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 B
C
Circuit 1 : a single bulb lit by a single battery
Circuit 2 : two bulbs in series lit by a single battery
Circuit 3 : two bulbs in parallel lit by a single battery
Circuit 1 Circuit 2
D
E
Circuit 3 c. To make a prediction, rank the bulbs A thru E from
most bright to least bright, indicating if there are any ties.
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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?
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. a) How do the brightnesses of bulbs A, B and C compare? 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-andbulbs 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
Bulb, 2.5 v, #21524
Bulb-socket, 97-2632
D-cell aluminum battery-holder with
Fahnestock clips, #11F
The Science Source
Carolina Biological Supply
Pkg. of 10 for $2.00
Pkg. of 30 for $21.95
Acme Model Engineering Co. @$1.10 for 45 or more
D-cell batteries
Hookup wire
1 farad capacitor,
#P6963-ND various
Digi-Key
@$2.50
About $9 for 100 feet
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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