Qualitative DC Circuits Teachers Day Handout American Physical ...

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Qualitative DC Circuits
Teachers Day Handout
American Physical Society


1. What's inside a lightbulb?

a. Make the bulb light, using only a battery, a bulb, and one wire.

b. Make a drawing of what you did in part a).

c. Make a large side-view drawing of the bulb.

d. On your drawing, clearly label how the filament is connected to the outside of the
bulb.

e. Explain how the filament wiring in the lightbulb and the wiring inside a flashlight
enable a flashlight to work. Don't forget the flashlight's on-off switch.


2. How bright is the bulb?

a. L. McDermott and P. Schaffer emphasize that there are some very persistent erroneous
ideas that students hold onto, such as
---Belief that current is “used up” in a circuit
---Belief that battery is a constant current source
---Belief that order of components and direction of current matter, etc.

b. One way to understand how persistent these ideas can be is review the now-famous
“five bulbs” example. Assuming identical and ideal batteries and bulbs, consider three
circuits:

A
C
B

c. To make a prediction, rank the bulbs A thru E from
most bright to least bright, indicating if there are any ties.
Circuit 1: a single bulb lit by a
single battery
Circuit 2: two bulbs in series lit by a
single battery
E
D
Circuit 3: two bulbs in parallel lit by a
single battery
Circuit 1
Circuit 2
Circuit 3
1

d. After your predictions are made, work with the others at your table to make the
circuits and see how your predictions work out.


Why pay so much attention to qualitative batteries and bulbs activities? L. McDermott and P.
Schaffer report that only about 15% get the brightness ranking right, and this number is
robust…no matter when you ask it (before or after traditional instruction), whether you ask
university graduates, calculus-based or algebra-based, or even science and science education
faculty (but not physics faculty).


3. What happens when you modify the circuit?











a) How do the brightnesses of bulbs A, B and C compare?

Arnold Arons (Teaching Introductory Physics,
John Wiley & Sons, 1997) developed questions
below, which applied to the circuit at the right.
Answer the questions, and then build the circuit
and see what happens.

b) What happens to the brightness of each bulb (does it increase, decrease, or stays the same?)
when bulb A is unscrewed and removed from its socket? What simultaneously happens to the
current at points 3, 4, and 5?

c) Replace bulb A in the circuit. What happens to the brightness of each bulb when bulb C is
unscrewed and removed? What simultaneously happens to the current at points 3, 4, and 5?

d) Replace bulb C in the circuit. What happens to the brightness of each bulb if a wire is
connected from point 1 to point 4? What simultaneously happens … to the current at point 3?
…the potential difference across bulb B? …to the potential difference across bulb C? …the
potential difference between points 1 and 5?


*Arons prefaces this question with this note: “Following are several sample problems on qualitative,
phenomenological aspects of simple D.C. circuits---aspects seriously neglected in most textbooks…Exposure to
several such questions is necessary before a majority of students become successful in the reasoning.”





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4. What does a capacitor do?

a. Find the black cylindrical component, with two wires sticking out. Be sure to build
your circuits so the shorter wire of the black cylinder, which comes out of the side
with the minus sign, is connected to the negative side of the battery. The black
cylinder is called a capacitor.

b. Make a series circuit with the capacitor and the battery, making sure that the negative
side of the capacitor is connected to the negative side of the battery. Now, remove the
battery and touch each end of the capacitor to the bulb-socket clips. What do you
observe about the bulb?

c. What does the capacitor seem to do?

d. This time, make a series circuit that includes the capacitor, the battery, and the bulb.
What do you observe? How do you explain it?

e. Remove the capacitor and touch it to the two clips on the bulbholder. What happens?
How do you explain it?

f. Repeat Parts d) and e), and vary the time you have the capacitor, the battery, and the
bulb connected. What pattern do you see? How can you explain this pattern?

g. What you did in Part a) is called "charging the capacitor," and what you did in Part b)
is called "discharging the capacitor."

h. Compare the effect of the bulb in this part to the effect of the bulb in the batteries-and-
bulbs circuits that you built in the beginning of this activity.


5. How can we use the capacitor to detect current?

a. If the bulb filament lights up, does that mean the current is running through it? Briefly
state your reasoning.

b. If the bulb filament does not light up, does that mean that no current is running
through it? Again, briefly state your reasoning.

c. Build some series circuits with more bulbs to make the bulbs progressively dimmer.
Stop when you build a circuit where the bulbs do not light up.

d. Try to charge the capacitor using the circuit with the bulbs that do not light up.

e. As before, discharge the capacitor through one bulb. What did you observe? How you
interpret your observation?

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f. Repeat the experiment in Parts d) and e), but vary the time you charge the capacitor.
How does the change in the time you charge the capacitor affect what you observe
when you discharge it through the bulb?

g. Compare and contrast your results to those of Part 4, Step b) above. How do you
explain the difference in the results?


6. How can discharging the capacitor affect a compass?

a. Build a circuit with a battery and bulb and two wires. Wrap the wire around the
compass. What happens to the compass when you complete the circuit?

b. Add the large coil to the circuit, and put the compass inside the coil. How do you
explain what happened?

c. Charge the capacitor, but without a bulb in the circuit. Discharge the capacitor through
the large coil, with the compass inside. What happens? How do you explain it?

d. Again, charge the capacitor. Connect the coil and bulb in series so you can discharge
the click on most recent e-mail capacitor through them. What happens to the compass
(inside the coil) when you complete the circuit? How do you explain it?


7. How can discharging the capacitor affect a speaker?

a. Charge the capacitor. Discharge it through a speaker. What happens?

b. How can you explain what happens?

c. What would happen if you briefly touched two wires from a battery to the speaker?
Make a prediction.

d. Now try, and record what happens. How do you explain your results?


8. Capacitor or battery?

a. How is the capacitor similar to a rechargeable battery?

b. How is it different from a rechargeable battery?

c. What experiment could you do to explore this question? (You can think up an
experiment using any materials you want.)


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9. How did the form of energy change?

a. Look back over the experiments you have done so far in this workshop. Identify those
where energy changes occurred—that is, where energy change from one form to
another.

b. For each of the experiments you identified, describe the energy changes that occurred.


10. Make up your own experiment and perform it.

a. Using any of the materials we've worked with today, make up your own experiment
and see what happens. Feel free to work with another group if you need extra
materials. Before you actually do the experiment, write down your hypothesis, your
prediction, and the reasons for your prediction. Note: the maximum voltage these
capacitors can stand is 2.5 V, so please build your circuits with just one battery.

b. Try to explain your results using the ideas we've been working with today.




























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Materials for Qualitative Electric Circuits

Item
Possible Sources
Cost
Bulb, 2.5 v, #21524
The Science Source
Pkg. of 10 for $2.00
Bulb-socket, 97-2632
Carolina Biological Supply
Pkg. of 30 for $21.95
D-cell aluminum
battery-holder with
Fahnestock clips, #11F
Acme Model Engineering Co.
@$1.10 for 45 or more
D-cell batteries

@$2.50
Hookup wire
various
About $9 for 100 feet
1 farad capacitor,
#P6963-ND
Digi-Key
Pkg. of 10 for $32.07
Pkg. of 100 for $255.10
2” speaker #958-8645
Allied Electronics
@$2.12

Materials notes:
• Each group gets six 1-foot wires, with the ends stripped (or an already-wound coil),
one D-cell in a battery-holder, three bulbs in bulbholders, one capacitor, and the
speaker.
• The bulb-holder and battery-holder are particularly durable, and both have Fahnstock
clips.



References

1. L. McDermott and P. Schaffer, American Journal of Physics, November 1992


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