NOTE: diodes are wired in opposing directions
John Scavo [Evergreen Park HS,
Physics]
Cub Scout Science
John passed around copies of an article, Amazing Science Tricks
by Michio
Goto, which appeared in the April 2004 issue of Boy's
Life® http://www.boyslife.org/,
official magazine of the Boy Scouts of America®. See
also the book
Amazing Science Tricks by Michio Goto: http://www.thejapanpage.com/html/book_directory/Detailed/329.shtml
. John called particular attention to the lessons
entitled Keeping
Water Separate, A Candle that Sucks Water, Bending
Light through
Water, and Toothpick Torpedo. He demonstrated the
bending of
light through water by poking a hole in a 2 liter soft drink bottle
with an awl,
and then filling it with water. When the bottle was placed on the
table (in
an aluminum oven pan!) in an upright position with the cap off, water
flowed out of the hole in a steady
stream. He held a flashlight at the level of the hole and on the
opposite
side of the bottle, and turned it on. Light shined through the
bottle, and
came out into the stream of water, and was totally reflected internally
along
the stream. Beautiful! He showed us the Toothpick
Torpedo.
First John dabbed a little shampoo on the blunt end of a
wooden
toothpick, and dropped the toothpick horizontally into a pan of water.
The toothpick began
moving in the direction of the sharp end. Why?
Shampoo
reduces the surface tension in the fluid near the blunt end of the
toothpick,
and thus the floating toothpick experiences an unbalanced force, and
goes
forward.
Isn't Science Amazing? Thanks, John
Peter Smagacz [Paul Robeson HS,
Physics]
Drift Velocity
Peter asked how fast electrons are traveling when a large
electric current is passing through a wire. Some people might
guess that
they move at the speed of light. Surprisingly, the
electrons
are slowly drifting through the wire, at less than one millimeter
per second,
when, say, current is passing through the starter motor in an
automobile. How come? The answer lies in the fact that
the density of electrons in a
conductor is very large (n = about 1029 per cubic meter),
so
that a very large current per unit area J is produced even when
the drift
velocity is fairly modest. Specifically, J = I / A = n e0
vD.
Thus, when vD = 0.001 m/sec, we get
If the battery cable has a cross sectional area A= 2´10-5 m2, the current flow would be I = J A = 320 amps. Peter illustrated this with an analogy by lining up some small stones (electrons) in a trough. When he pushed another stone into the trough at one end, a stone at the other end fell out. Thus, while the stones did not move rapidly, the effect of a stone entering at one end was rapidly communicated to a stone at the other end.
Thanks for sharing this, Peter!
Roy Coleman [Morgan Park HS,
Physics]
Diodes and Bulbs
Roy took 40 watt and 75 watt light bulbs, and
showed us that the
40 watt
bulb produced less light than the 75 watt bulb, when screwed
into a 110 volt
socket -- presumably because of the higher resistance of the 40
watt
bulb. Then he reminded us of Ann Brandon's demo
from the
last SMILE meeting, in which the lower wattage bulb produced
less light
than the higher wattage bulb, when placed in series across the 110
volt
supply. He then asked us whether the same thing would happen
here.
Curiously enough, when he placed two knife switches in series with the
bulbs,
one bulb would light only when the first switch was closed, whereas the
other
bulb would light only when the second was closed. How can that
possibly
be true?? Roy explained that he had "improved" the
switches and the bulb sockets by placing diodes under them, as shown in
the
circuit below (handout).
We see the light(s)! Thanks, Roy!
Bill Blunk [Joliet Central,
Physics]
Jacob's
Ladder + Signaling with Plane
Mirrors
Bill began by showing us Jacob's Ladder, which is
described on the Science
First website in the article Constructing a Jacob's Ladder http://members.tripod.com/shady_hollow/Projects/jladder.html.
A Neon Sign Transformer is used to set up a high voltage
arc
between a pair of vertical, nearly parallel wires. The wires were
about 30
cm long, and closer at the bottom end (about 5 mm),
tapering gradually
farther apart toward the top. The arc
begins at the bottom end of the wires, and it moves slowly and
erratically up the
wires, and then disappears at the ends --- as with the angels of
Jacob's
ladder. How come? The arc is hot, ionized air,
and --
being less dense than the surrounding air -- it is buoyed up, and
rises. Bill fanned the air around the
arc as it
began to move upward, and it reversed its course, because the ion path
was
pushed downward by fanning. Very interesting! Bill put
a piece of
paper through the arc, and moved it quickly around for a second or
two.
We saw that the paper had many small holes burned into it --- those are
produced
at the rate of 120 per second by the alternating
current. Fascinating!
Bill then reminded us of Colin Fletcher, whose travels through the Grand Canyon are described in the book The Man Who Walked through Time. He took a mirror along to signal aircraft by reflected sunlight, and so notify them of his location, so that they could drop food to him. However, he had great difficulty in hitting the aircraft with the reflected image of the sun, and was forced to build a large fire to identify his location. How could he have reflected the sunlight to the aircraft? Bill set up a central light source (a fiber optic illuminator -- a bright light), and passed around some signal mirrors, which are polished stainless steel plates with a hole at the center. The idea is to look through the hole in the mirror at the object to be illuminated (aircraft). While keeping the object in view, turn the mirror. Sunlight coming through the hole in the mirror appears as a spot of light. Turn the mirror so that you can see the spot on your face by its reflection from (your side of) the mirror. Adjust the mirror so that the bright spot falls on the hole of the mirror and in the line of sight of the airplane. Then, the reflected sunlight will hit the aircraft, as indicated in the diagram below:
(mirror)
| P (aircraft)
| /
(bright spot on face)* | * (image of bright spot)
\ | /
\ | /
\ | /
\|/
hole -->
/|\
/ | \
/ | \
(eye) O | \ (Sun)
(mirror)
Bill, this one really hit the spot!
Bud Schultz [West Aurora HS,
Physics]
Interactive
Physics Simulations -- SIMS
Bud has been using the Interactive Physics simulations
in his
classroom. He had loaded the module, Rocket Sled 2, into
his laptop
PC, and -- with the aid of a projector -- displayed the image of the
computer screen on the
blackboard screen. In this
module, the masses and initial speeds of two sleds could be varied, as
well as
the nature of the collision between the sleds. The sleds (one
lined up
behind the other) were initially
located near the edge of a canyon, and the objective was to see what
initial
conditions would enable one or both sleds -- one colliding from behind
into the
other -- to jump over the canyon and land on
the other side. Very often, one or both of the sleds went into
the canyon,
as Bud showed us.
Information on Interactive Physics Simulations can be obtained at the website http://www.design-simulation.com/IP/index.php. Other modules in the series include Coconut Kick, Newton's Mountain, and Asteroid. Bud showed us the last of these on his laptop. In particular, a projectile launched with the proper velocity will move in a circular orbit around the spherical asteroid because of gravitational attraction. What is the proper velocity?
Bud called attention to gaps in understanding of students (and some teachers!) in the application of mathematics to solve problems. He deals with this issue by giving extra-credit problems, such as the following:
You really took us over the edge! Good, Bud!
Ann
Brandon [Joliet West,
Physics]
Forming Images with Plane Mirrors
As we watched Ann used chalk to draw a semicircle on the front
desk --
concave facing us. She used a large wooden protractor as a guide.
She placed
about a dozen, small mirrors (10 cm high and 3 cm wide)
side-by-side
around and tangent to the chalk semicircle. The mirrors were held
vertically -- 10 cm side up -- on wooden blocks, facing the center of
the
semicircle. She then placed a lit candle at the center of
curvature. Ann picked out a person at the back of the
room, and
asked him to help her adjust each mirror, so that he could see the
reflected
image of the candle in each of the mirrors. We then saw that the
mirrors
had been re-arranged at regular intervals along a parabolic arc,
and the candle
was at the focus of that arc! Great! Thus, we can focus light
with an array of flat mirrors, by adjusting the location of each image
appropriately. We each walked by the location of the image,
to
verify that it had been focused there. We then placed a light
source at
the back of the room, and saw that reflected light bathed the candle
itself. Furthermore, there was a bright reflected image
produced by
each mirror, in which we could see the candle superposed on the
light
source. Neato!
Beautiful images, Ann!
Don Kanner was unable to attend to make his presentation on the Vandegraaff Electrostatic Generator. It will be scheduled at the beginning of our next class, Tuesday 20 April 2004. See you there!
Notes prepared by Porter Johnson