Doubting Ohm's Law: LED & 220Ω Resistor

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Discussion Overview

The discussion revolves around the application of Ohm's Law in a circuit involving a white LED and a 220Ω resistor connected to a 9V battery. Participants explore the unexpected current readings observed, questioning the validity of Ohm's Law in this context.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant doubts Ohm's Law due to measuring a current of 120-150 mA instead of the expected 41 mA based on V=IR.
  • Another participant requests clarification on the circuit setup and the type of multimeter used for measurements.
  • Some participants suggest that the LED may have a higher current draw than expected, questioning the specific type of LED used.
  • It is noted that the current through the circuit is influenced by both the resistor and the forward resistance of the LED.
  • One participant emphasizes that the voltage drop across the LED should reduce the current further, suggesting that a reading of 150+ mA is not plausible for any LED.
  • Participants recommend verifying the actual resistance of the resistor and the voltage of the battery to troubleshoot the issue.
  • A suggestion is made to check if the resistor is shorted out on the breadboard, which could explain the unexpected readings.
  • After re-evaluation, one participant reports that the current measurement aligns with the expected value of about 41 mA, indicating a resolution to their doubt.

Areas of Agreement / Disagreement

Participants express varying levels of doubt regarding the application of Ohm's Law in this scenario, with some suggesting potential errors in measurement or setup. The discussion includes multiple viewpoints on the behavior of LEDs in circuits, and while one participant resolves their doubt, consensus on the broader implications of the findings remains unclear.

Contextual Notes

Participants highlight the importance of circuit configuration and measurement accuracy, indicating that assumptions about the components and their behavior may affect the outcomes. The discussion does not resolve the theoretical implications of the observed current readings.

anj16
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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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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?
 


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
 


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

Integral said:
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.
 


davenn said:
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?
 


anj16 said:
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)
 


The resistance of the 220 ohm resistor comes to about 218 ohm.
 


Can we verify the battery's voltage?
 


anj16 said:
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
 
  • #10


davenn said:
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?
 
  • #11


@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
 

Attachments

  • IMAG0170.jpg
    IMAG0170.jpg
    22.6 KB · Views: 633
  • #12


BackEMF said:
Can we verify the battery's voltage?

The voltage of the battery is 9v
 
  • #13


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.
 
  • #14


K^2 said:
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.
 
  • #15


prob solved ...

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

Dave
 
  • #16


K^2 said:
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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