Download Discrepant Events in Physical Science

Discrepant Events in Physical Science
Presentation at Michigan Science Teachers Association
March, 2007
Carl Wozniak
Northern Michigan University
[email protected]
Here are some of my favorite demonstrations of discrepant events in physical science.
Alcohol and water miscibility
If you add 50 mL of water to 50 mL of water, you get 100 mL of water. If you add 50 mL
of ethanol to 50 mL of ethanol, you get 100 mL of ethanol. What happens if you mix 50
mL of water with 50 mL of ethanol? Actually, you get about 96 mL of liquid.
Why?
The water and ethanol molecules are different sizes, with the ethanol molecules being
smaller. Some of the ethanol fits in the spaces between the water molecules.
Think about two other materials: a liter of sand and a liter of rocks. If you pour the sand
into the rocks, the total volume will be less than two liters, because some of the sand fills
in the spaces between the rocks.
Color light addition with colored LEDs
A simple LED device has colored bulbs that are red, green, and blue. What colors will
you see reflected on a piece of paper when the lights are flashed in combination?
Surprisingly, not the colors that are emitted by the bulbs.
Why?
Colored light is additive. Green and red make yellow. Red and blue make magenta. Blue
and green make cyan. Green, red, and blue light together makes white light. Simple LED
devices can easily demonstrate the color combinations.
Sound amplification with thunder box
A cardboard cylinder has a piece of fiberglass attached to one end with a long coiled wire
attached to the center of the fiberglass. Small movements of the box produce low rolling
thunder sounds, and rapid oscillations produce crashes of thunder.
Why?
The thunderbox is a marvelous recreation of the human ear. Sound is caused by waves
displacing a medium. To hear sounds, small pressure currents in the medium around our
heads cause the tympanic membrane to vibrate. These vibrations are magnified by a
series of levers (ear bones) and a transferred through a fluid filled coiled tube (cochlea).
The cochlea has many tiny hair-like structures that move with the vibrations and turn the
vibrations into electrical impulses. These signals are transferred to the brain by the aural
nerve.
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Transverse and longitudinal waves with singing rod
An aluminum rod is held in the middle with one hand and
stroked along its length. A piercing shriek is heard.
Why?
The rods sing because of resonance, the building up of standing longitudinal waves.
These waves are created by the repeated sliding of your hand down the length of the rod,
which causes vibrations within the metal. This action is helped by the addition of rosin,
which causes your thumb and forefinger to stick to the rod more and vibrate as they
traverse the length of the rod. The pitch of the sound can be varied by changing where
one holds the singing rod or by changing the length of the rod itself. (Source.)
A ring tone you can’t hear
Recently, a cell phone ring tone has made the rounds that students can hear, but most
teachers can’t. You’re students could receive calls in the middle of class and likely you
wouldn’t even know it, except for the other students wondering what was making all the
noise.
Why?
You can find a video story and sample here. Why does it work? As we get older (over
40), our hearing worsens. The hair cells become damaged as we age, and the high pitch
of the ring tone can no longer be detected.
Lenz’s Law demonstration
This is a great extension after a pendulum lab. Once students are familiar with the
potential/kinetic equilibrium and periodicity of the pendulum set up a pendulum with a
magnet for a bob. The pendulum works normally on the table surface. But if you put a
conductive metal (copper sheeting works well) underneath the bob and the pendulum is
set in motion, the pendulum rapidly comes to a stop.
Why?
Lenz’s Law: An electromagnetic field interacting with a conductor will generate
electrical current that induces a counter magnetic field that opposes the magnetic field
generating the current.
If a magnet is moved past a conductive material, two things happen. First, the moving
magnetic field cuts through the conductor and induces eddy currents in the conductor.
This was discovered by the English scientist, Michael Faraday. Next, the eddy currents in
the conductor generate their own magnetic field, which opposes the magnetic field of the
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magnet. In 1834, Russian physicist Heinrich Lenz discovered this directional relationship
between the induced magnetic fields and current, which is known as Lenz's Law.
Information from NDT Resource Center.
Energy balls and circuits
Form a circle with everyone touching hands. One person
has a small ball in their hand and asks the person next to
them to touch a metal contact on the ball. The ball begins
to flash and make noise. Ask any person to break the
chain and the noise and flashing stop. So what is flowing
through our bodies?
Why?
Energy balls are ping pong ball-sized plastic balls with
two metal contacts. When both contacts are touched in a completed circuit the balls light
up and make noise. Our bodies are moderate conductors of electricity (all that salt water).
A common misconception is that electrons are zipping around the circle that is formed by
everyone holding hands. Actually, an electric current is zipping around, but this is not the
same thing as electron flow.
A good visual analogy is to set up a long PVC pipe horizontally with an elbow at each
end pointing upward. Put a short extension pipe on one of the elbows so that the opening
on one side is higher than the other. If you fill the pipe completely full of water, so that
water starts to overflow the lower end, you have something akin to a circuit. If you pour
more water into the taller end, the water immediately starts to flow out the lower end. The
water molecules that you just poured aren’t magically flowing through the pipe and
exiting the other side. Rather, the water molecules along the entire pipe bump into each
other and cause an almost instantaneous expulsion on the other end.
Energy balls are great for demonstrating series and parallel circuits, and will easily work
with a group of 30 students.
Rotation of a Cylinder along Two Axes
Cylinders have the ability to easily spin
along two axes at the same time. To
make a cylinder spin along both axes,
put the cylinder on the table, place your
finger on the X and rapidly push your
finger down while at the same time
pulling it toward you.
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When you launch the cylinder, it spins about its long axis (spin axis 1) and rotates about a
line perpendicular to this axis (spin axis 2). As it rotates about its center, the cylinder
forms a blurry circle on the table top. As the cylinder spins, the top of one end moves in
the same direction as the end that is rotating, while the top of the other end moves
opposite the rotation. The arrows in Figure 2 show these relationships.
The two arrows within the cylinder show how it spins. The two arrows outside the
cylinder show how it rotates. On the right end the two motions cancel each other, and
when the mark on the spinning cylinder is at the top, it actually stops momentarily. On
the left end the two motions add, and when the mark on the spinning cylinder comes to
the top, it moves twice as fast as it would with either motion alone
Figure 2
Human eyes can see the stopped mark easily, while the extra-fast moving mark is a blur.
The number of X’s or O’s that you see is equal to the number of spins per rotation.
(Information from Exploratorium Website.)
Sources:
Arbor Scientific
Edmund Scientifics
Educational Innovations
Lab Warehouse
Sci-bay
Steve Spangler
Surplus Shed
United Nuclear