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-   -   I am starting to doubt Ohm's law. I would like someone to point out why I am wrong. (http://www.physicsforums.com/showthread.php?t=658414)

anj16 Dec11-12 09:00 PM

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

Integral Dec11-12 09:07 PM

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?

davenn Dec11-12 09:14 PM

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

anj16 Dec11-12 09:16 PM

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

Quote by Integral (Post 4193830)
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 (Post 4193830)
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 Dec11-12 09:18 PM

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

Quote by davenn (Post 4193837)
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 Dec11-12 09:19 PM

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

Quote by anj16 (Post 4193822)
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 Dec11-12 09:21 PM

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.

BackEMF Dec11-12 09:28 PM

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?

davenn Dec11-12 09:31 PM

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

Quote by anj16 (Post 4193843)
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 Dec11-12 09:39 PM

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

Quote by davenn (Post 4193859)
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 Dec11-12 09:50 PM

Re: I am starting to doubt Ohm's law. I would like someone to point out why I am wron
 
1 Attachment(s)
@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

anj16 Dec11-12 09:52 PM

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

Quote by BackEMF (Post 4193853)
Can we verify the battery's voltage?

The voltage of the battery is 9v

K^2 Dec11-12 09:56 PM

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.

anj16 Dec11-12 10:02 PM

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

Quote by K^2 (Post 4193889)
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 Dec11-12 10:37 PM

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

davenn Dec11-12 10:38 PM

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

Quote by K^2 (Post 4193870)
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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