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Resistance of the LDR

When the lamp was at a distance of 10cm from the LDR, the reading on the voltmeter was 2.4V.

Calculate the resistance of the LDR at this distance.

I've written down Ohms Law, R = V/I and I know V = 2.4 as stated in the question, but how do I find current from the distance?? Or am I missing something crucial?
Reply 1
Assuming you are attempting the question with this diagram:

diagram.jpg

You are given the EMF of the circuit, so we can calculate the total current of the circuit using I=V/R, with the reading of the voltmeter provided (2.4V) and the total resistance of the circuit (75 ohms). I =2.4/75 = 0.032A. Then we can calculate the difference in voltage from total volts - voltmeter volt to calculate the voltage across LDR (6.0V - 2.4V = 3.6V). Then use ohms law again to calculate the resistance of LDR (R = V/I = 3.6/0.032 = 112.5ohms)

The distance in this question is not very relevant, and is probably there to throw you off. Always refer to the previous part of the question if you feel as if you are missing something crucial, a voltage and a distance can get you no where near to a resistance alone! Hope this helps.
Original post by .Umar.
Assuming you are attempting the question with this diagram:

diagram.jpg

You are given the EMF of the circuit, so we can calculate the total current of the circuit using I=V/R, with the reading of the voltmeter provided (2.4V) and the total resistance of the circuit (75 ohms). I =2.4/75 = 0.032A. Then we can calculate the difference in voltage from total volts - voltmeter volt to calculate the voltage across LDR (6.0V - 2.4V = 3.6V). Then use ohms law again to calculate the resistance of LDR (R = V/I = 3.6/0.032 = 112.5ohms)

The distance in this question is not very relevant, and is probably there to throw you off. Always refer to the previous part of the question if you feel as if you are missing something crucial, a voltage and a distance can get you no where near to a resistance alone! Hope this helps.

Ah damn it, never knew the circuit diagram was relevant here! Cheers, I've got it now
(edited 2 years ago)
How can the total resistance of the circuit be 75ohms but the ldr had a resistance of 112,5 ohms?
I think it was a mistype, 75ohms is the resistance across the resistor

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