LDR circuit electricity question

[Zeynaz]

Homework Statement

Switch S is closed and light falls on the LDR. The variable resistor R3 is set such that the voltmeter reads 0V. The resistance of R3 then is 720 ohms.

a) show that the resistance of the LDR is 720 ohms.

Relevant Equations

V=IR

So far what i figured out is that R2 and R3 are in parallel, R1 and LDR in parallel. And these two parallel circuits are parallel to each other ( tell me if I am wrong)

The voltmeter has to read 0 but I am not sure how i can achieve that. Is there a rule where when the resistance of each side is equal they cancel each other?

For the lower parallel half i found the total R by using resistance for parallel circuits. This gives me R-lower=300 ohms.

Also voltage of R3 and R2 has to be equal to each other because they are in parallel.

So how should I proceed?

 

[scottdave]

If do a search for Bridge Circuit, you can find several resources. Here is one that goes through the method. https://courses.engr.illinois.edu/ece110/sp2019/content/courseNotes/files/?nodeMethod

 

[Zeynaz]

Alright! thanks

 

[scottdave]

You may see it called Wheatstone Bridge.