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

  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
  2. jcsd
  3. Integral

    Integral 7,351
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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. davenn

    davenn 3,970
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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. 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. 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. 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. 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. 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. davenn

    davenn 3,970
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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. K^2

    K^2 2,470
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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. 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. 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. K^2

    K^2 2,470
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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. 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. davenn

    davenn 3,970
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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. davenn

    davenn 3,970
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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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