Manual

-13-
Connect the wires according to the Wiring Checklist.
Take a look at the schematic. There is a low 3.3KΩ
resistor and a high 100KΩ resistor in parallel (connected
between the same points in the circuit). How bright do
you think the LED will be? Press the switch and see if
you are right. The LED is bright, so most of the current
must be flowing through the smaller 3.3KΩ resistor. This
makes perfect sense when we look at the water diagram,
with most of the water flowing through the pipe with less
rocks. In general, the more water pipes (or resistors)
there are in parallel, the lower the total resistance is
and the more water (or current) will flow. The relationship
is more complicated than for resistors in series and is
given here for advanced students:
R
Parallel
=
For two 10KΩ resistors in parallel, the result would be
5KΩ. The 3.3KΩ and 100KΩ in parallel in our circuit now
give the same LED brightness as a single 3.2KΩ resistor.
To demonstrate this, disconnect the wires from the
100KΩ resistor and connect them to the 10KΩ; press the
switch and the LED should be just as bright. The total
resistance is now only 2.5KΩ, but your eyes probably
won’t notice much difference in LED brightness. Now
disconnect the wires from the 10KΩ and connect them to
the 1KΩ; press the switch. The total resistance is now
only 770Ω, so the LED should now be much brighter.
EXPERIMENT #4: Parallel Pipes
R
1
x R
2
R
1
+ R
2
Wiring Checklist:
o 27-to-56
o 55-to-52-to-43 (this will take 2 wires)
o 51-to-42-to-3 (2 wires)
o 4-to-26
Water Diagram
On/Off
Valve
Water
Meter
Rocks
Pump
Schematic
Rocks