Here I am, the new guy on the block.
I have been "reading the mail " for about a week now and I see it is time
to jump in.
I scanned the section on capacitors in my Radio Amateur's Handbook
(vintage material.here, from 1970) and ran it through the optical
character recognition stuff and below is an excerpt.
I have been using capacitors off and on since I was 11 years of age (and
that was a long time ago) in "ham" radio, and at work building control
units for linear accelerators and electron beam pattern generators used
to make the patterns for integrated circuits.
The electret does maintain a "charge" on its own but it is generally only
good for setting up an electrical field that is used by something else to
accomplish some sort of work
I think the most common use is in the electret microphone. As I
understand the concept, one plate of the electret is free to move and
vibrate as the sound waves hit it. As the plate flexes closer to the
other one, the effective voltage across the capacitor rises slightly and
lowers as the plate flexes away. This varying voltage is then amplified
to whatever level is needed.
The electrets that were used in school for demonstration purposes can't
power anything practical, in fact, the volt meter that is used to measure
the voltage across the electret has to have a very high input resistance
so that it draws an absolute minimum of current, otherwise it would lower
the voltage so much that a false reading occurs.
I don't yet see how they could power a telephone system. But,now, you
guys have me thinking about it and now its going to bug me until I start
playing with one. As if I didn't have enough to do. HA
************************************************************
CAPACITANCE
Suppose two flat metal plates are placed close
to each other (but not touching) and are con-
nected to a battery through a switch, as shown in
Fig. 2-8. At the instant the switch is closed, elec-
trons will he attracted from the upper plate to the
positive terminal of the battery, and the same
number will be repelled into the lower plate from
the negative battery terminal. Enough electrons
move into one plate and out of the other to make
the e.m.f. between them the same as the e.m.f. of
the battery.
If the switch is opened after the plates have
been charged in this way, the top plate is left
with a deficiency of electrons and the bottom
plate with an excess. The plates remain charged
despite the fact that the battery no longer is con-
nected. However, if a wire is touched between the
two plates (short-circuiting them) the excess
electrons on the bottom plate will flow through
the wire to the upper plate, thus restoring elec-
trical neutrality. The plates have then been dis-
charged.
The two plates constitute an electrical capaci-
tor; a capacitor possesses the property of storing
electricity. (The energy actually is stored in the
electric field between the plates.) During the time
the electrons are moving-that is, while the capac-
itor is being charged or discharged-a current is
flowing in the circuit even though the circuit is
"broken" by the gap between the capacitor plates.
However, the current flows only during the time
of charge and discharge, and this time is usually
very short. There can be no continuous flow of
direct current "through" a capacitor, but an alter-
nating current can pass through easily if the
frequency is high enough.
The charge or quantity of electricity that can
he placed on a capacitor is proportional to the
applied voltage and to the capacitance of the
capacitor. The larger the plate area and the
smaller the spacing between the plates the greater
the capacitance. The capacitance also depends
upon the kind of insulating material between the
plates , it is smallest with air insulation, but sub-
stitution of other insulating materials for air may
increase the capacitance many times. The ratio
of the capacitance with some material other than
air between the plates, to the capacitance of the
same capacitor with air insulation, is called the
dielectric constant of that particular insulating
material. The material itself is called a dielectric.
************************************************************
I scanned the ol' college physics book concerning absolute zero and have
some
editing to do before I can include it in the next blurb.
Anyway, gotto go.... Be well....
David
E-Mail Addresses: (Primary) bolin666@juno.com (no attachments)
(Secondary) bolin666@sj.bigger.net
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