way to present the question:

Use a construction that provides a means of ‘feeling’ the temperature of three different materials: all three of differing thermal conductivities – low, medium, and high – a simple box having squares of ceramic, wood, and aluminum.  The ceramic and aluminum pieces exhibit nearly the same thermal conductivity, but the aluminum has just a little better thermal conductivity.  The wood has a relatively low thermal conductivity.

This box is 17” wide, 8” high, and 3” deep.

In The Classroom, Discuss:

Thermal energy in materials

Temperature (as a measure of thermal energy)

Conductivity (of thermal energy)

[note that you haven’t yet presented the biological aspect of the students’ upcoming measurement of what’s warmer or cooler]

The Exercise:

Begin by drawing a three column ‘Vote List’ on the classroom board (ALUMINUM – WOOD – CERAMIC).

[this is a little of a deliberate distraction, but flip the switch to ON and count to five seconds before starting]

Have the students, one-by-one, ‘test’ (by touching) the three materials and marking their vote on the board.

Total the votes.

Ask the class if this makes sense… that the aluminum (or perhaps the ceramic) piece is the coolest?  Ask, “Why is this so?”… “Is it because the metal or ceramic is heavier?”… Or, “Is it because the wood is darker in color?”

Now, turn the box to show its insides – that there’s nothing inside the box.  The three pieces of material MUST be at the same temperature! – Room Temperature.  If any students still doubt you, use an IR thermometer to prove your point!

And Finally:

The really fun part of the lesson:  Ask the class, “Why did so many people think that one material is ‘cooler’ than the other two materials?”  Don’t give the answer – guide the discussion – let the class discover the answer.

Marty Sagendorf is a retired physicist and teacher; he is a firm believer in the value of hands-on experiences when learning physics.  He authored the book Physics Demonstration Apparatus.  This amazing book is available from Educational Innovations – it includes ideas and construction details for the creation and use of a wide spectrum of awe-inspiring physics demonstrations and laboratory equipment.  Included are 49 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 » | Biology, Elementary Level, High School Level, Middle School Level, Physics | Tagged: , , , , | Permalink
Posted by Tami O'Connor

Teaching Energy Using Dropper Poppers

December 30, 2009

by: Tami O’Connor

One of the units I enjoyed most as a middle school teacher was the section on energy.  The many awesome hands-on experiments generated such a series of oohs and aahs that it made my already-enjoyable days even more enjoyable!  One of my favorites was a lesson that dealt with the Law of Conservation of Energy.  A consequence of this law is that energy cannot be created, nor can it be destroyed.  (The students would have already explored potential and kinetic energy before the following activity.)

I initiated this lesson reviewing what happens with energy in a closed system.  The students clearly remembered comparing the amount of potential energy to kinetic energy using the example that the height of a roller coaster’s first hill is always greater than the height of any of the remaining hills.  It is, of course, possible to have a little hill followed by a higher hill as long as the roller coaster is going faster at the top of the little hill than the next higher one.  The students were generally able to explain the transfer of energy including heat energy and sound energy in the overall system.

I would then take out a normal playground ball and a meter stick and ask the the students to predict the height the ball would bounce if dropped from a meter off the ground.  Most students accurately predicted that the ball would not bounce as high as the height at which it was initially dropped.  Of course, we would then test our hypothesis.  A few students in each class would always insist that the ball could bounce higher than the height at which it was dropped, so I would invite them to show me how it could happen.  Inevitably, the student would add energy to the system by throwing the ball down to the ground rather than simply dropping it.  This was a great opportunity for discussion and was a topic that we would tap into later in the lesson.

I would then pull out my complete collection of balls that ranged from the hard, less bouncy baseballs to the rubber and highly bouncy super balls and have the students explore on their own.  Though there were noticeable differences in the elasticity of the  balls in my collection, none of them bounced higher than the height at which they were dropped.

My next demonstration utilized a racquetball that I had cut in half… well, actually a little less than half.  I would again ask my students to predict how high the  half-ball would bounce.  The answers varied, but by this time, not one student predicted that it would bounce higher than its drop point.  As before, we tested their hypotheses before moving on to the next step.

Because the racquetball is very flexible, I was able to turn the half-ball inside out thus storing elastic potential energy.  Once again, I asked the students to predict what would happen when I dropped the ball.  Based on their recent experience, they all answered that the half-ball would bounce lower than its drop point.  Of course, because I stored elastic potential energy in this system, once the half-ball hit the ground, it popped right side out and was propelled significantly higher than the point at which it was dropped.  Talk about a discrepant event!

Thank goodness Educational Innovations sells Dropper Poppers.  This product eliminates the time and difficulty of cutting racquetballs in half, not to mention the expense of purchasing racquetballs really intended for use in the court!

Dropper Popper Activities

When this small, “half-ball” is turned inside out and then dropped onto a hard, flat surface, it releases the stored energy and “jumps” higher than the point from which it was released.

EXPLANATION
• Elastic potential energy is energy that is stored as a result of deformation of an elastic object such as a spring or a rubber band.
• Gravitational potential energy is energy that is stored as a result of an object’s position above the ground.

ACTIVITY #1 How High Will a Ball Bounce
Showing your students a regular ball such as a small super ball, basketball, or ping pong ball, survey the class to determine the height at which they predict the ball will bounce if dropped without additional energy. You may be surprised to learn that some students will predict that the ball will bounce higher than the point from which you drop it.

Drop the ball. Students will discover that the ball will never reach the height from which you dropped it. The Law of Conservation of Energy states that energy cannot be created nor destroyed; it can only be transferred as alternate forms of energy. The energy that initially went into the system was transferred out as sound energy and heat energy. The ball will never bounce higher than the initial drop point because the energy that comes out of a system can never exceed the energy that goes in.

Explain to your students that the ball’s energy was stored due to its position above the ground. Because of the force due to gravity, the ball falls down as it is attracted to the earth.

ACTIVITY #2 The Dropper Popper
Show your students the Dropper Popper (POP-100), and ask them to predict the height at which the popper will bounce if you drop it straight down. Drop the popper without turning it inside out and observe the height at which it returns.

Turn the Dropper Popper inside out and explain that by doing work on the popper you are storing energy in it. Have the class predict again the return height of the popper after it is dropped. Drop the popper with the “bulge” pointing upward. When the popper hits the ground the stored elastic energy will be released and will cause the popper to bounce higher than the point from which it was dropped.

ACTIVITY #3 Ping-Pong Ball – Be sure all students wear protective eye wear.
This activity is truly best when each student has his/her own Dropper Popper and a Ping-Pong Ball, (PNG-100). Have the students store energy in the popper by turning it inside out. Then place the ping-pong ball in the “bowl” of the popper. Drop the popper onto a hard surface in such a way that the ping-pong ball remains above the popper and inside of its “bowl”.  The bulge should be on the bottom of the popper so the ping-pong ball fits securely inside.  The height your ping-pong ball will fly will be truly impressive!
• Have students estimate how high the ball travels.
• Change the height at which you drop the popper and determine if the height the ping-pong ball travels is based more on gravitational or elastic potential energy.

An additional demonstration of the Law of Conservation of Momentum and Energy can be shown using the AstroBlaster (SS-150).  This device has several balls threaded on a plastic shaft.  When dropped straight downward onto a hard surface, the top ball can rebound to a height equal to five times the original drop!

6 Comments | Energy, Experiments, Middle School Level, Physics | Tagged: , , , | Permalink
Posted by Tami O'Connor

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