Physics 2BL
• BATTERIES AND LIGHT BULBS •
Rev 5.2.1
Introduction
Scientific Inquiry
The primary mode of inquiry in this lab is observation. Before we can form models of the natural
world, we need to accurately chronicle the phenomena in which we are interested. Then, as we
form models, we demand their consistency with our observations.
An important component of observation is choosing an appropriate level of detail. There is
always a compromise between being complete (guaranteeing you have all facts that could
possibly be of interest) and being overwhelmed (having too much irrelevant information). For
example, if you are interested in modeling traffic flow, and you observe the number of cars on a
street, you will want to record the day and time of your observations, but the phase of the moon
is probably not worth recording. As you do the experiments in this lab, you will get a feel for
which factors are most relevant. But details that initially seem insignificant sometimes turn out to
be important, so it is better to err on the side of recording too much. You get a sinking feeling
when you have to throw out data because you are unsure of the conditions under which it was
taken. There are no hard rules, however, and the course will help you learn the ‘art’ of observing.
Physics
In this lab you will explore the concept of the circuit using batteries, wires, and one light bulb. It
is a good idea to read all the steps in each part before you start. This lab relates to material in
Halliday, Resnick, and Walker, chapters 26-28, and you should review those chapters as needed.
Writeup Procedure
This lab consists of seven related mini experiments. A list of materials and instructions are given
below for each experiment. As you carry out the experiments, record your observations in your
notebook. You can do this however you see fit, but a picture is worth a thousand words, and in
this lab it may be easiest to draw diagrams. Try to be neat, but don’t let that get in the way of
including relevant information, as discussed above.
A few questions follow each experiment. Answer these in your notebook as you go. They will
guide you in drawing conclusions about the physics of circuits, as well as ensure you achieve an
appropriate level of detail in your observations.
After you finish the experiments, revisit your answers to the pre-lab homework. If you now
would answer some of the questions differently, or if you have additional insight into the
questions, write down your revised answers in your notebook. You should finish this write up
and turn in your lab book before you leave.
Acknowledgements
Material in this lab has been adapted from McDermott, Tutorials in Introductory Physics.
© 2003 UCSD-PERG
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Physics 2BL
• BATTERIES AND LIGHT BULBS •
Rev 5.2.1
Pre-Lab Homework
These Pre-Lab Homework problems are to be done before you get to lab. They are predictions –
your hypotheses – of what might happen. You will actually perform each of the experiments and
discover if your predictions were correct. Write down what you honestly think will happen, so that at
the end of the experiment you can compare your ideas with what you saw. Your TA’s will be able to
better help you in lab if they can see from your Pre-Lab what ideas you have about what is going on.
You will not be graded down for wrong predictions that show effort, but you will lose points for
missing predictions.
1.
Imagine that you have one battery, one light bulb and one piece of wire that cannot be cut. Sketch at
least one way to connect these components in order to get the bulb to light up.
2.
Imagine you have two batteries, one light bulb, and as much wire as you want. Draw a picture of how
you would connect the two batteries in series with the light bulb.
3.
Imagine that you have two batteries, one light bulb, and as much wire as you want. Draw a picture of
how you would connect the two batteries in parallel with the light bulb.
4.
How many D-cell batteries (1.5 V each) do you need to get 12 V? Would you put them in series or
parallel?
5.
What is a capacitor and what is its function? Does charge actually flow through a capacitor? In what
way does a capacitor function like a battery?
6.
Find the equivalent capacitance for two identical parallel plate capacitors in parallel. Do the same for
two parallel plate capacitors in series.
7.
Write Ohm’s law and then state it in your own words. What does it mean for a resistor to be Ohmic?
8. What is a resistor? Does charge actually flow through a resistor? How does a resistor affect current?
How is a light bulb similar to a resistor? How is it different? Are light bulbs Ohmic?
Experiment A: One Battery, One Bulb, and One Wire
This experiment should be performed in groups of two. The following materials and procedure
apply for each group of two.
Materials:
• One battery (not in its holder)
• One light bulb (not in its holder)
• One piece of wire
Procedure:
1. If the battery and/or light bulb is already in a holder, remove it from its holder, and set the
holder aside.
2. Using only the materials specified above, determine an arrangement that will make the bulb
light up.
3. Confer with the rest of the group (or the TA) until you have discovered and tested all four
possible arrangements.
© 2003 UCSD-PERG
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Physics 2BL
• BATTERIES AND LIGHT BULBS •
Rev 5.2.1
Questions:
A1.
A2.
Draw at least one way to make the bulb light up.
How are the arrangements that made the bulb light similar? How are they different than
the ways the bulb did not light up?
Experiment B: One Battery, One Bulb, and Two Wires
As in Experiment A, this experiment is to be performed in groups of two.
Materials and Procedure:
• One battery and battery holder
• One light bulb and light bulb holder
• Two wires (already connected to the battery holder)
Using just these materials, determine an arrangement to make the bulb light up.
Questions:
B1.
B2.
How is this setup different from Experiment A? What advantages are there to this setup?
(i.e., Why do we do this at all?)
Comparing your results with Experiment A, are the requirements the same for lighting
the bulb?
Experiment C: Two Batteries in Series with One Bulb
Procedure:
1. In your group, make two different setups:
(a) Connect one battery to make the bulb light up (as in Experiment A or B).
(b) Connect two batteries in series with the bulb to make it light up.
2. Check with your TA to make sure you have the series circuit set up properly before
answering the questions.
Questions:
C1.
C2.
C3.
C4.
C5.
Draw the series setup (b).
Which bulb is brighter, the bulb in the set-up with one battery, or the bulb with two
batteries in series?
How does the current through the light bulb correlate to the brightness of the bulb?
Compare the current through the light bulb in the series circuit to the circuit with just one
battery.
Compare the current supplied by the battery in the series circuit to the circuit with just
one battery.
© 2003 UCSD-PERG
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Physics 2BL
• BATTERIES AND LIGHT BULBS •
Rev 5.2.1
Experiment D: Two Batteries in Parallel with One Bulb
Procedure:
1. In your group, make two different setups:
(a) Connect one battery to make the bulb light up (again, as in Experiment A or B).
(b) Connect two batteries in parallel with the light bulb to make it light up.
2. Check with your TA to make sure you have the parallel circuit set up properly before
answering the questions.
Questions:
D1.
D2.
D3.
D4.
D5.
D6.
D7.
Draw the parallel set-up (b).
Which bulb is brighter – the bulb in the setup with one battery, or the bulb with two
batteries in parallel? Or, do they have the same brightness?
For both circuits (a) and (b), compare the voltage across the light bulb.
Compare the current through the light bulb in the parallel circuit to the circuit with one
battery. Is this consistent with your answer to D3?
Compare the current supplied by each battery in the parallel circuit to the circuit with one
battery.
Why is this sort of circuit (batteries in parallel) desirable or advantageous?
Why is the circuit in Experiment C (batteries in series) advantageous?
Experiment E: One Capacitor and one bulb
Materials:
2 Capacitors (1F)
Two batteries
Light bulb
Wires
Stopwatch
Procedure:
1. Connect two batteries in series to the capacitor. Make sure that positive terminal of the
capacitor is connected to the positive end of the batteries in series. Do the same for the
negative terminal. Allow at least 30 seconds to charge the capacitor.
2. Remove the battery and connect the light bulb. Measure how long it takes for the light to
go out.
Questions:
E1.
E2.
What happens to the capacitor when a battery is connected to it? Describe how the
current changes with time.
What happens to the capacitor when a light bulb is connected to it? Describe how the
current changes with time.
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Physics 2BL
E3.
• BATTERIES AND LIGHT BULBS •
Rev 5.2.1
How does the voltage across the capacitor before it is discharged compare with the
battery?
How do your observations compare to your results from part D? Discuss why these two
experiments differ.
E4.
Experiment F: Two capacitors in parallel and one bulb
Procedure:
1. Connect the two capacitors in parallel and charge as before with two batteries.
2. Remove the batteries and connect the light bulb. Measure the time it takes for the light to
go out.
Questions:
F1.
How does the current supplied by each capacitor in the parallel circuit compare with the
current supplied by a single capacitor?
How does the voltage across each capacitor before it is discharged compare with the
voltage across the capacitor in part E?
How does the time compare with that measured in experiment E? If the time is linear in
the capacitance, does this agree with your result from the prelab for two capacitors in
parallel?
F2.
F3.
Experiment G: Two capacitors in series and one bulb
Procedure:
1. Connect the two capacitors in series and charge as before with two batteries
2. Remove the batteries and connect the light bulb. Measure the time it takes for the light to
go out.
Questions:
G1.
G2.
G3.
How does the current supplied by each capacitor in the series circuit compare with the
current supplied by a single capacitor?
How does the voltage across each capacitor before it is discharged compare with the
voltage across the capacitor in part F?
How does the time compare with that measured in experiment F? Does this agree with
your result from the prelab for two capacitors in parallel?
When you are finished exploring and experimenting with different circuits, please clean up all
your equipment and place it in the box as you found it.
© 2003 UCSD-PERG
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