Elenco Electronic Component Kit - Manual (Page 7)

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CAPACITORS, What do they do?
Capacitors are components that can store electrical
pressure (Voltage) for long periods of time. When a
capacitor has a difference in voltage (Electrical
Pressure) between its two leads it is said to be
charged. A capacitor is charged by forcing a one
way (DC) current to flow through it for a short period
of time. It can be discharged by letting an opposite
direction current flow out of the capacitor. Consider
for a moment the analogy of a water pipe that has a
rubber diaphragm sealing off each side of the pipe
as shown in Figure 9.
If the pipe had a plunger on one end, as shown in
Figure 9, and the plunger was pushed toward the
diaphragm, the water in the pipe would force the
rubber to stretch out until the force of the rubber
pushing back on the water was equal to the force of
the plunger. You could say the pipe is charged and
ready to push the plunger back. In fact, if the
plunger is released it will move back to its original
position. The pipe will then be discharged or with no
charge on the diaphragm.
Capacitors act the same as the pipe in Figure 9.
When a voltage (Electrical Pressure) is placed on
one lead with respect to the other lead, electrons are
forced to “pile up” on one of the capacitor’s plates
until the voltage pushing back is equal to the voltage
applied. The capacitor is then charged to the
voltage. If the two leads of that capacitor are
shorted, it would have the same effect as letting the
plunger in Figure 9 move freely. The capacitor
would rapidly discharge and the voltage across the
two leads would become zero (No Charge).
What would happen if the plunger in Figure 9 was
wiggled in and out many times each second? The
water in the pipe would be pushed by the diaphragm
then sucked back by the diaphragm. Since the
movement of the water (Current) is back and forth
(Alternating) it is called an Alternating Current or
AC. The capacitor will therefore pass an alternating
current with little resistance. When the push on the
plunger was only toward the diaphragm, the water
on the other end of the diaphragm moved just
enough to charge the pipe (transient current). Just
as the pipe blocked a direct push, a capacitor clocks
direct current (DC). An example of alternating
current is the 60 cycle (60 wiggles each second)
current produced when you plug something into a
wall outlet.
SUMMARY: A capacitor stores electrical energy
when charged by a DC source. It can pass
alternating current (AC), but blocks direct current
(DC) except for a very short charging current, called
transient current.
CAPACITORS
Pipe Filled with Water
Rubber Diaphragm
Sealing Center of Pipe
Plunger
Figure 9