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Phy 122
Your 'question form' report has been received. Scroll down through the document to see any comments I might have inserted, and my final comment at the end.
** Question Form_labelMessages **
Capacitor Charging Lab
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I have run into 2 problems with the Capacitor Charging Lab
1. Early in the lab it says
Report the time, in seconds, required for the bulb to stop glowing, and the time at which the capacitor first reaches the 'halfway point' (i.e., the time required to reach half the generator voltage). Report in the first line, in comma-delimited format. Starting in the second line describe how the voltage changes, specify any additional measurements you made, and explain why you think the voltage behaves in this manner.
2. The part that says:
Next you will use the TIMER to measure current vs. clock time, again starting at 4 volts.
You should have noted the maximum current in your previous trial. If not, over-charge the circuit a little and in short 'bursts' controlled by your switch, allow it to discharge to 4 volts, reading the current in every 'burst'. You will in this way get a very good idea of what the current will be when you begin the discharge.
When you measured the voltages vs. clock time you used voltages of 4, 3.5, 3.0, 2.5, 2.0, 1.5, 1.0, .75, .50, .25 volts--about 10 voltages with an easy-to-remember scheme. Decide what currents you will read, write them down and prepare to click the timer when each current occurs.
When you are ready you may conduct your measurements.
If necessary you can repeat your trials until you are sure you have good data.
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For #1 - I still can't get my head around this Halfway concept. I understood the generator voltage to be the voltage at the start of my cranking, but the voltage is always increasing, so it never decays to a halfway point. What am I missing here?
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I might have given you a misleading answer before, thinking of a different activity in which the voltage decays. In this case the voltage approaches a limit, and you're looking for the time required to get halfway there.
More specifically:
The voltage across the capacitor will start at zero and will approach the voltage of the generator as a limit.
The question is how long, from the start of cranking, it takes for that voltage to build to half the generator voltage.
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For #2 - I've gotten through most of the lab fine, but now that I have to charge the capacitor, but everything in series and track the current vs clock time, I keep running into a problem. I overcharge the capacitor to about 5Volts. Then I disconnect the generator and set up everything in parallel. By the time I have everything set up again (switch is off) the voltage is down to about 2.3V. I don't see what I'm doing wrong and this would seem to be way off from the 4.5 - 5 Volt range where I'd like to be to burst off and get down to 4.0 volts.
Any ideas?
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The capacitor will slowly leak charge. It shouldn't get down to 2.3 volts that quickly, though. This is not to say that it's not possible, but it's worth being sure. To ge sure I would suggest that the last thing you connect should be the capacitor. You can keep one lead on the capacitor and have the other disconnected, ready to connect.
You could also check the capacitor for leaks by charging it while connected to the voltmeter. The voltmeter will draw very little current, so if the voltage changes at a noticeable rate you know you have a leak.
That being done, you should have the TIMER ready to go. You can charge the capacitor then, instead of taking time to remove a clip, you can just unplug the leads from the generator, then connect the one wire necessary to complete the already-set-up circuit, and hit the TIMER at every chosen voltage.
You could just use the 2.3 volts, but the lower the voltage the harder it is to get accurate readings, so I agree that the 4.5-5 volt range is desirable.
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