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course Phy 202
capacitors and current __ voltage __ energy#$&*
Phy 202
Your 'capacitors and current __ voltage __ energy' report has been received. Scroll down through the document to see any comments I might have inserted, and my final comment at the end.
** Capacitors and Current __ Voltage __ Energy_labelMessages **
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1 hour 3 min
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Capacitors and Current, Voltage and Energy
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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 alight 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.
Note video clip(s) associated with this experiments on the CD entitled EPS02. The link is Experiment 17: Capacitors and Current, Voltage, Energy . The link will not work within this document; go to the CD, run the html file in the root folder which contains 'experiments' in the filename, and click on the link.
You should review Experiment 16, and be sure you understand how cranking rate and mechanical force are related to voltage and current.
Connect the leads of the generator to the large capacitor, as shown on the videoclip. 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?
• What happened after the handle was released and how could you possibly explain this?
• 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?
Answer these questions as indicated below:
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Your answer (start in the next line):
The amount of force necessary to crank the handle became less the longer I cranked the handle. I believe that the amount of current flowing through the circuit was lessening as I cranked the handle. After the handle was released, it continued to rotate on its own. I believe this may have happened because there was a build up of energy through the circuit. I believe that the evidence that the capacitor in some way stored at least part of the energy I produced because it became easier to crank the handle. There had to be some of the energy built up in the circuit for the handle to continue rotating even after I stoped rotating it.
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The following is not a generally a safe thing to do with a capacitor, since a capacitor can and very quickly release its energy into the wire. This sudden influx of energy could easily cause the wire to melt before it has time to dissipate the energy, and could create a safety hazard.
It is safe to do this with the capacitor supplied with your kit, using the wires and other supplies in the kit, because this capacitor discharges relatively slowly:
• Take one of the thin wire leads and clamp the ends to a posts of the capacitor so that current can flow from one capacitor terminal to the other.
• After about 10 seconds remove the lead.
Note also that your capacitor is designed for low-voltage use only, within the voltage limits indicated on the capacitor. You will not easily exceed the safe voltage with your hand-cranked generator, as long as you use it in the indicated manner (i.e., cranking it by hand).
Now place a bulb in the holder and connect one of the tabs on the holder to one post of the capacitor using a thin wire lead. Connect one of the leads of the generator to the remaining tab of the bulb holder and the other to the remaining post of the capacitor, so that current must pass through the bulb to get to the capacitor.
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?
• After you stop cranking, what happens to the generator and what happens to the bulb?
Answer these questions as indicated below:
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Your answer (start in the next line):
As I continue cranking i notice that the force I have to exert is less and less the longer i crank the handle. The bulb burns less and less bright as I crank and it eventually stops burning completely. When I stop cranking, the generator handle continues to rotate on its own for several rotations. The bulb goes out during this time but comes back on once the handle stops rotating. When the bulb comes back on, it is first fairly bright but gets less and less bright until it burns out over a 15-20 second time span
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What happened to the voltage produced by the generator as you continue cranking?
• Does the voltage increase, decrease, or remain the same?
• How can you tell?
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Your answer (start in the next line):
The voltage stays the same.I believe it stays the same because I am cranking the generator at a constant rate.
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What happens to the current passing through the circuit as you continue cranking?
• 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?
Your answer (start in the next line):
The current decreases. I can tell this because it becomes easier to crank the handle. I can also tell because the light becomes more dim.
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Sketch an approximate graph showing how the current through a capacitor behaves over time, at a constant cranking voltage. Describe your graph, in detail, as indicated below:
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Your answer (start in the next line):
The current is represented on my y axis and time is represented on the x axis. My graph shows that over time, the current in the capacitor decreases since the brightness of the light decreases.
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description of current thru capacitor vs. t:
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?
• Based on the force required to crank the generator, what happens to the current through the light bulb?
• Are your answers to these two questions consistent?
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Your answer (start in the next line):
I conclude that the voltage stays the same since I continue cranking the handle at the same rate. Based on the force required to crank the handle, i believe that the current decreases since it becomes easier to crank the handle. My answers to these two questions are consistent.
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what happens to voltage across bulb:
what happens to current through bulb:
consistent?
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?
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Your answer (start in the next line):
I believe that the voltage across the capacitor stays constant as I crank the handle.
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If the current through the bulb decreases, what does this tell you about the voltage across the bulb?
What is your conclusion about the voltage across the capacitor?
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&&&&The voltage will remain constant. If the cranking is kept at a constant rate, so will the voltage.
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This is true of the voltage produced by the generator.
However the current through the bulb decreases, so it isn't true of the voltage across the bulb. Nor is it true of the voltage across the capacitor.
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Determine the cranking rate necessary to make one of the 6-volt bulbs burn with a medium intensity--not just barely glowing, but not really bright either. Note that you can easily crank the generator fast enough to burn the bulb out, so be careful that doesn't happen.
Now discharge the capacitor (to discharge, connect the ends of a wire to the capacitor's two posts, or terminals, and leave it for about 10 seconds).
Set up a circuit in which the bulb and capacitor are in parallel:
• Run a lead from one post of the capacitor to the tab on the bulb holder. Then run another lead from the other post to the other tab.
• Attach the generator to the posts of the capacitor.
Crank at the same rate as before, not varying the rate at which you crank.
• Does the bulb get brighter or dimmer, or does it remain the same?
• Does the cranking get harder or easier, or does it remain the same?
as indicated below, describe your circuit and answer the above questions.
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Your answer (start in the next line):
I connected the capacitor and bulb in a parallel circuit and then attached the clamps of the generator to each post on the capacitor. When When I crank at the same rate, the bulb seems to get brighter and the generator handle gets easier to crank.
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(cranking rate at which bulb has medium intensity, parallel bulb and capacitor initially uncharged) brighter, dimmer, or same:
cranking harder, easier, or same:
Discharge the capacitor.
Crank the generator at the same rate as before. Keep cranking for at least a minute. Then let go of the generator, while keeping it attached to the capacitor.
• What happens to the light when you release the generator?
• What happens to the generator when you release the generator?
• What do you think is happening in the capacitor?
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The light continues to burn but it becomes less and less brigh tuntil it is not burning at all. The generator handle continues to rotate when I let go but it rotates more and more slowly and then stops. I believe that capacitor is releasing the energy it stored.
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after minute of cranking, what happens to light when generator is released:
what happens to the generator:
what you think is happening in the capacitor:
Discharge the capacitor.
Repeat once more, and time how long it takes for the bulb to stop glowing. Give the time, in seconds, as indicated below:
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Your answer (start in the next line):
37 sec
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how long for bulb to stop glowing generator connected:
Discharge the capacitor.
Repeat again, but this time after cranking (again at the same rate) for about a minute, as quickly as possible unhook one of the generator leads, and determine how many seconds are required for the bulb to stop glowing.
Indicate your result as indicated below.
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Your answer (start in the next line):
61 sec
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how long for bulb to stop glowing generator disconnected:
Discharge the capacitor.
Repeat again, but this time after cranking for about a minute, hold the handle of the generator but do not disconnect it. Determine how many seconds are required for the bulb to stop glowing.
Indicate your result as indicated below.
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Your answer (start in the next line):
about 10 sec
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how long for bulb to stop glowing holding handle
Compare the behavior of the circuit when the generator is simply released, or released and held, or unhooked?
• Describe any differences in the brightness of the bulb.
• Describe any differences in the time required for the bulb to stop glowing.
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Your answer (start in the next line):
I did not notice much of a difference in the brightness of the bulbs. I did notice that the time required for the bulb was least when i help the generator handle, was in-between when I released the handle, and was the most when i unhooked one of the lead of the generator from the capacitor.
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differences in brightness for three discharges
differences in time to stop glowing
Discharge the capacitor.
Use the same setup, but attach a second 6-volt bulb in series with the first (don't use the 14-volt bulb), so that the capacitor is in parallel with a series combination of two bulbs.
Crank so that both bulbs glow, with the brighter of the two glowing the same as before.
• Does the two-bulb series combination require faster cranking or slower, compared to the single bulb?
• Does this combination require more force or less than for the single bulb?
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Your answer (start in the next line):
The two bulb series combination requires faster cranking. The combination also seems to require slightly less force.
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(capacitor in parallel with two-bulb series) Is cranking faster, slower or the same as with single bulb
more force or less
Crank the new system for about half a minute (this will tire your arm; most students can handle a minute but some can't; if half a minute is too much, then 20 seconds will suffice), at the same rate as before, then as before allow the system to discharge. Observe how the brightness of the bulb changes and how long it takes for the bulb to stop glowing. Do this in each of the three ways you did before, being sure to discharge the capacitor before each trial:
• Simply let go of the generator handle.
• Disconnect the generator from the circuit.
• Leave the generator connected and hold the handle.
as indicated below give a full report of your observations. Compare to the behavior of the single-bulb circuit:
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Your answer (start in the next line):
The amount of time it took for the bulbs to stop buring for this process, compared to the last process with just one bulb, was about the same. It took 37s when the handle was released, 13s when the handle was held, and it took 71s for the lights to stop burning when one lead of the generator was disconnected from he capacitor.
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three modes of discharge, compare to behavior of single-bulb circuit
Finally, repeat the entire process once more, but this time have the two 6-volt bulbs connected in parallel. You should be able to handle a minute of cranking but if not reduce to half a minute, and note that you have done so. Again discharge the capacitor before each trial.
Give a complete but succinct report as indicated below:
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Your answer (start in the next line):
I was not able to get this to work. I thought that I may have connected to circuit incorrectly but I tried it several times and was not able to get either light to burn. I cranked as hard as I was able to but still no light.
&&&&I got this to work, each bulb seemed to be the same intensity, and this time the amount of time it took for the bulb to stop glowing took less time than the original time.
here are my times: 11 s, 15 s, 5 s&&&&
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Can you describe the circuit as you set it up?
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results when capacitor, first bulb, second bulb in parallel:
Give your best explanation of any differences in brightness, cranking rate, amount of force required to crank, and discharge time:
• In which system do you believe you 'stored' the most energy in the capacitor, which the least, and why do you think this was so?
• Which system produced the most light after being released, which the least, and why do you think this was so?
• Which system took longest for the bulb to stop 'glowing', which stopped more quickly, and why do you think this was so?
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Your answer (start in the next line):
I believe that the most energy was stored in the series circuit with the two bulbs and the least energy stored was in the parallel one bulb circuit. The bulbs in the two bulb series circuit burned slightly longer. The parallel circuit with one bulb seemed to produce the most light and the series with two bulbs seemed to produce the least light. In the parallel circuit more energy was flowing and therefore produced more light. The series connection with two bulbs seemed to last a little longer than the parallel connection with one bulb. This was the case due to the fact that the energy had to pass through one bulb and then the second so it took longer for the built up energy to run out.
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Very good overall.
There are a couple of questions I want you to reconsider, and a setup I'll want you to describe in more detail.
&#Please see my notes and submit a copy of this document with revisions, comments and/or questions, and mark your insertions with &&&& (please mark each insertion at the beginning and at the end).
Be sure to include the entire document, including my notes.
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There is still one question that remains unresolved. Check my new note, inserted along with one of your revised responses.
Use #### for insertions in this revision.
&#Please see my notes and submit a copy of this document with revisions, comments and/or questions, and mark your insertions with &&&& (please mark each insertion at the beginning and at the end).
Be sure to include the entire document, including my notes. &#
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