Owner's manual

EXPERIMENT #4: PARALLEL PIPES

Remove the resistors used in Experiment #3; the other parts are used here. Insert the new parts 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:

x R

Parallel

______________

+ R

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, remove the 100kΩ resistor and replace it with the 10kΩ (in the same holes); 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 remove the 10kΩ and replace it with the 1kΩ; press the switch. The total resistance is now only

770Ω, so the LED should now be much brighter.

Parts Needed:

• a 9V battery or power supply

• Switch

• one 1kΩ resistor (brown-black-red-gold)

• one 3.3kΩ resistor (orange-orange-red-gold)

• one 10kΩ resistor (brown-black-orange-gold)

• one 100kΩ resistor (brown-black-yellow-gold)

• one LED

• 2 wires

Wiring Checklist ( indicates same position as last

experiment):

Insert red battery wire or positive power supply (P. S.) into j4 and

black battery wire or negative power supply (ground) into (–)3.

Insert switch into f4 and f5.

Insert the LED into g20 and g21 (“flat” side goes into g21).

Insert a short wire between f21 and (–)21.

Insert the 3.3kΩ resistor into i5 and i12.

Insert the 100kΩ resistor into j5 and j12.

Insert a short wire between h12 and j20.

BATTERY

or POWER

SUPPLY

+9V

3.3KΩ

WIRING DIAGRAM

100KΩ

Note: From now on there will be less description for frequently used parts.