Reading of a voltmeter having resistance R across a resistance

  • #1

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
 

Answers and Replies

  • #2
phinds
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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?
 
  • #3
cnh1995
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V' = Ir/(R+r) .
This looks dimensionally incorrect. The RHS has dimensions of current.
 
  • #4
CWatters
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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.
 

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