# Potentiometer and EMFs with internal resistance

## Homework Statement

In a potentiometer 99cm of wire of resistance 99ohms is connected with a battery of 50V and 1ohm internal resistance.Find emf of battery giving zero deflection for a length of 13cm.

## Homework Equations

Sadly,the formula I know is only for ideal emfs,which states that:
If two batteries with emf E1 and E2 obtain null point at L1 and L2 lengths of the potentiometer wire respectively,then : E1/E2=L1/L2

## The Attempt at a Solution

The question doesn't mention connecting 2 emfs,just 1 case,1setup,hence the formula I know can't be used here.

Hence,my problem.

A potentiometer is supposed to compare emfs based on where their null points are obtained,right?Then,how am I supposed to find the emf with just 1 setup(that too batteries with internal resistance)?
(Maybe I misunderstood the language of the question.Any other interpretations would be useful)

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scottdave
Homework Helper
It says:

In a potentiometer 99cm of wire of resistance 99ohms is connected with a battery of 50V and 1ohm internal resistance. Find emf of battery giving zero deflection for a length of 13cm.
So you have a known battery of 50 volts, and a battery of unknown EMF. The question is, do we assume the unknown battery also has 1 ohm internal resistance?
That would be a decent assumption to go on (and should be stated in your solution steps.
Since 99cm is 99 ohms, you have 1 ohm/cm so you could just add 1 cm of virtual wire to simulate the internal resistance. You could do this for both sides so that the potentiometer is now 101 cm long, and the setting is at 14 cm.

The question is, do we assume the unknown battery also has 1 ohm internal resistance?
I doubt it.The unknown emf needs to be ideal.

cnh1995
Homework Helper
Gold Member
A potentiometer is supposed to compare emfs based on where their null points are obtained,right?Then,how am I supposed to find the emf with just 1 setup(that too batteries with internal resistance)?
(Maybe I misunderstood the language of the question.Any other interpretations would be useful
I think there are two batteries. One with emf of 50V and internal resistance 1 ohm and the other with unknown emf.
You are supposed to find the emf of the unknown battery for which the null point is obtained at 13cm.

• scottdave
scottdave