CASTLE Unit 4 worksheet 5. Electrical nodes
COMPILATION. CASTLE Unit 4 worksheet 5. Electrical nodes
Date: Mon, 15 Oct 2007
From: Marci Denney
Hi! I'm trying to color code worksheet 5 of CASTLE Unit 4, and am perplexed as to number
3 -- the wires on either side of the middle bulb. Having checked the circuit, I know the bulb
doesn't light, but I don't know what to do with the colors.
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Date: Tue, 16 Oct 2007
From: Vonnie Hicks
The short answer is that all three bulbs are connected in parallel, and the middle bulb should
light equally with the other two. The wire to the left of the bulb is blue, and to the right, red. It
is very easy to have a loose connection or broken wire within a clip lead that might give an
erroneous result when connecting the circuit.
What CASTLE needs to include a little better is the concept of an electrical "node" which is
the name of what gets color coded in circuits. The term is a better fit than 'wire' because nodes
include all of the conducting parts in direct contact with each other, - wires, metal clips used to
hold and connect to bulbs and batteries, parts of bulbs and batteries themselves. Nodes end at
light bulb filaments, cells, insulators, and open places in a circuit. I will generally introduce
nodes to my class by holding up two clip leads and saying "here I have two nodes", then clip
them together and say, "and now I have one node!". Then I will bring out my wad of keys on a
metal ring, saying "here is another node, so now I have two again", then clip the leads onto the
key ring and say, "and now I am back to one." Nodes are drawn in a schematic diagram as if
they are wires, even when no wire is involved, as in the node that includes only the tip of a light
bulb and positive end of a D-cell. Before analyzing any circuit, I ask the students, "How many
nodes are in this circuit?". In the case of your circuit problem, as well as all others in that
worksheet, there are only 2 nodes. I generally ask students to ignore nodes within the battery
packs for the sake of this question.
Electrically, a node is an "equipotential" in a circuit, which is why a node can only have one
color. If there is a difference in electrical pressure within an ideal node (that is, one with truly no
resistance), then charge will immediately flow from high to low until the pressure-and color-is
uniform. It then becomes easy to recognize parallel connections as connections between the same
two nodes, and series connections as requiring an additional node for each additional element.
Color coding is easiest if one starts at the positive end of the battery pack, and colors red
throughout that whole node, including all metal parts touching each other directly. Then move to
the negative end of the battery pack and do the same with blue. For C4 W5, that is as far as you
will ever get, and for later worksheets, it makes identifying those troublesome middle nodes.
I discovered the concept of a node in some Lillian McDermott materials in the early 1980's,
and it greatly simplified my reading of circuits. I was horrified to find the term absent from the
final Physics by Inquiry materials. I nearly had Mel Steinberg convinced to use it in the
CASTLE materials, but his committee decided not to make such a radical change. Still, my
students and I have found that almost any circuit, no matter how complicated, becomes simpler if
we first look for the nodes. Hope this helps******************************************************************************
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CASTLE Unit 4 worksheet 5. Electrical nodes
Date: Tue, 16 Oct 2007
From: Ron McDermott
You get a pressure/voltage/color change when you pass through a bulb. Wires that go
directly from the battery up to a bulb (or bulbs) are either red or blue. In diagram 3, EVERY
wire in the diagram can be traced back directly to the battery without having to pass through a
bulb (you just have to find the right path to follow). All the wires, therefore, are either red or
blue depending on the side of the battery to which they are connected). All the bulbs show a
pressure/voltage/color drop red to blue, so all the bulbs will be brightly lit. What you have here
is a very complicated looking parallel circuit.
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