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Homework Help: How to measure capacitance of capacitor using these materials only?

  1. Sep 15, 2010 #1

    How can I measure the capacitance of a capacitor, using the following:

    a battery,

    a digital voltmeter (which records voltage as a function of time),

    a digital ammeter (which records current as a function of time),

    an industrial capacitor,

    two copper plates,

    and several bits of copper wire.

    I know that the capacitance is how much charge can be held between the two plates of a capacitor, but I never really needed to look into a way of doing it manually using single components.

    I also have to construct a second capacitor from the copper plates, whose capacitance equals that of the industrial capacitor.

    If you could help me with a bit of information on that, I would highly appreciate it.

    Thank you
  2. jcsd
  3. Sep 15, 2010 #2

    Chi Meson

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    Science Advisor
    Homework Helper

    Some things to keep in mind:

    The capacitance indicates how much charge is stored for a given voltage.

    Current is the quantity of charge per unit of time, so if your ammeter records the current as a function of time, you should be able to find the total charge that has flowed.

    Now, how would you measure the voltage?
  4. Sep 15, 2010 #3
    Chi Meson, thank you for you reply and time.

    Using C = Q/V, and amps being related to charge, we can say that Voltage = Q/C, so to work out the voltage, we would get the value of the reading of the ammeter and divide it by the value of the capacitance? Is that so?

    In which case, to work out the value of the capacitance, wouldn't it just be the value of the reading of the ammeter divided by the voltage of the battery? But that seems wrong, because it would be much too easy a question to ask, especially since I have to write an essay response for this to explain how I would do it.
  5. Sep 16, 2010 #4
    bump, can anyone please help? I need to formula a 500 word essay on this and im not sure where to start.
  6. Sep 16, 2010 #5
    I am not sure how can you use the copper plate, maybe it intends to function like a rheostate but it doesnt seem that way.
    If you dont even have a time watch, i think there isnt much you can do.
    If you do have one, you may try working out a I-t graph, which give you the total charge stored, and than mark the maximum voltage.
  7. Sep 16, 2010 #6
    But aren't the units of voltage and amps measured in units per second? Wouldn't that suffice as a tool to measure time, in some indirect way?
  8. Sep 16, 2010 #7
    The problem is, Q=CV is only true during steady state, if you are to calculate statis of a charging or discharging capacitor, you must put together an exponetial graph.
  9. Sep 16, 2010 #8
    Hmm, it seems I am out of luck then - as my knowledge on how to do this is very limited when it comes to building a device or devising a method on how to do it with the given materials. This assignment is ridiculous!
  10. Sep 16, 2010 #9
    actually building a theoretical capacitor isnt very hard, given that you have the capacitance you need, a parallel plate capacitor can vary its capacitance by adjusting the distance between two plate when connected to a voltage supply.
    Oh, I just notice you have digital meters, does it means you have the I-t graph and V-t graph, if it is, than you can work out the capacitance pretty easily.
    Last edited: Sep 16, 2010
  11. Sep 16, 2010 #10
    Hi, thanks for your reply.

    Well, I believe we do have the I/t and V/t graphs as they are not analogue. But I am still missing the point on how to do it. But thanks for the tip on varying the distance between the plates to alter capacitance, i'll be sure to use that when writing the final essay.
  12. Sep 18, 2010 #11
    The are under the graph I-t is the total charge stored and the area under the graph V-t is the power delivered by the power cell, which i remember to be twice the amont of energy transferred to the capacitor
  13. Jan 12, 2011 #12
    I don't understand what a digital voltmeter (which records voltage as a function of time), and a digital ammeter (which records current as a function of time) are. I always considered a voltmeter as giving instant voltage and an Ammeter giving amps/second. For faster changing voltages and currents I've always used oscilloscopes.
    Without a proper time function you can't solve your problem.
    You can try this;
    Short the capacitor out. Connect your power source through a resistor and a switch to the capacitor. Measure the voltage of the power source. Connect the voltmeter across the capacitor and then short the switch and time how long it takes the voltage to reach about .98 of the supply voltage. If the capacitor charge too quickly, increase the resistor value so that is takes more than 10 seconds to reach your value. The time will be close to the product of the resistance and capacitance times 5. That should give you an idea of what the capacitance it. The larger your time, the more accurate will be your result. You can use a 555 timer chip to automate this process.
    If you want to make a capacitor you need to know that plate area is directly proportional to capacitance. Distance between plates is inversely proportional to capacitance. The material between the plates affects capacitance directly (a constant). Also consider how stiff the material between plates needs to be. Good luck
  14. Nov 14, 2012 #13
    I have the same kind of task. I have to design a circuit that measures capacitance. Thee limitations are that I have not studies transistors or inductors or diodes or all that and my knowledge is pretty limited. All i know is basic electronics and logic. Can you please help me design such a circuit?
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