Elenco Basic Electronic Experiments - Manual (Page 18)

INTRODUCTION TO CAPACITORS
Capacitors: Capacitors are electrical components that can store electrical pressure (voltage) for periods of time. When
a capacitor has a difference in voltage (electrical pressure) across it, it is said to be charged. A capacitor is charged by
having a one-way current flow through it for a short period of time. It can be discharged by letting a current flow in the
opposite direction out of the capacitor. In the water pipe analogy, you may think of the capacitor as a water pipe that has
a strong rubber diaphragm sealing off each side of the pipe as shown below:
If the pipe had a plunger on one end (or a pump elsewhere in the piping circuit), as shown above, 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 pressure on the diaphragm.
Capacitors act the same as the pipe just described. When a voltage (electrical pressure) is placed on one side with respect
to the other, electrical charge “piles up” on one side of the capacitor (on the capacitor “plates”) until the voltage pushing
back equals the voltage applied. The capacitor is then charged to that voltage. If the charging voltage was then decreased
the capacitor would discharge. If both sides of the capacitor were connected together with a wire then the capacitor would
rapidly discharge and the voltage across it would become zero (no charge).
What would happen if the plunger in the drawing above was wiggled in and out many times each second? The water in
the pipe would be pushed by the diaphragm and 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 side of the diaphragm moved just enough to charge the pipe (a transient or temporary current). Just as the pipe
blocked a direct push, a capacitor blocks a direct current (DC). Current from a battery is an example of direct current. An
example of alternating current is the 60 cycle (60 wiggles per second) current from the electrical outlets in the walls of your
house.
Construction of Capacitors: If the rubber diaphragm is made very soft it will stretch out and hold a lot of water but will
break easily (large capacitance but low working voltage). If the rubber is made very stiff it will not stretch far but will be
able to withstand higher pressure (low capacitance but high working voltage). By making the pipe larger and keeping the
rubber stiff we can achieve a device that holds a lot of water and withstands high pressure (high capacitance, high working
voltage, large size). So the pipe size is determined by its capacity to hold water and the amount of pressure it can handle.
These three types of water pipes are shown below:
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SOFT
RUBBER
LARGE CAPACITY
LOW PRESSURE
STIFF
RUBBER
LOW CAPACITY BUT
CAN WITHSTAND
HIGH PRESSURE
STIFF
RUBBER
HIGH CAPACITY AND
CAN WITHSTAND
HIGH PRESSURE
TYPES OF WATER PIPES
RUBBER DIAPHRAGM
SEALING CENTER OF PIPE PLUNGER
PIPE FILLED WITH WATER
A Rubber Diaphragm in a
pipe is like a Capacitor