7/27/2019 Discrepan

1/4

Discrepant Events in Physical Science

Presentation at Michigan Science Teachers AssociationMarch, 2007

Carl WozniakNorthern Michigan University

cwozniak@nmu.edu

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 mLof 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 fillsin 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 willyou 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 LEDdevices 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 wireattached 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 ourheads 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

mailto:cwozniak@nmu.edumailto:cwozniak@nmu.edu

7/27/2019 Discrepan

2/4

2

vibrations into electrical impulses. These signals are transferred to the brain by the aural

nerve.

Transverse and longitudinal waves with singing rod

An aluminum rod is held in the middle with one hand andstroked 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 theytraverse 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 cant hear

Recently, a cell phone ring tone has made the rounds that students can hear, but most

teachers cant. Youre students could receive calls in the middle of class and likely you

wouldnt even know it, except for the other students wondering what was making all thenoise.

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.

Lenzs Law demonstration

This is a great extension after a pendulum lab. Once students are familiar with thepotential/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?

Lenzs Law: An electromagnetic field interacting with a conductor will generateelectrical 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.

http://www.arborsci.com/Data_Sheets/Files/SingingRods.pdfhttp://www.cbsnews.com/stories/2006/06/12/tech/main1701359.shtmlhttp://www.arborsci.com/Data_Sheets/Files/SingingRods.pdfhttp://www.cbsnews.com/stories/2006/06/12/tech/main1701359.shtml

7/27/2019 Discrepan

3/4

3

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

magnet. In 1834, Russian physicist Heinrich Lenz discovered this directional relationshipbetween the induced magnetic fields and current, which is known as Lenz's Law.

Information fromNDT 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 tothem 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 bothcontacts 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 thatwater 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 arent magically flowing through the pipe andexiting 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

http://www.ndt-ed.org/TeachingResources/NDT_Tips/LenzLaw.htmhttp://www.ndt-ed.org/TeachingResources/NDT_Tips/LenzLaw.htm

7/27/2019 Discrepan

4/4

4

Cylinders have the ability to easily spin

along two axes at the same time.Tomake a cylinder spin along bothaxes, put the cylinder on thetable, place your finger on the X

and rapidly push your fingerdown while at the same timepulling it toward you.

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 inthe 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, andwhen 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 Xs or Os that you see is equal to the number of spins per rotation.

(Information from Exploratorium Website.)

Sources:

Arbor Scientific

Edmund ScientificsEducational InnovationsLab Warehouse

Sci-bay

Steve Spangler

Surplus ShedUnited Nuclear

http://www.exploratorium.edu/snacks/spinning_cyl/index.htmlhttp://www.arborsci.com/http://scientificsonline.com/http://www.teachersource.com/http://www.labwarehouse.com/http://www.sci-bay.com/http://www.stevespangler.com/http://www.surplusshed.com/http://www.unitednuclear.com/supplies.htmhttp://www.exploratorium.edu/snacks/spinning_cyl/index.htmlhttp://www.arborsci.com/http://scientificsonline.com/http://www.teachersource.com/http://www.labwarehouse.com/http://www.sci-bay.com/http://www.stevespangler.com/http://www.surplusshed.com/http://www.unitednuclear.com/supplies.htm