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course Phy 122
3/29 2253 hrs
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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 longer that I cranked the handle, the less mechanical resistance was felt. I think the capacitor started to build up a charge and with more electrical resistance, there was less mechanical resistance.
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In fact, the capacitor is building charge, which builds a voltage that opposes that of the generator. The result is less voltage across the bulb, hence less current.
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After the handle was released, the generator kept cranking, almost as though the capacitor was now acting as the motor driving the generator with its stored energy.
The continuation of the generators rotation. With the capacitor willing to dump its energy back into the system, the mech resistance was very little allowing the crank to easily rotate.
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force on handle, what happened to current:
behavior when handle released, explanation:
evidence of stored energy:
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):
The mech force required to make the light burn diminished as the charge built up and the although the required mech force diminished, the bulb stopped lighting.
After I stopped cranking, the generator kept turning. When I physically prevented the generator from turning, the light kicked back on.
your brief discussion/explanation:
The charge needed to go somewhere, when I stopped it from turning the generator, it went to the light bulb. When I let the generator turn, it started cranking again and the light went out. Basically I diverted the flow of electricity.
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There's no place for the current to go except through the capacitor, bulb and generator, in series. So you can't divert it.
The generator is cranking due to the current passing through it. This cranking causes it to generate a 'back voltage' that opposes that of the capacitor, which again reduces the voltage across the bulb.
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observations of force and bulb with continued cranking:
observations of generator and bulb after cranking stops
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):
With no change in the amount or rate of crank, the voltage should stay the same.
Voltage is directly proportional to the work.
@&
Voltage does stay the same, as you say.
Current, which is felt as mechanical resistance, is also involved in figuring the work.
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your brief discussion/explanation:
explained in the question above.
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Does the voltage increase, decrease, or remain the same?
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):
I believe the current decreases
The ease of the crank mechanical resistance.
your brief discussion/description/explanation:
NA
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----->>>> Does the current increase, decrease, or remain the same?
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):
Since voltage remains same, my graph is between time/current. Time would be the x-axis and the current would by my y-axis. The graph shows a downward sloping line from left to right because as time increases, the current decreases.
your brief discussion/explanation:
above
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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):
The voltage should remain the same.
The current diminishes as well.
Consistent.
@&
The voltage of the generator remains constant.
However the decreasing mechanical resistance, which is also accompanied by a dimming bulb, reveals a decreasing current. This will only happen if the voltage across the bulb decreases.
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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):
The voltage across the capacitor remains constant as well.
your brief discussion/explanation:
The voltage across the system should remain constant because the crank rate didn’t change.
@&
The voltage of the generator remains constant, as you say.
The voltage across the bulb decreases.
What does this imply for the voltage across the capacitor?
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what happens to voltage across capacitor:
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):
The bulb seem to remain at the same brightness.
The cranking go easier, just not as quickly as it did in series, and the bulb seemed to get brighter.
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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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Your answer (start in the next line):
The light continued to burn, growing dimmer until it went out.
The generator continues to turn, slowly coming to a stop.
The capacitor is releasing its stored energy into the system.
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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):
13 sec
your brief discussion/explanation:
The capacitors energy is released into the system until it is out of charge.
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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):
It took about 20 seconds until there was no more orange left in the filament.
your brief discussion/explanation:
same as above. It took more time since I disconnected the load of the generator from the system.
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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):
The light in this instance when dim and out very quickly.
your brief discussion/explanation:
The larger the system, the easier it is for the capacitor to bleed down all of its stored charge, I believe.
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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 didn’t notice any difference in the brightness in the bulb.
The time was shortened by good amount when the generator was prevented from turning.
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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):
It seemed to me to be about the same.
I used the same amount of force but it stands to reason that more force would be necessary to work this system without a capacitor. With the capacitor, it seemed the same.
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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 behavior was pretty much the same.
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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):
The behavior is what you would expect. Same as above. The lights lit up and when I took one lead off of the capacitor, from the generator, the light slowly dimmed until they went out.
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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):
Between the single capacitor and bulb in parallel
Two bulbs in parallel with the capacitor. The room was lit up more.
Single bulb in parallel.
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*#&!
&#Good work on this lab exercise. See my notes and let me know if you have questions.
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