-12-
Connect the wires according to the Wiring Checklist and
press the switch. The LED is on but is very dim (this will
be easier to see if you wrap your hand near the LED to
keep the room lights from shining on it). Take a look at
the schematic. There is a low 3.3KW resistor and a high
100KW resistor in series (one after another). Since the
LED is dimly lit, we know that the larger 100KW must be
controlling the current. You can think of this as where two
sections of the pipe are filled with rock, if one section is
much longer than the other then it controls the water flow.
If you had several rock sections of different lengths then
it is easy to see that these would add together as if they
were one longer section. The total length is what matters,
not how many sections the rock is split into. The same is
true in electronics - resistors in series add together to
increase the total resistance for the circuit. (In our circuit
the 3.3KW and 100KW resistors add up to 103.3KW).
To demonstrate this, disconnect the wires from the
100KW resistor and connect them instead to the 10KW,
press the switch; the LED should be easy to see now
(total resistance is now only 13.3KW). Next, disconnect
the 10KW resistor and connect the 1KW in its place. The
LED is now bright, but not as bright as when you used the
1KW in Experiment #1. Why? Because now the 3.3KW
is the larger resistor (total resistance is 4.3KW).
Also, in Experiment #2 you saw how the 1KW resistor
would dominate the circuit when the variable resistor was
set for 0W and how the variable resistor would dominate
when set for 50KW.
EXPERIMENT #3: Resistors in Series
Wiring Checklist:
o 27-to-56
o 55-to-42
o 43-to-51
o 52-to-3
o 4-to-26
Water Diagram
On/Off
Valve
Water
Meter
Rocks
Pump
Schematic
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