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Lab 5: Simple Electrical Circuits
Introduction:
In this laboratory you will explore simple DC (direct current) electrical circuits. The primary
goal of the lab will be to develop a model for electricity. A model is a clear mental picture that
helps to explain how a physical system behaves and allows you to correctly predict what will
happen in new situations. Note that a model is not a complete theory. A theory is a complete
explanation in terms of known physical laws and mathematical equations. A model is the
conceptual basis on which the complete theory is constructed.
The method we will use to help you develop your model is a three-step procedure:
predict/test/evaluate. You probably already have some concept, some idea of what electricity is
and how it behaves. It may be right or wrong. Whatever it is, it is your initial model. You will
be asked to predict what you think will happen in a given electrical circuit, based on this model.
You will then test your prediction by making measurements on a real circuit. This, essentially, is
how all good science is done. We make predictions based on what we think is a correct
explanation of a system, then experiment to verify or refute our prediction. If the prediction was
correct, that says the model is at least partially right. If the prediction was wrong, that says the
model is either wrong or incomplete, and must be revised. The goal, of course, is to develop a
model that always results in correct predictions. When we have done that, we can then say with
some justification that we understand the system. So don’t worry if your predictions are wrong at
first. You will not lose points for incorrect predictions. In fact, correcting an erroneous model
often results in more effective learning than simply verifying a correct one. So use this
opportunity to make some mistakes and to learn from them.
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1.
A Simple Circuit
a.
Making a Bulb Light
Your first objective is to make a bulb glow using a power supply, wires, and a bulb in a socket.
A power supply is much like a battery, it is a source of Electromotive Force (EMF). EMF is the
term we use to refer to the potential difference (or “voltage”) that a battery develops across its
terminals. For example, an ordinary flashlight battery (D-cell) has an EMF of 1.5 Volts.
Procedure:
Turn on the power supply, move the STBY-ON switch to the ON position, make sure the meter
range switch is in the down (0-25V) position, and set the power supply to 5 Volts. You can now
use the STBY-ON switch to turn the supply on or off without having to reset the voltage each
time. Check that the current range control is set to 2A and the small center knob is turned fully
clockwise. The power supply is now a source of 5 Volts of EMF.
You are now ready to try and light the lightbulb. Connect a red wire to the positive terminal of
the power supply and a black wire to the negative terminal and turn on the power supply by
moving the STBY-ON switch to the ON position. Now experiment and see what it takes to light
the bulb. Don’t worry if the current limiter happens to turn off the power supply. Just reset it by
pushing the red button near the current limiter knob, and resume your experimenting. When you
have gotten the bulb to light, draw a picture on the worksheet of the circuit you used. Be sure to
clearly indicate what wires were connected where. As a precaution against possible damage to
the apparatus, always move the STBY-ON switch to the STBY position after you are finished
experimenting.
Further Investigation:
Hopefully you found that in order to light the bulb you needed to create a closed path for the
current to travel around. But what about the details? In particular, how is the filament (the part
of the bulb that actually glows) connected to the external wires from the power supply?
Procedure:
Use the magnifying glass to inspect the bulb. Identify the filament and see how it is connected to
the base of the bulb.
Now unscrew the bulb from the socket. Turn the power supply on and try to make the bulb light
by touching the connecting wires directly to the bulb itself. See just where on the bulb you have
to make the connections in order to get the bulb to light. Answer the worksheet question about
how the bulb works.
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b.
Measuring Current
To light the bulb, you needed a complete circuit in which current can flow. But how does the
current behave as it travels around the circuit? Is the current the same in the wire leading from
the positive terminal of the power supply to the bulb as it is in the wire leading from the bulb to
the negative terminal, or does the bulb somehow “use up” current? Let’s apply our
predict/test/evaluate method and find out. On the worksheet, write your prediction of what
happens to the current in the circuit you have just been working with.
Screw the bulb back into the socket and construct the circuit shown in Figure 1 below. Connect
the multimeter as shown in the figure in order to measure the current flowing in the wire which
leads from the positive terminal of the power supply to the bulb.
Turn on the power supply and record the reading on your multimeter. Now turn the power
supply off, disconnect the multimeter, and construct the circuit shown in Figure 2. below.
Turn on the power supply and again record the reading on your multimeter.
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Now evaluate your results. Were the two readings the same, or did they differ significantly?
Note here that some slight variation in the last one or two digits of the meter reading is to be
expected. No two measurements, even of the same quantity and made with the same instrument,
can be exactly the same. There is always some uncertainty in any measurement. What you are
looking for is significant differences (say greater than five or ten percent) between the two
readings. If no significant differences are noted, you can conclude that the two measurements in
fact agree. Record your evaluation on the worksheet. Was your prediction correct? If not, how
do you have to modify your model in order to account for this new information? Use the space
provided on the worksheet to explain what modifications (if any) are necessary.
2.
A Slightly More Complicated Circuit
Now consider the circuit shown in Figure 3. below.
Such a circuit is called a series circuit because the two bulbs are connected one after the other
rather than side by side. How will this circuit behave? In particular (assuming the power supply
is still set to 5 Volts) will the current flowing in this circuit be the same as in the one-bulb circuit,
or not? Will the first bulb somehow “use up” some of the current so the second bulb gets less, or
will the current through both bulbs be the same? Make your prediction on the worksheet.
Now, making sure that the power supply is still set at 5 Volts, turn on the power supply and
observe the behavior of this circuit. Record what you see. Now, using the same measurement
technique you used in part 1, measure and record the current in all three wires of your series
circuit.
Now evaluate your results. Was your prediction correct? If not, what additional modifications in
your model are necessary. Explain in the space provided on the worksheet.
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3.
Potential Differences in a Series Circuit
Remove the multimeter and reconstruct your original series circuit, as shown in Figure 3. You
should now have an accurate model of how the current in a series circuit behaves, but what about
potential difference? The power supply develops a potential difference of 5.0 Volts across its
terminals. In a circuit consisting of only a single bulb, this potential difference is obviously also
developed across the terminals of the bulb. But what about in a circuit consisting of two or more
bulbs in series? On the worksheet, predict, based on your model, how you expect the potential
differences across the two bulbs to behave. Will the potential difference across each bulb be 5
Volts (since that’s what the power supply is set at), or will something else happen?
Now use your multimeter to measure and record the potential differences across the two bulbs.
Also record, for comparison, the potential difference across the terminals of the power supply.
Note that you may wish to measure this with the multimeter rather than simply relying on the
power supply’s voltmeter (which may not be as accurate).
Now evaluate your results. Was your prediction correct or not. Explain in the space provided on
the worksheet.
4.
Resistance
Resistance is defined as the ratio of the potential difference across the terminals of an electrical
device to the current flowing
through it. In equation form:
R=
V
i
Since a certain current flows through a bulb when a certain potential difference exists across its
terminals, the resistance of the bulb can be easily determined using the above equation. For
example, you probably found, for a single bulb, that a potential difference of 5 Volts resulted in a
current of about 0.14 Amperes. So the resistance of the bulb, under these conditions, is about 36
 (ohms). But is the resistance of the bulb always this value? To answer that question, more
data is needed. If current through a device is measured for several different values of potential
difference, and the results graphed, we can learn more about the behavior of bulbs and other
kinds of resistive devices. Figure 4, on the next page, shows several possible graphs of potential
difference versus current through an electrical device.
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If the graph is a straight line, then the resistance of the device is the same regardless of how much
current is flowing through it. Such a device is said to be ohmic. If the graph is not a straight line,
then the resistance of the device depends on operating conditions and the device is referred to as
non-ohmic. Lets investigate the behavior of two kinds of resistive devices, the bulb, and a
resistor designed for use in electronic circuits. We’ll use the computer to make simultaneous
measurements of current and voltage and create graphs of our data for each of these devices.
The computer can measure current and voltage by means of two devices called, respectively, a
current probe and a voltage probe. These connect to a dual channel amplifier which interfaces
them to the computer.
Note: The current probe and the dual channel amplifier are easily damaged by either
excessive current or excessive voltage, so great care must be taken in their use. Please be
sure to observe all the operating precautions described below.
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Set the power supply to 0 Volts (but do not turn it off completely), make sure the meter range
switch is in the 0-25V position, place the STBY-ON switch to DC ON, reset the current range
control to 0.5 A, and make sure the small inner knob is fully clockwise. Now assemble the
circuit shown in figure 5. Make sure the current probe is connected with the correct polarity.
The voltage probe is just a pair of clip-leads connected to the dual channel amplifier. Connect
them across the terminals of the bulb just as you would any voltmeter. You are now ready to
investigate the resistive characteristics of the bulb.
Procedure:
Start the program Logger Pro. Click on File and open the folder “Current and Voltage Probes”.
In it, find and open the file Simplecircuits1. You should see a graph of Voltage versus Current
displayed. If this does not happen, consult your instructor.
After making sure the power supply is at 0 Volts, but turned on, press the “Collect” button on the
computer display. Then press the “Keep” button. This will put the first data point on your graph.
Now increase power supply voltage to 1 Volt. Again press “Keep”. Increase to 2 Volts. Press
“Keep”. Continue this process, putting data points on your graph, up to and including a supply
voltage of 10 Volts. Then press “Stop”. Immediately return the power supply to 0 Volts and
move the STBY-ON switch to STBY.
Look at your graph. Which of the graphs in Figure 4 does it most resemble? Use the “tangent”
button on the computer display to find the slope of your graph for the first, fifth and last data
points on your graph, and record these values on your worksheet. Print a copy of your graph.
Now connect the voltage probe leads across the terminals of the resistor, and repeat the above
procedure. Remember to go no higher than 10 Volts on the power supply, and turn it off as soon
as you are finished.
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Now disassemble your circuit and use your multimeter to directly measure the resistances of the
bulb and the resistor. Record your values on the worksheet.
What do you conclude about the behavior of the bulb and the resistor? Is either of them ohmic?
If so, which one? Or are both ohmic? If either one is not, how does its resistance depend on the
amount of current flowing through it?
Conclusion:
You have now had some direct experience with electrical circuits. Perhaps you have found that
what you already knew about electricity was correct, or perhaps you now know something you
didn’t know before. In either case, use the space provided on the worksheet to summarize your
conceptualization of the behavior of electric circuits. Was “something” flowing? Where is the
source of energy? Where was the energy transformed? If your initial model (conceptualization)
was correct, describe how the experiments you conducted verified it. If you had to revise your
model, describe what your misconceptions were and what new understanding you have replaced
them with.
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Simple Electric Circuits
Worksheet
Name_______________________
Name_______________________
1a. A simple circuit
Draw the circuit (using circuit elements) you used to light the bulb.
Draw an accurate cutaway sketch of the bulb showing clearly where on the bulb you had to touch
the power supply wires in order to make the bulb glow. Clearly indicate how these points are
connected to the filament.
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1b. Measuring Current
Prediction:
Measurements:
Current in wire from positive terminal to bulb:___________________
Current in wire from bulb to negative terminal:___________________
Evaluation:
10
2.
A Slightly More Complicated Circuit
Prediction:
Observations:
Measurements:
Current in first wire:____________________
Current in second wire:__________________
Current in third wire:____________________
Evaluation:
11
3.
Potential Differences in a Series Circuit
Prediction:
Measurements:
Bulb One Potential Difference:________________
Bulb Two Potential Difference:________________
Power Supply EMF:_________________________
Evaluation:
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4.
Resistance
Resistance values:
Bulb:
First data point:____________________
Fifth data point:____________________
Last data point:____________________
Resistor:
First data point:____________________
Fifth data point:____________________
Last data point:____________________
Direct resistance measurements (with multimeter)
Bulb:____________________
Resistor:_________________
What are your conclusions about how the bulb and resistor behave? Record them here.
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Conclusion:
Summarize your conceptualization (model) here:
Final Question:
What makes a light bulb light? Current or Voltage? Explain.
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