Batteries and Bulbs
Your friend needs more light in his workroom, so he decides to add another light fixture
to his track lighting. However, he is concerned that adding another light may dim the
lights that are already in the track. When he proceeds with the addition of another light,
he notices that none of the lights are dimmer then before. He wonders what type of
circuit the track lighting uses and has asked you for help. You decide to build a model of
circuits using two bulbs and compare the brightness of the bulbs in these circuits to a
circuit with a single bulb. You know that the circuit, where bulbs are as bright as your
reference circuit, is equivalent to the circuit that your track lighting uses. Before you
actually build your model, you try to predict which bulbs will be brighter. The bulbs you
use in your circuits will be identical and so will the batteries.
?
How does the brightness of the bulb depend on the arrangement of the bulbs in
the following circuits?
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Predictions:
To a good approximation, the brightness of a bulb is directly related to the electrical
current traveling through it. A brighter bulb has more current. The amount of current
through the bulb depends on the voltage across the bulb. Before setting up any circuit,
predict the brightest of the bulb in comparison to bulb A in circuit 1. (The notation used
in the circuits are bulbs are labeled A, B, C, etc. and voltage points are labeled 1, 2, 3,
etc. Also, there are junctions in the circuit where the current divides and travels through
branches. An example of a junction is in circuit 4 where bulbs H, F, and the battery are
connected together at a place to the right of point 9. This connection is called a junction
or branch point.
Using the symbols > < and =, rank the brightness of the five identical bulbs in circuits 2,
3, 4, and 5 in comparison to bulb A in circuit 1.
Circuit 2
Circuit 3
Circuit 4
Circuit 5
Bulb B is
Bulb D is
Bulb F is
Bulb I is
Bulb C is
Bulb E is
Bulb G is
Bulb J is
Bulb H is
Bulb K is
Using the digital multimeters to measure electrical quantities.
First notice that current is measured at a point in the circuit whereas potential
difference is measured between two points in a circuit.
To measure current:
Symbol for ammeter:
You must unhook the circuit at the point where you want to measure the current and
insert the multimeter as shown in the example below. (Notice that the multimeter has
two connections.) In this example, the current through point 11 is being measured. You
must select the ammeter function on the multimeter by pushing the button marked “A”.
Since this is a DC circuit, make sure the AC/DC button is out. Finally, choose the scale
that the meter should read. In most cases you will choose 200m (milliamps) or 2000m
(milliamps).
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To measure potential difference:
Symbol for a voltmeter:
You must hook one connection from the multimeter to one side of the circuit element (in
the example below this is a bulb) and hook the other connection on the other side of the
circuit element. In this example the potential difference across bulb G is being measured.
You must select the voltmeter function on the multimeter by pushing the button marked
“V”. Since this is a DC circuit, make sure the AC/DC button is out. Finally choose the
voltage scale to be measured. Since the batteries that you are using are about 1.5 Volts, y
Procedure:
1. Using two battery holders and three light bulbs. Build circuit 1 as your reference
circuit. Build circuit 2 and compare the brightness of bulbs A, B, and C. Use the
ammeter to measure the current at points 1 in circuit 1 and 2, 3, and 4 in circuit 2. A
common misconception about current is that it is “used up” as it travels through a bulb.
Use the voltmeter to measure the voltage between points 2 and 3, points 3 and 4, and
points 2 and 4. Comment on your observations
Current
Voltage
Point 1
Point 2
Point 3
Point 4
Points 2 & 3
Points 3 & 4
Points 2 & 4
Observations:
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2. Replace circuit 2 with circuit 3. Compare the brightness of bulbs A, D, and E.
Repeat the current and voltage measurements as was done in procedure 1.
Current
Voltage
Point 5
Point 6
Point 7
Point 8
Points 5 & 6
Points 6 & 8
Points 5 & 7
Points 5 & 8
Observations:
3. Now replace circuit 3 with circuit 4 and, as you have done with the other circuits,
compare the brightness of each bulb and the current and voltage at each point to the bulb
and current in the reference circuit (circuit 1).
Current
Voltage
Point 9
Point 10
Point 11
Point 12
Points 9 & 10
Points 10 & 12
Points 9 & 11
Points 9 & 12
Observations:
4. Do the same comparisons for circuit 1 and circuit 5.
Current
Voltage
Point 13
Point 14
Point 15
Points 13 & 14
Points 14 & 15
Points 13 & 15
Observations:
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5. Challenge Game (extra credit!)
Light a bulb with one battery and one wire. Draw a sketch of how you did it.
Questions:
1. How do your predictions compare to your results?
2. By applying the conservation of charge, explain what happens to the current at a
junction. What happens to the current at a bulb? Describe what happens to the current as
it goes through a circuit.
3. Describe the relationship between the potential difference across each bulb and battery
in each circuit.
4. Which circuit would you recommend to your friends to solve the track lighting
question? Where would you add a third bulb to this circuit so that the resulting circuit
had three bulbs of the same brightness as bulb A?
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