Very good work. Let me know if you have any questions.
The hand-cranked generator is connected to a large-capacity capacitor and the difficulty of cranking changes as time passes. This cranking difficulty vs. elapsed time is noted. The general nature of the current flow vs. time (i.e., increasing or decreasing) is inferred. The capacitor is connected in series and in parallel with a light bulb and the behavior of current vs. elapsed time inferred in each case; the effect of the light bulb is noted. The charged capacitor is allowed to discharge through the generator, then after recharging it is allowed to discharge through the light bulb; the nature of the capacitor is speculated upon.
Now connect the leads of the generator to the large capacitor, as shown on the video clip.
• Crank the handle of the generator at a constant rate of approximately two revolutions per second and keep cranking. After about a minute release the handle and see what happens.
• What happened to the amount of force necessary to crank the handle? What do you think was therefore happening to the amount of current flowing in the circuit?
The handle became easier to crank, so the amount of current was decreasing in the circuit.
• What happened after the handle was released and how could you possibly explain this?
The handle continued to turn after it was released, my explanation would be that some current was still be pushed through the circuit from the capacitor and caused the crank to continued to rotate.
• What evidence do you have that the capacitor in some way stored at least part of the energy you produced when you turned the crank?
The continued rotation of the crank would seem to be evidence that the energy produced was stored and then released.
This circuit is a series circuit consisting of the generator, the bulb and the capacitor.
• Crank the handle of the generator at a rate that causes the bulb to burn, but neither very brightly or very dimly. Continue cranking the handle at the same rate regardless of what happens. After about a minute, release the crank and see what happens.
• As you continue cranking, what do you notice about the force you have to exert, and what do you notice about the bulb?
As I continued to crank, it became easier to turn the generator and the bulb continued to decrease in brightness.
• After you stop cranking, what happens to the generator and what happens to the bulb?
After cranking stopped, the generator continued to turn and the bulb no longer burned.
• What happens to the voltage produced by the generator as you continue cranking?
The voltage, which I kept constant by turning the crank at the same rate, seemed to be maintained because there was still “push” when I stopped cranking.
• Does the voltage increase, decrease, or remain the same? How can you tell?
The voltage remained the same because I turned the crank at roughly the same rate, which is proportional to the voltage.
• What happens to the current passing through the circuit as you continue cranking?
The current, proportional to how difficult it is to crank, was lowered as I continued to crank.
• Does the current increase, decrease, or remain the same? How could you tell if you weren't looking at the light? How can you tell by looking at the light?
The current decreases during the process, if I was to not look at the light I would know this because the crank became progressively easier to turn, if I were looking at the light I would know this because it gradually became dimmer.
• Sketch an approximate graph showing how the current through a capacitor behaves at a constant voltage.
An approximate graph of this would have a negative slope from high near time = 0 to low near time = 60 sec, because it begins at a higher current (harder to crank initially), and the current decreases over time because it becomes easier to crank while continuing to crank at roughly the same rate.
You have directly experienced the fact that the brightness of the light bulb depends on the voltage across the bulb (i.e., the faster you crank the generator when it is connected to a single bulb the brighter the bulb burns).
• What therefore do you conclude happens to the voltage across the bulb as you continue cranking the capacitor-and-bulb circuit?
The voltage seems to be stored or maintained through the circuit because of the generator continuing to turn after release at close to the same rate initially that was being turned by hand.
• Based on the force required to crank the generator, what happens to the current through the light bulb? Is this consistent with your answer to the preceding question?
The current decreases, which would be consistent with the previous answer because if the voltage were in some way stored then the current would not be pushed through and the bulb would burn dimmer.
The total voltage across the capacitor and bulb remains constant as long as the generator is cranked a constant rate.
• Based on what you think happens to the voltage across the bulb as you continue cranking, what do you think happens to the voltage across the capacitor?
It seemed the voltage through the bulb was decreasing so the voltage across the capacitor must have been increasing since the crank was turned at a constant rate.