1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Capacitor help?

  1. Mar 3, 2006 #1
    I have conducted an experiment in school I have some problem with it . I connect a resistor 6600 ohm to a 6V direct current and I read the miliammeter and it show about 0.9 mA . But after I connect a capacitor 3300 F parallel to the resistor , the miliammeter increase to more than 1 mA.Why the current increase when I connect the capacitor parallel to resistor . Furthermore, the experiment procedure told me to reduce the resistance to get 1 mA but I have to increse the resistance in order to get 1 mA. What wrong with my experiment or the experiment procedure is wrong .
     
  2. jcsd
  3. Mar 3, 2006 #2

    Integral

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Initially the capacitor will be a very low resistance parallel path, so you would expect see a higher total current. Within a few seconds, depending on the cap, the current should drop to that passing through the resistor. You will need to reduce the resistance to get a steady state current of 1mA, a 6k Ohm resistor should give a steady state of 1 mA. Your transient current will be higher.
     
  4. Mar 3, 2006 #3
    I have wait more than 5 minutes but the miliammeter still show more than 1 mA .
     
  5. Mar 3, 2006 #4

    Integral

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Measure every few seconds.. (maybe 30sec) see if you can see the change, it should be dropping. This is assuming that you indeed have it wired as you say. And as I said it also depends on the capacitance.
     
  6. Mar 3, 2006 #5
    I measure every few seconds for more than 5 minutes but the reading doesn't change at all . I connected another miliammeter series to the capacitor wait until this miliammeter reach zero but the miliammeter connected series with resistor still doesn't change at all , still more than 1 mA.What is the problem?
     
  7. Mar 4, 2006 #6

    Integral

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Can you post a diagram of your circiut?
     
  8. Mar 6, 2006 #7
    I have attach the diagram of the circuit please help me .
     

    Attached Files:

  9. Mar 6, 2006 #8

    Integral

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Where in the circuit are the ammeters?
     
  10. Mar 8, 2006 #9
    I am sorry I forgot t draw the miliammeter . I am now attaching the newer image of the circuit . As I said the miliammeter is always reading bigger than 1 mA eventhough the other miliammeter(mA2 in the circuit )is already 0 mA.Please help me.
     

    Attached Files:

    Last edited: Mar 8, 2006
  11. Mar 10, 2006 #10
    Integral are you there ? Why you don't answer my question .Somebody please answer my question
     
  12. Mar 10, 2006 #11
    I don't understand what this experiment is trying to do. The moment you close the switch on the uncharged capacitor, all the current will initially go through the capacitor, it would be like connecting wire between two ends of your power source. What will this achieve?
    What is this experiment's aim?
    Sam
     
  13. Mar 10, 2006 #12

    BobG

    User Avatar
    Science Advisor
    Homework Helper

    Hopefully, you meant 3300 pF instead of 3300 F, otherwise it will take about 3 and a half years for your capacitor to discharge (plus, I couldn't even begin to imagine how big, physically, a 3300 F capacitor would be).

    If you're getting more than 1 mA through the resistor, your voltage at the node has to be higher than 6 V (try measuring it). One thing to check is that your capacitor is fully discharged before you start. You should have zero volts at the node before you close the switch.
     
  14. Mar 10, 2006 #13

    Integral

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    [​IMG]

    To do the experiment you have described you will need an Ammeter as shown in my attachment. Please confirm the values of your circuit components including correct units. Actual measurements of the voltage and resistance would be nice.
     
  15. Mar 10, 2006 #14
    the capacitor is we(i and ngkamsengpeter) use is not 3300pF and also not 3300F but is 3300microfarad.
     
  16. Mar 10, 2006 #15
    But my book doesn't show the diagram as your attachment . Actually my experiment is used to calculate the time constant and only has one miliammeter(mA in the diagram I have attached ) and don't have the mA2.This was totally different from your diagram . Is it my book wrong .
     
  17. Mar 10, 2006 #16

    lightgrav

    User Avatar
    Homework Helper

    If your milliameter is reading more than 0.9 mA, then either
    1) your voltage source has more that 6V potential difference
    2) your resistor has less than 6600 Ohm resistance (even with the meter)
    3) your mA meter is slightly out-of-calibration,
    . . . maybe even all 3.

    Check the source's Voltage ... especially if it a battery!
    Check your meter's reading with nothing connected.
    Check your resistance with an Ohm-meter.

    This is a relative measurement anyway ...
    don't get hung up on 10% difference from "lab instructions" values !
     
  18. Mar 11, 2006 #17
    it is really 6V,
    and the resistor(if not mistake)is about 8800ohm,
    the miliammeter is no problem(new)~
     
  19. Mar 14, 2006 #18
    lifesfun is correct since before I connect the capacitor it shows 0.9mA with resistance 6600 ohm . So, the resistance and the voltage should be no problem but as I mentioned , I don't know why the current increase to more than 1 mA when I connect a capacitor parallel to the resistance .
     
  20. Mar 15, 2006 #19
    can anyone please answer my question.I know that the current increase is due to the internal resistance of the capacitor but I let the capacitor to fully charge and I think that after fully charge the current will back to 0.9mA but it is not the case , the ampere doesn't change even 0.1mA.Please help me !!!
     
  21. Mar 23, 2006 #20
    anybody please reply th question above~~
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Capacitor help?
  1. Help with capacitors (Replies: 4)

  2. Help with Capacitors (Replies: 2)

Loading...