# Finding the emf of a cell

Gold Member

## Homework Statement

Say we have a circuit where a cell with internal resistance r is connected to a voltmeter with extremely high resistance. This means practically all of the p.d would be across the voltmeter as the ratio of its resistance to the internal resistance is, say, infinitely high.
So the voltmeter reading equals emf of cell?

Is this reasoning correct?
Thanks!

(V1/V2=R1/R2)

## The Attempt at a Solution

(n/a)

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Mister T
Gold Member

## Homework Statement

Say we have a circuit where a cell with internal resistance r is connected to a voltmeter with extremely high resistance. This means practically all of the p.d would be across the voltmeter as the ratio of its resistance to the internal resistance is, say, infinitely high.
So the voltmeter reading equals emf of cell?

Is this reasoning correct?
Thanks!
I can't tell from what you've written. Which part is the question, which part is the answer, and which part is the reasoning.

Gold Member
I can't tell from what you've written. Which part is the question, which part is the answer, and which part is the reasoning.
Well, I am saying that, for the circuit described, the voltmeter reading will equal approximately the emf of the cell. Is this right? Thank you!

Mister T
Gold Member

Gold Member
Well, I thought that if the voltmeter reading was infinitely high:

We know:

emf = terminal voltage - (current * internal resistance)
Hence, if we increase the resistance of the load by putting a voltmeter with infinitely high resistance, current will be zero, so (current * internal resistance) will be zero, so the p.d. across the internal resistance would be zero, so the terminal voltage would equal the emf, and as the voltmeter reads the terminal voltage, the voltmeter reading will equal the emf.

Is that right? Thanks

CWatters