ISPP REMINDER
April 2006
OUR NEXT MEETING. . .
. . . is at Northwestern University
Monday
May 8
6:30 pm
A map and directions are enclosed
THE FREE GIVEAWAY . . .
. . . Can you guess what it will be?
FUTURE MEETINGS. . .
June 6 (Tuesday) Museum of Science and Industry Ruth Goehmann
AT OUR LAST MEETING. . . .
Mike Kash (Lake Forest College) called the meeting to order. Gerry Lietz (DePaul University) announced our next meeting at Northwestern University. Gerry got a letter from Charles Mattes who retired and moved to Colorado and says hello. Roy Coleman (Morgan Park High School) related that he had received a latter from Ian McGuiness in Scotland. Gerry asked if there were any new attendees and recognized Ed Shaughnessy from Lake Forest Academy and Arie Van Ek from Illiana Christian High Debbie Lojkutz (Joliet West High School) handed our new teacher bag to our new teacher Rob Lang who came with John Baier from Glenbard South. Debbie mentioned that Joliet Township High School was looking for teachers for the fall She gave the web site www.jths.org where one could download a job application.
Mike Kash (Lake
Forest College) showed
us the arrangement of electrical components he uses in class to
demonstrate the
exponential rise of the voltage across an inductor in an LR circuit. He
noted
that when using toggle switches to initiate transient currents, the
switch
bounces as it is closed leading to unwanted spurious signals. Mike
suggested a
strategy to arrange the circuit so that the transient is initiated with
the
opening of a switch. He drew an RL circuit on the board to illustrate
his
point. When the switch is closed the R3, R2 and L components are
shorted out.
Only when the switch is opened do these components see the power supply
and
react. Mike passed out a handout describing the circuit and details of
the
measurement. It also contained a plot of the data obtained by a
computer
interface with data capturing software. We could see the exponentially
increasing voltage as the coil was powered up. Mike used a computer
program to
fit the curve of the voltage vs. time and obtained a time constant from
which
he calculated the inductance. The results agreed with the labeled
values within
2%. Mike attributed the good fit to the fact that the coil had an air
core and
no iron. He had measured the inductance to be on the order of 25 mH
with a 10
Ω D.C resistance. The inductor (part # RL-1239-33000) was purchased
from
Renco Electronics 595 International Place, Rockledge FL 32955 (tel:
312-627-1000. They have a 25 piece minimum order. Then Mike revealed
that he
had purchased a number of coils so that each of us could have one as
our free
give-away for the meeting. He also passed out a handout describing all
of the
circuits he had shown us. Thanks, Mike.
Then Mike rearranged the circuit to show the exponential decrease in voltage across the coil as the power was removed. Mike explained that the circuit was assembled on a unique plug in board that was designed by people in the Department here at Lake Forest College. Lastly, Mike replaced R1 in the second circuit with a capacitor to show the ringing of the RCL circuit with the characteristic exponentially decreasing oscillating voltage. Neat!
Bailey
Donnally (Lake Forest College) mentioned that the method of data taking
was so
precise that it could actually tell the difference between a good
capacitor and
a bad capacitor. A cheap paper capacitor will not give a good
exponential
decay. Interesting?
Art Schmidt (Northwestern University) mentioned how miniaturization was affecting electronics these days. As an example he showed us a new radio controlled airplane called an “Air Hog” that he bought at Toys R US for $40. The plane is made of Styrofoam and has a wingspan of a mere 20 cm. At that size one could fly the plane in a typical lecture hall. He tried to fly it in the room but bounced off the rear wall with no apparent damage. Art explained that the controls offer some opportunities to discuss physics. It has two electric motors with props. The props rotate in opposite directions. Turning is accomplished by varying the speeds of the motors. Gerry Lietz (DePaul University) wondered if the different rotational speeds and resulting reaction torque also affected the roll of the plane, banking it in the right direction for the turn. The plane does not have ailerons that provide this control on a real airplane. It would be interesting to work out the dynamics to see if it did. Changing the speeds of both motors together controls the pitch. Higher motor speed make the plane climb etc.
Art brought out an oldie. He had a flat ceramic magnet and a ball bearing which was attracted to the magnet. He stuck the bearing to the magnet and then allowed a paperclip to be magnetically attached to the ball. He asked to which would the ball bearing remain attached when he pulled on the paper clip. Would the ball remain attached to the magnet or go with the paperclip. It would seem that the ceramic magnet would win because it was the stronger magnet. In fact, the paper clip was only magnetic because of the induced magnetism due to contact with the ceramic magnet through the ball bearing. What do you think? When he applied a force, pulling on the paperclip the ball bearing parted from the ceramic magnet and hung attached to the paperclip. Interestingly, the ball remained magnetically attached to the paperclip even after the ceramic magnet that induced the magnetism in the paperclip and ball was removed. Also, when the ball bearing finally was detached from the paperclip it could not be reattached. Was it that the point of attachment was too hard to relocate or did some of the magnetic dipoles on the paperclip and bearing simply unalign snapping back to a previous random orientation? Art tried to repeat the experiment with a strong rare earth magnet. In this case the ball remained attached to the rare earth magnet and not with the paperclip. We tried using a larger steel ball with the same results. The ball stayed with the rare earth magnet.
Art asked if anyone had ever used Kinetic Books Physics texts on CD for computers. They have both a calculus based and non-calculus based Physics text each for around $30, which is quite a savings over traditional hard cover texts. The CD book also makes good use of the graphics capability of the computer with animated examples. Art had polled some students about how they would feel about using a text on CD. One objection was that a student liked to highlight important details in her text. Art had discovered that there was a highlighting capability with the CD text as well.
John Milton (DePaul University) told us that Janet Landato is retiring from Harper College and is clearing out some of her leftover giveaways. She had sent John a box of syringes of different sizes with Tygon tubing connectors. By mating two different sized syringes together with the Tygon tubing one could demonstrate Pascal’s Principle and force amplification. As one would push on the smaller syringe one could feel the larger force that was transmitted through the pressure of the air to the larger syringe. John passed out three different size syringes. Thanks Jan.
John had
come upon a sale of plastic Easter eggs at Walgreen’s (a bag for $2)
that
reminded him of an exercise to help students understand the method used
in the
Millikan Oil Drop Experiment to determined that charge is quantized and
what
the charge on an electron was. Millikan noted that the net charge
induced on
oil droplets occurred in integer multiples of a single value of the
electron
charge which is now accepted as 1.6 x 10 –19 Coulombs. The
simulated
experiment involved filling the various plastic eggs with one, two and
three
washers. By making measurements of the masses of many eggs the students
could
deduce the basic unit of mass for one washer as well as determine the
number of
washers in each egg. John thought of another experiment using coffee
filters.
One could sandwich multiple coffee filters together in combinations of
two,
three and four filters. Then by dropping each combination of filters
and
measuring the terminal velocity of each a student could deduce the mass
of a
single filter as well as number of filters making up each sandwich.
John pulled out a small disc and a cylinder each having identical diameters and set them on a ramp and asked which one would reach the bottom of the ramp first. We asked if they had the same mass and John responded with another question, ”Does it matter?” The solid cylinder won. John mentioned that often texts would point out that an object sliding down without friction would beat all others rolling down. It is difficult to find an object that slides without friction. A piece of dry ice would work if you had a ready supply. John discovered a more convenient object, a Kick Disc which he had gotten from Pasco for around $40. It is a battery powered indoor hockey puck that we have seen at previous meetings. He set the Kick Disc and the solid disk next to each other on the ramp and released them. The Kick Disc won handily.
John had bought two Kick Discs to show elastic collisions. Roy Coleman (Morgan Park High School) mentioned that he wraps a strip of Velcro around each of a pair of discs to demonstrate inelastic collisions.
John introduced his next item by relating how John Thompson in his acoustics class at DePaul posed the question of what really happened when two people talk to each other through cups connected by a string. What kind of wave is transmitted, transverse or longitudinal? Does there have to be tension in the string for it to work?
John knew a retired violin player and asked him if the squeak made by an inexperienced player was due to stroking the strings with a component along the length of the string instead of at right angles to the string. Thus generating a longitudinal wave instead of a transverse wave. He did not get a satisfactory answer. Larry Alofs recalled Ed McNeal saying exactly that.
Mike Kash
set up a microphone to see if he could detect the sound in one cup
generated by
John talking into the other cup. They got it to work.
Gerry recalled seeing a cartoon where the string from one cup dropped off and lay on the ground between the two cups. Gerry assured us that it wouldn’t work that way. We discussed ways in which it could be determined which of the types of waves were present on the string.
Eileen Wild (retired) demonstrated projectile motion using some toys she got from Happy Meals. A frog launched small discs with considerable speed. A catapult presented an interesting dilemma. One usually associates a maximum projectile range for a launch angle of 45o. However because of the arrangement of the spring launching mechanism the catapult lever had to be pulled back further than 45o to get the devise to impart any appreciable velocity to the launched projectile. Eileen also had a snail which could be launched from it shell and traveled a few centimeters. Eileen thought that the toys were a good way to stimulate discussions of motion.
Rich Goberville
(Joliet Central) held
up a clear plastic box to which he had attached several A.C. light
sockets and
two switches. The simple wiring showed that all four components were
wired in
series, an unusual arrangement for an A.C. circuit where lights are
typically
wired in parallel. In each socket was a colored light one red and one
blue.
Each switch also had a color tag, one red and one blue. With a gleeful
twinkle
in his eye Rich closed the red switch nearest the red light and the red
light
lit. It should be noted that the blue switch which normally would need
to be
closed to complete the circuit, remained open. How was the circuit
being
completed? He showed that the operation was symmetric by opening the
red switch
and closing the blue switch. The red light went out and the blue light
lit. He
opened the switch. Now he was ready to give us the real shocker. Some
of us had
seen this demonstration before but we were not prepared for what was to
follow.
He switched the lights in their sockets so the red light was now
nearest the
blue switch. When he closed the red switch the red light lit even
though it was
in the other socket. You might have guessed there were some hidden
diodes in
the circuit. Rich drew a circuit diagram showing how the circuit
worked.
Normally one hides the diodes in the switches and in the light sockets.
Rich
went through considerable work to take apart the bulb base and solder a
diode
into the light bulb base. It took him several light bulbs before he
succeeded
in sawing off the tip of the base without severing the electrical
connection. He then slipped a diode into the base that he then glued
back into position.
Really sneaky, Rich! We really were bamboozled! Thanks for the great
variation
on an old demo. We all agreed you have exceeded your mentors.
John Baier (Glenbard South High School) recalled once having a light bulb with a magnet mounted in the gas envelope such that the filament looped around it. When used normally with A.C. power the filament would oscillate back and forth in response to the cycling current in the filament. John would apply D.C. current and show that the filament would just deflect off to one side. Unfortunately, the bulb broke and he has not been able to find a replacement. He asked if anyone knew of a source. We’ll be looking. It sounds like a neat thing to have in one’s demo toolbox.
Larry
Alofs (Kenwood Academy (retired)) used one of Janet’s
syringes to point out some curious
phenomena. He extended the plunger of the syringe out as far as it
would go
without pulling it free and then put his finger over the end of the
syringe. He
then pushed on the plunger and noted how the force needed to move the
plunger
increased as he pushed it in compressing the gas in the syringe. He
drew a
rough sketch of the force as a linear function of the piston motion
noting that
the force could get to be considerable quickly as the pressure built
up.
Then he took his finger off the end and pressed the plunger all the way in. Now he replaced his finger over the end and tried to pull on the plunger drawing a vacuum against his finger. He recommended that we try it and feel how the force seems to plateau out at some peak value as the plunger is pulled beyond a point, in this case about half way out. He asked if we could explain this. The situation was not exactly the reverse of the case of compression. Some of us guessed that the force was not due to a suction of the gas inside the syringe chamber as much as it was the pressure of the atmosphere pushing on the piston from outside. Once the air inside had expanded to the point of not exerting any pressure from within the force of the air on the outside took over. This force depended on the atmospheric pressure and the area of the piston, both of which are constants. Neat. Larry outlined an experiment where he mounted a sealed syringe vertically with the plunger down. He attached a weight hanger to the syringe handle and added masses until the plunger moved at a constant speed pulling the plunger out. By measuring the area of the syringe plunger and the weight of the masses he could determine the pressure of the atmosphere.
With
that,
Mike Kash adjourned the meeting. Submitted by Art Schmidt
Remember that at
the Northwestern meeting we will be asking for
nominations for the John Rush Award that is given out at the June
meeting. The John Rush award is given to one of our members for
exciting ideas and 'phun' contributions to the group. John taught for
many years at Eisenhower High School in Blue Island, and coined the
word 'pneumenon', a new phenomena that you haven't experienced.
On the first day of class John would show his students a strange
contraption and challenged them to discover what it was. Please bring
suggestions. Thanks.
For any information regarding ISPP contact Gerry Lietz at DePaul University, Physics Department, 2219 N. Kenmore Chi. IL 60614 phone: 773-325-7333 e-mail glietz@depaul.edu. ISPP home page: http://condor.depaul.edu/~glietz/ispp/ispp.html
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