Re: fluid analogs of battery, resistor, capacitor, inductor

[William Beaty <billb@ESKIMO.COM>]

> It's common to mention the pump as a fluid analog of the battery, and
> a narrow pipe as the analog of a resistor, but I have cooked up a
> couple of others --- one for the capacitor and one for the inductor.

Hi Mike!  A couple of comments about:

  http://www.rose-hulman.edu/~moloney/PH112/FluidElecModel.htm

A water pump can be a confusing electrical analogy because real water
pumps are usually CONSTANT FLOW devices, whereas batteries are CONSTANT
PRESSURE devices.  If you plug the outlet or inlet of a water pump, its
motor gets hot and smokes.  If you connect the outlet of a water pump to
its inlet, there is a constant flow rate of water and nothing bad occurs.
This behavior is opposite that of a battery.  A battery "likes" to have
its inlet/outlet disconnected and plugged.  And if you connect the inlet
of a battery to its outlet, the "fluid" suddenly flows very, very fast,
and the battery becomes hot inside and may burst.

A constant-pressure water pump is a strange device.  It only runs itself
just enough to maintain a certain pressure difference across its inlet and
outlet.  If you plug either hose of a constant-pressure water pump, the
pump will stop and sit quitely, with no overheating.  If you allow a small
leak to occur between inlet and outlet, the pump will "see" this, and
start running just fast enough to keep the pressure up regardless of the
leak.  If you connect the output solidly to the input, the pump will speed
up until it operates far beyond its design ratings, the bearings will heat
up, and the stored fuel will be rapidly exhausted.

However, you can create a simple and fairly obvious constant-pressure
water pump.  Your "capacitor", if one of the springs is removed, becomes
much like a battery (you'd have to use specialized constant-force
springs.)  Or perhaps remove ALL springs, turn the device upright, and
place a heavy weight on the piston.  We get an added bonus: this "pump"
has limited energy storage, just like a battery.  To "charge" this
"battery," pump the fluid out of the top and into the bottom, so the
piston is driven upwards.  To use this "battery" as a power supply,
connect it to a resistor.  The fluid then will flow slowly around the
circuit, through the resistor, and the weighted piston will slowly descend
until the "battery" is "dead," and needs to be "recharged".  If at any
time you should pinch the tube closed at any point, the circuit will be
"open", and the piston will halt it's descent.


On your other analogies: they are very much like I use to (mentally)
explain circuits to myself.  Not being a classroom teacher, I've never
tried them out on students.  I've always been tempted to make some
GIF-animations of these things for my website.  Or better yet, interactive
Java applets, with "dirty liquid" flowing visibly through the pipes.

Your capacitor is a better analogy than we might think.  Capacitors really
do have a theoretical upper limit, where the "piston" is driven all the
way to the end of the "cylinder".  In a real capacitor, what would happen
if ALL of the conduction-band electrons were taken from one plate and
placed on the other plate?  The "empty" plate might become an insulator.
Obviously this cannot be done with a metal-plate capacitor, since
dielectric breakdown would occur long before we reached the enormous level
of P.D.  where the electron-deficit began to affect the conductivity of
the metal.  (Yet if our capacitor was some sort of super-ultra-giga
capacitor having dielectric and metal layers just one atom thick, perhaps
we COULD reach the limits of energy storage even with relatively small
voltages.)

One last note:

"fluid" flows in pipes.  What is the name of the stuff that flows in
wires?  Charge.  Not current.  Electrons are particles of charge-stuff,
not particles of "current."

I believe that many people have a misconception about circuits: an
incorrect idea that a substance called "current" exists, and that this
substance flows within wires.  Yet when a switch is opened, this
"current-substance" somehow vanishes.  Does it race back into the battery
when the circuit is broken?  That's not so clear!  K-6 textbooks support
this misconception by teaching kids that the stuff within the wires is
named "current electricity."  Electrons are small bits of "current
electricity"?  No!  They are small bits of electric charge.

In reality, the name of the flowing substance is "charge", and whenever a
circuit is broken, the "charge" simply stops moving.  This is sensible,
since all wires behave as pre-filled pipes.  But the "pre-filled pipes"
concept becomes impossible if we give the name "current" to the electric
fluid.  Nobody can visualize a chunk of disconnected copper wire being
always filled with a substance named "current."


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William J. Beaty                                  SCIENCE HOBBYIST website
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