Reading of a voltmeter having resistance R across a resistance

[Shivang kohlii]

Homework Statement

The potential difference across resistance R carrying current I is V = IR . Now if the potential difference is measured via voltmeter of resistance r , the reading on voltmeter is V' . Prove that V' = Ir/(R+r) . For what value of r does the voltmeter measure true value

Homework Equations

V=ir

The Attempt at a Solution

Initially , V = IR
FINALLY, Rnet = R × r / ( R + r)
V' = I' Rnet , where I' is telhe new current in circuit..
I don't know how to proceed

 

[phinds]

Prove that V' = Ir/(R+r)

That would be hard to prove since as a general statement, it's not true. You need to draw a circuit diagram and show the resistors. Are they in series? Are they in parallel? Are they even in the same part of the circuit?

 

[cnh1995]

V' = Ir/(R+r) .

This looks dimensionally incorrect. The RHS has dimensions of current.

 

[CWatters]

Looks like a typo in the solution. Missing R?

I believe you are also meant to assume the total current flowing stays the same. So I'=I.