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I am starting to doubt Ohm's law. I would like someone to point out why I am wrong.

  1. Dec 11, 2012 #1
    As the title states I am really doubting Ohm's law. This is why: On a breadboard I placed a white LED and a 220Ω resistor with the 9v battery all in series. The total current flowing through the circuit shouldn't be more than 41mA by V=IR, but my multimeter points between the range of 150-120(Edit: mA). Can someone tell me why???

    Thank you.
     
    Last edited: Dec 11, 2012
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  3. Dec 11, 2012 #2

    Integral

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    Re: I am starting to doubt Ohm's law. I would like someone to point out why I am wron

    We can't help without knowing what you are doing. A circiut diagram including how the meter is connected would help. Try to diagram what you have on the breadboard.

    120-150 whats? What kind of meter are you using?
     
  4. Dec 11, 2012 #3

    davenn

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    Re: I am starting to doubt Ohm's law. I would like someone to point out why I am wron

    what sort of white LED is it give us a part number ?
    it may be one that draws more current

    Hint.... If you ever want to doubt a known physical law, assume it's something you are doing wrong, not the law ;)

    Dave
     
  5. Dec 11, 2012 #4
    Re: I am starting to doubt Ohm's law. I would like someone to point out why I am wron

    Sorry about that I have edited it in the original post.

    Just so you know the correct way of connecting a meter in a circuit to measure the current is to connect it in series. which I did.
     
  6. Dec 11, 2012 #5
    Re: I am starting to doubt Ohm's law. I would like someone to point out why I am wron

    I agree with you which is why I thought someone could point out where I am wrong. And about drawing more current wouldn't the current be restricted by the resistor?
     
  7. Dec 11, 2012 #6
    Re: I am starting to doubt Ohm's law. I would like someone to point out why I am wron

    1) Measure the actual resistance of the "220 ohm" resistor.
    2) Check the accuracy of the meter (use another meter - digital might be better as it would probably change the circuit less than and analgoue one)
     
  8. Dec 11, 2012 #7
    Re: I am starting to doubt Ohm's law. I would like someone to point out why I am wron

    The resistance of the 220 ohm resistor comes to about 218 ohm.
     
  9. Dec 11, 2012 #8
    Re: I am starting to doubt Ohm's law. I would like someone to point out why I am wron

    Can we verify the battery's voltage?
     
  10. Dec 11, 2012 #9

    davenn

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    Re: I am starting to doubt Ohm's law. I would like someone to point out why I am wron

    yes, but the calculated total current through the circuit is a combination of the total resistance of the resistor and of the forward resistance of the LED

    you still didnt tell us what sort of LED ??

    Dave
     
  11. Dec 11, 2012 #10

    K^2

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    Re: I am starting to doubt Ohm's law. I would like someone to point out why I am wron

    Unless LED develops a negative voltage drop, that seems kind of irrelevant, no? 9V / 220 Ohm = 41mA.

    Any LED will actually reduce voltage by about 1 - 2 volts. So the current with an LED should be even less than 41mA. 150+ mA cannot be explained by any kind of LED.


    anj16, can you take a picture of your setup?
     
  12. Dec 11, 2012 #11
    Re: I am starting to doubt Ohm's law. I would like someone to point out why I am wron

    @Dave about the part# for the LED, I have no clue. I bought in bulk off Ebay.
    I have attached a picture of the setup
     

    Attached Files:

  13. Dec 11, 2012 #12
    Re: I am starting to doubt Ohm's law. I would like someone to point out why I am wron

    The voltage of the battery is 9v
     
  14. Dec 11, 2012 #13

    K^2

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    Re: I am starting to doubt Ohm's law. I would like someone to point out why I am wron

    Isn't the entire + line connected on the breadboard? If so, your resistor is shorted out. Try checking resistance while it is in the breadboard.
     
  15. Dec 11, 2012 #14
    Re: I am starting to doubt Ohm's law. I would like someone to point out why I am wron

    Thank you so much!! I never thought about that. I think I need a break from this ;)

    EDIT: Also the doubt I had is gone because I re-measured the current and it comes to about 41 mA.
     
  16. Dec 11, 2012 #15

    davenn

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    Re: I am starting to doubt Ohm's law. I would like someone to point out why I am wron

    prob solved ....

    the old saying ... a pic is worth 1000 words ;)

    Dave
     
  17. Dec 11, 2012 #16

    davenn

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    Re: I am starting to doubt Ohm's law. I would like someone to point out why I am wron

    yup fair comment :)

    D
     
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