# light bulb circuits

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LIGHT BULB CIRCUITS
A “complete circuit” is needed for electricity to flow through a circuit. This can be
accomplished by using a conducting wire to connect either the positive or negative terminal
of a power supply to a circuit element, and then connect a wire from the circuit element to
the other terminal of the power supply.
In this activity you will construct a complete circuit that causes Christmas tree light bulbs
to glow and then investigate the brightness of the otherwise identical light bulbs as
additional bulbs are connected in various circuit arrangements.

Objectives:
 Create a working light bulb circuit
 Discover what happens to the brightness of otherwise identical bulbs
a. as more and more are connected in series,
b. as more and more are connected in parallel,
c. as multiple bulbs are connected in combination arrangements

Equipment:
Size D dry cells, cell holders, connecting wires, Christmas tree lights

I use old Christmas tree lights to do this activity. You can get them free if you ask
your students to donate their old strands, or you can buy them for about half a cent
per light when they are on sale after Christmas. You can cut them apart and strip
the leads so that students can twist them together. No special alligator clips are
needed. Be sure not to mix bulbs from different strands since unless you test them
to ensure that they are identical. If the bulbs within a group are not identical, you
will probably get unexpected results. I do not recommend spending the \$1-2 each
for bulbs and holders sold by the science supply catalog vendors. I do recommend
purchasing some of the inexpensive cell holders from science supply vendors. You may
also wish to obtain a few connecting wires and/or connecting terminals.

Procedure:

Note: Screen shots of circuits shown in this document were taken
from a free electrical circuit simulation found on the World
Wide Web at http://www.physicslessons.com/exp22b.htm.

This simulation is great and can be used in investigations on its own.

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Light Bulbs in Series Arrangements
1. Use your materials to connect a circuit
with one glowing light bulb. Notice its brightness.
2. Keeping the same number of dry cells
(effectively yielding the same “voltage”),
connect another bulb in series with the first one.
3. How does the brightness of the second bulb
compare with the first one? Same
4. Compare the brightness of these two with the brightness of the original single
bulb. Although each of these bulbs have the same brightness, they are dimmer
than any single bulb by itself
5. Add a third bulb in series with these two.
6. Compare their brightness with each other and with the circuit containing one bulb
and with the circuit containing two bulbs. All same brightness, but much dimmer
than before
7. Observe what happens as you continue to add
more bulbs in series.
8. Complete this statement: Whenever light bulbs
are connected in series, each bulb is
__equally________ bright, but the brightness
___decreases_____ as more bulbs are added
in series.
9. Complete this statement: Adding more dry cells in series with a light bulb circuit
will make each of the bulbs __dimmer________________.

Light Bulbs in Parallel Arrangements

1. Use your materials to connect a circuit
with one glowing light bulb. Notice its brightness.
2. Keeping the same number of dry cells (effectively
yielding the same “voltage”), connect another bulb
in parallel with the first one.
3. How does the brightness of the second bulb compare
with the first one? Same brightness
4. Compare the brightness of these two with the brightness of the original single
bulb. Both equally bright as single bulb

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5. Add a third bulb in parallel with these two.
6. Compare their brightness with each other
and with the circuit containing one bulb and
with the circuit containing two bulbs.
All same brightness and equal to single bulb
7. Observe what happens as you continue to
add more bulbs in parallel. Still all same brightness and equal to single bulb
8. Complete this statement: Whenever light bulbs are connected in parallel, each
bulb is __equally___ bright, and the brightness __remains the same______ as
more bulbs are added in parallel.
9. Complete this statement: Adding more dry cells in series with a light bulb circuit
will make each of the bulbs __brighter_____________.

Light Bulbs in Combination Arrangements

Combination circuits are circuits which contain some bulbs in series and some in parallel.
See if you can build combination circuits identical to the ones shown in these simulation
screen shots. Identify each bulb by a letter of the alphabet and then write in a
comparison of light bulb brightness in each arrangement. For example, if Bulb A is
brighter than Bulbs B and C, which are equally bright, you would write A > B = C.

Brightness order and percentage of total current passing through each bulb is listed.
Note than these percentages may not be used to compare the brightness of a bulb in
one circuit with the brightness of bulbs in another circuit. Also, twice the current
does not correlate to twice the power or brightness.

1.    A (100%) > B (50%) = C (50%)

A
B

C

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2. A (100%) = D (100%) > B (50%) = C (50%)

A
B               D

C

3. A (66.7%) > B (33.3%) = C (33.3%)

A

B               C

4. A (100%) > B (66.7%) > C (33.3%) = D (33.3%)

A
B

C           D

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5. A (100%) = E (100%) > B (66.7%) > C (33.3%) = D (33.3%)

A                            E
B

C                   D

6.   A (54.54%) > E (27.27%) = F (27.27%) > B (18.18%) = C (18.18%) = D (18.18%)

A

B           C       D

E               F

Discussion:

We normally associate brightness with the power of a light bulb. For example, a 100-Watt
incandescent bulb is generally brighter than a 60-Watt incandescent bulb. When
otherwise identical light bulbs are connected in various circuit arrangements, the more
current flowing through the bulb, the more power it has and the brighter it is.

As more and more bulbs are connected in series, the overall resistance of the circuit
increases, which causes the overall current to decrease. This results in dimmer bulbs.

Since the same current flows through each bulb in a series arrangement, each bulb will be
equally bright.

When a parallel circuit has one single identical bulb in each branch, the resistance in each
branch is the same so that the same current flows through each bulb. This makes each
identical bulb in a parallel circuit have the same brightness.

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As more and more parallel branches with a single bulb are added in, the brightness of the
individual bulbs remain unchanged. The reason for this is that the total resistance of a
parallel circuits drops as more branches are added in. This drop in total resistance results
in an increase in the total current, which divides evenly among the branches. For example,
a parallel circuit with 3 branches will have 1/3 the resistance and 3x the total current as a
circuit with one single bulb. When this 3x larger current divides itself evenly among the 3
branches, each branch ends up with the same current it would have had if it was the only
bulb present.

When considering various circuit arrangements, the old adage that “current follows the
path of least resistance” holds true. When comparing bulbs in combination arrangements,
it is useful to “trace” the current as it flows around the circuit. As the current reaches
places where it can split, we can be sure that more current will pass through the paths of
least resistance, making bulbs in these sections brighter than the others. The current will
split proportionally to its resistance, meaning that in a parallel branch with two series
bulbs in one branch and a single bulb in the other (just like combination circuit #3), twice
as much current will flow through the branch with the single bulb (66.7% of the total
current) since it has one-half as much resistance as the other branch, which receives
33.3% of the current.

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