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


by anj16
Tags: ohm's law
anj16
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Dec11-12, 09:00 PM
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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.
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Integral
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#2
Dec11-12, 09:07 PM
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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?
davenn
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Dec11-12, 09:14 PM
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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

anj16
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Dec11-12, 09:16 PM
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I am starting to doubt Ohm's law. I would like someone to point out why I am wrong.


Quote Quote by Integral View Post
120-150 whats? What kind of meter are you using?
Sorry about that I have edited it in the original post.

Quote Quote by Integral View Post
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.
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.
anj16
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Dec11-12, 09:18 PM
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Quote Quote by davenn View Post
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
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?
BackEMF
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Dec11-12, 09:19 PM
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Quote Quote by anj16 View Post
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.
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)
anj16
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#7
Dec11-12, 09:21 PM
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The resistance of the 220 ohm resistor comes to about 218 ohm.
BackEMF
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Dec11-12, 09:28 PM
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Can we verify the battery's voltage?
davenn
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Dec11-12, 09:31 PM
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Quote Quote by anj16 View Post
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?
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
K^2
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Dec11-12, 09:39 PM
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Quote Quote by davenn View Post
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
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?
anj16
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#11
Dec11-12, 09:50 PM
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@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 Thumbnails
IMAG0170.jpg  
anj16
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Dec11-12, 09:52 PM
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Quote Quote by BackEMF View Post
Can we verify the battery's voltage?
The voltage of the battery is 9v
K^2
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Dec11-12, 09:56 PM
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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.
anj16
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Dec11-12, 10:02 PM
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Quote Quote by K^2 View Post
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.
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.
davenn
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Dec11-12, 10:37 PM
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prob solved ....

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

Dave
davenn
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Dec11-12, 10:38 PM
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Quote Quote by K^2 View Post
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.
yup fair comment :)

D


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