Voltages across Capacitor, Bulb and Generator
Now take some measurements:
-
Without otherwise changing the circuit (i.e., leaving the series circuit as
it was) attach the multimeter as a voltmeter across the capacitor using an appropriate range
(remember that when attaching a voltmeter, you must attach it in
parallel with the circuit element, in this case with the capacitor--the
current must branch, with one branch consisting of the voltmeter and the
other of the capacitor).
- Crank the generator at a rate that initially lights the bulb, but not too brightly, and use the BEEPS program to maintain a constant cranking rate.
- Observe how the voltage across the capacitor changes with time, and how the brightness of the bulb changes with capacitor voltage.
- Particularly observe the time at which the bulb stops glowing and the time at which the voltage across the capacitor passes the point where it is equal to half the generator voltage.
If necessary repeat the process a couple of times, being sure to discharge the capacitor first and making sure the meter is set to measure volts, not amps.
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.
Your answer (start in the next line):
#$&*
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time to stop glowing, time for capacitor voltage to reach half of generator
voltage; how voltage changes, explanation; brief
discussion/description/explanation
Now repeat, but this time put the voltmeter in parallel across the bulb rather than the capacitor.
Report the time, in seconds, required for the bulb to stop glowing, and the time at which the bulb voltage first reaches the 'halfway point'. Report in the first line, in comma-delimited format. Starting in the second line describe how the voltage changes, specify any additional measurements you made, and explain why you think the voltage behaves in this
manner.
Your answer (start in the next line):
#$&*
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(voltmeter across bulb) time to stop glowing, time for capacitor voltage to
reach half of generator voltage; how voltage changes, explanation; brief discussion/description/explanation:
Sketch a rough graph depicting generator voltage vs. clock time, bulb voltage vs. clock time, capacitor voltage vs. clock time and bulb current vs. clock time.
Describe your graph and describe in as much detail as you can how you believe these quantities are related to one another:
Your answer (start in the next line):
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graphs of generator voltage, bulb voltage, cap voltage, bulb current vs. clock
time, relationships; brief discussion/description/explanation:
You have made measurements of generator voltage, bulb voltage and capacitor voltage vs. clock time. Now measure current vs. clock time.
- Set the meter to the highest milliamp setting and place it in series between the bulb and the capacitor, so that all the current flowing out of the bulb goes through the meter and into the capacitor.
- Remember when you start cranking to start slowly, with your eye on the meter, so you don't damage it.
- If you have the meter connected incorrectly it either won't ready any current, or it will quickly overload.
Does the current behave in a manner consistent with your graph? Answer this question below, describe the behavior of the current with respect to clock time
(including your data), and explain why you think the current behaves in this manner.
Your answer (start in the next line):
#$&*
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measured bulb current vs. clock time, data, explanation; brief discussion/description/explanation:
Voltages while Discharging thru Bulb
You need a switch for the next activity. You can make a low-voltage switch out of two of the aluminum strips in your lab kit, and one of the magnets. Note that this 'switch' is not safe for high voltages, but at the voltages normally produced by the generator it is as safe as any connection in the circuit.
- Simply run the strip along the tabletop, place the magnet, lying on one side, on top of the strip about 1 cm from one end.
- Fold the 1 cm of aluminum up along the side of the magnet and tape its very end to the magnet
(if you don't have tape you can probably use a paper clip or another flat
metal object to hold the strip to the magnet--just 'sandwich' the strip
between the metal object and the magnet; in the first figure below a paper
clip is used to hold the strip to the magnet).
- Clip a wire lead to the other end of the strip.
- Clip another wire lead to a second strip.
- Clip the other end of each wire lead to the generator. Place the magnet on one part of the table, the other aluminum strip on another part of the table, so that the two aluminum strips are not touching.
- Crank the generator and notice that, as you would expect, no current flows.
- Now place the magnet on top of the second aluminum strip, in such a way that the strip along the bottom of the magnet makes contact with the second strip. Crank the generator. You will probably notice that current flows. If not, adjust the system as necessary to that current will flow.
(The second figure below shows the magnet lying flat on the second strip,
with the first and second strips in contact).
- You can easily and quickly pick up the magnet and place it either in contact with the second strip, or out of contact, thus allowing current to flow or preventing it from flowing.
First Figure: The paper clip on top of the magnet is attracted to the
magnet and 'sandwiches' the aluminum strip between the clip and the magnet.
The first strip (clipped to the yellow lead) and the second strip (clipped to
the black lead) are not in contact and there is no way for current to flow from
the yellow lead to the black lead.
Second Figure: The first strip (clipped to the yellow lead and held to
the magnet by the paper clip) is held in contact with and the second strip
(which lies flat in the table beneath the magnet) by the weight of the magnet.
Current can now flow from the yellow lead through the 'switch' to the black
lead.
Note that the magnet is used mainly for its flat surface and its weight,
which is sufficient to ensure contact between the aluminum strips. Its
magnetic properties are not important to the circuit; the only use made of
magnetic properties is to attract the paper clip and thus hold the strip in
place. Any other object with a flat surface would work as well,
provided the aluminum strip is taped or otherwise attached.
Place the capacitor in series with your 'switch' and the generator.
- In
the above picture, the black lead could be attached to one post of the
capacitor, one lead of the generator to the other post, and the other lead of
the generator to the yellow lead in order to complete a series circuit.
- The switch would be 'open' in the first picture, and would not permit current to
flow. In the second picture the switch would be 'closed'.
Set the meter to read voltage, and place it in parallel with the capacitor.
- Crank the generator at a rate that will generate about 3 volts, and continue
cranking for about 30 seconds, or until the generator 'feels' like it is
generating no current (i.e., until the handle turns as freely as if the leads
are disconnected).
- Keep an eye on the meter, when the 'zero' current is reached, quickly break the circuit
by picking up the magnet and placing it away from the second strip.
Describe below what happens to the voltage as you crank the generator, then as you break the circuit, and also after you break the circuit.
Your answer (start in the next line):
#$&*
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voltage as you crank, as you break, after you break circuit; brief discussion/description/explanation:
Set the 'beeps' program to give you a signal every 2 seconds. You will connect the 'switch' on a beep, then every 2 seconds you will write down the voltage indicated by the meter at the 'beep'. Prepare to do this: Get paper and pencil ready, be sure the meter is located so you can easily read it while writing, and visualize the process. If you miss a reading, mark an 'x' on your paper so you will have a record of the elapsed time.
You will continue until the meter fails to change for 3 consecutive 'beeps'.
- When you are prepared, you may proceed.
If you don't think you got accurate data, you may easily repeat the process of charging the capacitor, disconnecting the generator, connecting the bulb and switch, and recording your voltages. It only takes a minute or two, so if necessary repeat a few times until you are sure you have good data.
In the first line below, indicate the number of readings you made. Starting in the second line, provide a table of voltage vs. clock time, reporting one clock time and one voltage, in that order, in each comma-delimited line. If you missed a reading, the 'x' you marked will remind you to increment the clock time by an extra 2 seconds between readings.
Your answer (start in the next line):
#$&*
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of readings; table of v vs. t; brief discussion/description/explanation:
You will again discharge the capacitor through the bulb, and will this time determine how long it takes for the bulb to stop glowing.
- Charge the capacitor to 4.0 volts. The easiest way to do this might be to slightly overcharge the capacitor, then connect the bulb and use the switch to reduce the voltage a little bit at a time.
- Use the TIMER program to time the discharge. Click the TIMER at the same instant you 'close' the switch, and click again at the instant the bulb stops glowing.
Repeat this procedure 3 times, obtaining 3 discharge times. Give your 3 times in the first line, in comma-delimited format. In the second line estimate, based on your experience, how consistent you think the discharge time is; specify what you think is the mean time of discharge and the uncertainty, in the format a sec +- b sec.
Your answer (start in the next line):
#$&*
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three discharge times until bulb dims; mean time of discharge, uncertainty; brief discussion/description/explanation:
Now repeat the procedure, starting at 4.0 volts, but use the TIMER program to record the clock times at which the voltage reaches 3.5 volts, 3.0 volts, 2.5 volts, 2.0 volts, 1.5 volts, 1.0 volt, .75 volt, .50 volt and .25 volt.
Give your voltage vs. clock time table below, with one clock time and one voltage per line, in comma-delimited format.
Your answer (start in the next line):
#$&*
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voltage vs. clock time data table; brief discussion/description/explanation:
Sketch a good graph of voltage vs. clock time, and sketch the curve you think best represents the voltage of the system vs. clock time.
Using your graph, estimate as closely as you can the voltage at which the bulb stops glowing.
Give your estimate in the first line below, and in the second line give the left and right endpoints of the smallest voltage interval you believe is 95% certain to contain the correct voltage.
Your answer (start in the next line):
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voltage at which glow stops; interval of 95% certainty; brief
discussion/description/explanation
Repeat the discharging process once more, this time starting about 1 volt higher than the highest voltage in the interval you reported above.
- After charging to this voltage, set up the bulb circuit as before.
- This time you are going to attempt to time how quickly the bulb begins to glow after you close the 'switch', and how long it takes to reach its peak brightness.
- When you have the TIMER ready, quickly close the circuit and at the same instant click the TIMER. As soon as you see a glow, click the TIMER again.
When the bulb has reached its peak brightness and first starts to dim,
click the TIMER again
and at the same instant 'open' (i.e., disconnect) your 'switch'.
- Write down the voltage, the time delay between closing the circuit and the beginning of the glow, and the time delay between this instant and the instant the bulb begins dimming.
- If necessary wait until the bulb has been dim for at least 10 seconds. Then repeat the process of closing the circuit, timing the delay and the onset of dimming, and writing down your results.
- Continue in this manner until the bulb no longer glows when you close the circuit.
In the first line below report the number of times you closed the circuit and observed a glow. Starting in the second line give a table for each set of observations, giving in each comma-delimited line a voltage, the time delay until the first glow, and the subsequent time delay until a visible dimming. Starting in the first line below your table, discuss how you think your reaction time and the limits of your ability to detect differences in brightness might have affected your attempts to accurately observe the delay between closing the circuit and first glow, and between first glow and peak glow.
Your answer (start in the next line):
#$&*
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delay between closing of circuit, bulb glow, peak brightness, perceptible
dimming; brief discussion/description/explanation:
Current while Discharging thru Bulb
Repeat the discharge beginning at 4.0 volts, but this time after charging the capacitor and verifying its voltage, you will connect the meter in series with the capacitor and bulb.
- While making the connections, keep the meter set to measure voltage. You are unlikely to damage the meter when it is set for voltage.
- Be very sure that the 'switch' is open, so that no current can flow until you 'close' it.
- Before closing the switch, double-check to make sure that the circuit is in series.
- With the circuit still set to measure voltage, close the circuit and observe the meter. If it measures 0 and the bulb doesn't light, then you probably have everything in series. In either case, open the switch again.
- If the meter measured 0 and the bulb didn't light, then you probably had everything right. In either case, double-check your circuit to be sure that it is just a single 'chain', with only one 'loop'.
- When you are sure everything is set up correctly, with the switch 'open' so no current can flow, switch your meter over to the highest current setting (e.g., 200 mA).
- Using the timer, see how long it takes for the bulb to stop glowing. Also keep an eye on the meter so you get an idea of how the current is changing; be sure to note the current at the beginning of the discharge.
Report in the first line how long it took for the bulb to stop glowing. In the second line state whether the time required for the bulb to stop glowing is significantly different when the meter is connected in series as an ammeter, vs. in parallel as a voltmeter, and support your statement with data. Starting in the third line, describe how the current changed with time, and whether there was any apparent difference between the behavior of the current and the behavior of the voltage in preceding trials.
Your answer (start in the next line):
#$&*
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how long to stop glowing; different for ammeter than voltmeter in previous(?);
data;
current vs. clock time, any significant difference; compared to voltage vs.
clock time;
your brief discussion/description/explanation:
Next you will use the TIMER to measure current vs. clock time, again starting at 4 volts.
- You should have noted the maximum current in your previous trial. If not, over-charge the circuit a little and in short 'bursts' controlled by your switch, allow it to discharge to 4 volts, reading the current in every 'burst'. You will in this way get a very good idea of what the current will be when you begin the discharge.
- When you measured the voltages vs. clock time you used voltages of 4, 3.5, 3.0, 2.5, 2.0, 1.5, 1.0, .75, .50, .25 volts--about 10 voltages with an easy-to-remember scheme. Decide what currents you will read, write them down and prepare to click the timer when each current occurs.
- When you are ready you may conduct your measurements.
- If necessary you can repeat your trials until you are sure you have good data.
Report in the first line the number of currents read. Starting in the second line give you table of current in mA vs. clock time.
Your answer (start in the next line):
#$&*
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(reading currents, short intervals) How many currents did you read?; table of
current vs. clock time; brief discussion/description/explanation:
Construct a good graph of current vs. clock time, and sketch the curve you believe best represents the current of the system as a function of clock time.
Resistance vs. Current
Using your graphs of voltage vs. clock time and current vs. clock time, determine the current at 4.0, 3.5, 3.0, 2.5, 2.0, 1.5, 1.0 and .5 volts:
- Using the voltage vs. clock time graph you can easily determine the clock time at which each voltage occurred.
- Using the current vs. clock time graph you can easily determine the current at each of these clock times.
In the table below, give in the first comma-delimited line the clock time at which each of the specified voltages was observed. There are 8 voltages, so you should give 8 clock times. The first voltage was observed when you closed the circuit, so your first clock time will be 0. In the second line give the current at each of these clock times. Starting in the third line give a table of current vs. voltage, using the usual convention (one voltage then one current in each comma-delimited line):
Your answer (start in the next line):
#$&*
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(previously observed) clock times for voltages at half-volt intervals 4.0 to
.5 volts; current at each clock time; current vs. voltage table; brief discussion/description/explanation:
Assuming that the capacitor has no resistance and no 'reluctance' to
release its charge, what therefore is the resistance at each of the observed
voltages? Give a table of resistance vs. current, in units of ohms vs. amps, using the usual format for a table. In the first line below the table explain how you obtained your results.
Your answer (start in the next line):
#$&*
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table of resistance vs. current; brief discussion/description/explanation:
Almost all the resistance in this circuit is in the filament of the bulb. Sketch a graph of resistance vs. current and give your description of your graph, and a statement of how you believe the resistance in a bulb filament is related to current.
Your answer (start in the next line):
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description of resistance vs. current graph; brief
discussion/description/explanation
Discharge through Wire Lead; Resistivity of Wire Filament
Conduct one more quick test.
- Charge the capacitor to 4 volts, keep the voltmeter attached, and prepare to time some events that will probably happen pretty quickly.
- Attach one your leads to one terminal of the capacitor.
- Prepare to place the other end of the lead on the other terminal, and prepare to start the TIMER at the instant you do so.
- You will attempt to determine the clock times at which the capacitor voltage reaches 2 volts, 1 volt, .5 volts and .25 volts.
- Place the lead in contact with the terminal, so that the capacitor will begin to discharge, and at the same instant click the TIMER.
- At the specified voltages, click the TIMER again.
Report voltage vs. clock time, using the usual format. Beginning in the first line below the table, report how you think the results obtained here might impact the results you obtained for resistance vs. current, and how much you think those results might have to be modified.
Your answer (start in the next line):
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attempt to time discharge of capacitor alone, impact on previous resistance
vs. current results; brief discussion/description/explanation:
In your textbook or on the Internet, look up the concepts of 'resistance' and 'resistivity' with the goal of learning why the resistance of the bulb changes with current. Is resistance directly related to current, or are there one or more other quantities which change(s) with current and affect the resistance? Report the results of your research
: