From: SFPhysics@aol.com
Date: Mon Apr 23 2001 - 14:06:49 PDT
From: SFPhysics@aol.com Message-ID: <70.9ec9889.2815f369@aol.com> Date: Mon, 23 Apr 2001 17:06:49 EDT Subject: Capacitors.... roll your own!
> My physics class is currently studying capacitors. I do not know a
> demonstration or any student activities or experiments to do to demonstrate
> capacitance. Does any one have any good, not too equipment intensive,
> ideas? Yibi Smith, Dixon High School.
>>
Greeting Yibi:
There are two very cheap and easy demonstrations of capacitance. The first
is the Exploratorium Snack where you use a discarded 35 mm plastic film "can"
to make a capacitor that will hold a high voltage static charge. I am not
sure where in the Exploratorium web site this is or if it was just in the old
Snack Book. The outside of the case is covered with aluminum foil
Scotch-taped on while the inside has a layer of foil or sometimes is filled
with saltwater instead of foil. Through the lid a paper clip is pushed to
make contact with either the saltwater or the inner foil. A respectable
charge made be placed on this new version of the Leyden Jar and will deliver
quite a shock so I let the students use NE2 neon bulbs taped on sticks to
discharge them. I have scaled these little units up to small plastic bottled
water size containers but they become quite dangerous as they get larger.
With a bottled water capacitor charged from a Van DeGraaff generator you can
draw some very loud and bright sparks from a charge you placed on the unit
many minutes before.
Another important aspect of capacitors is their ability to pass Alternating
Currents easily and the demo for this is simple with no danger associated
with it. I let my students use waxed paper, the kind from Safeway, etc., and
aluminum foil to make a demonstration capacitor. Using two strips of
aluminum foil, one on each side of a strip of waxed paper, roll up the three
layers so that the foil on one side never touches the foil on the other side.
If you make the waxed paper larger than the foils it will be easy to do
this. Leave a connecting tab on both foils or as the roll is finishing put 2
bare copper wires taped separately against each of the foils. Once you are
finished reasonably tightly rolling up the three layers you can Scotch tape
your new capacitor to stay together. I use a small diameter wood dowel in
the center to roll up the unit and the aluminum foil tabs or wires are taped
to the opposite dowel ends to provide connection points.
To demonstrate that no direct connection exists I like to have the students
use a simple battery and light bulb continuity checker. If the lamp comes on
then the foils are touching and the unit must be unrolled and checked for the
short before being rolled up again and tested. Once it has been determined
that no direct connection exists I let the students apply a sound source (a
modified radio with the speaker disconnected) to one side of their capacitor.
The other lead of the capacitor is connected to an audio amplifier/speaker
with a high impedance input. A common ground from radio to amp/speaker is
run. After the students turn on the radio and the amp the music comes out of
the amp with no direct connection in between. Then they disconnect the
capacitor to see what happens.
Not so cheap but totally safe: One last fun thing to do with low voltage on
high value capacitors is to charge them with a battery then discharge them
with a lamp. Radio Shack was selling capacitors with a value of 1 FARAD
which is phenomenal to us antique guys who thought 100 microFarads was big
time capacity. Charge the 1 Farad capacitor with a low voltage Christmas
tree lamp cut from a string in series with a pair of double A cells set in
series for 3 volts. The lamp will light as the capacitor charges then dim
and go out as full charge is achieved. Disconnect the charging circuit and
put a lamp across the capacitor terminals. It will light up then slowly dim
until it is out in a few seconds. I have the students play with this
charge/discharge cycle then record the times of each. They will love the
charge/discharge sequence and can spend most of a period exploring that. The
creative students will put two capacitors in parallel to see what happens. I
had one class where 8 of these high value capacitors were paralleled by the
students and they found out what the increased capacity would do. The
capacitors are polarity sensitive so that is something to communicate to the
experimenters.
A "Genicon" type hand generator can be used to charge these high value caps
then when you let go the crank keeps turning as they discharge. However,
they are 50 cents worth of plastic gears and a cheap toy DC motor wrapped up
in a $40 dollar plastic handle. If you can get two of these through your
budget they will last for years and ultimately justify their cost. I liked
to make my own with gears from Edmund Scientific but had never found a nice
handle to use on my clunky cheap homemade ones.
Fiziks = great fun ! ! ! ;-)
Al Sefl
The difference between Physicists and boys is the price of their toys........
PS: You can purchase "The Dissectible Capacitor" and even make one out of
plastic soap buckets; but, beware, they will hold enough of a high voltage
charge to knock your posterior end into the next room! Nevertheless, it too
is a great demo.
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