How to Determine the EMF of a Battery Using Two Voltmeters?

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Homework Help Overview

The discussion revolves around measuring the electromotive force (emf) of a battery using two voltmeters, which yield different readings when connected individually and together. The subject area includes concepts of electrical circuits, voltmeters, and the behavior of ideal versus real components.

Discussion Character

  • Exploratory, Assumption checking, Conceptual clarification

Approaches and Questions Raised

  • Participants explore the differences between ideal and real voltmeters and batteries, questioning how these differences affect the readings. There are attempts to set up equations based on the voltmeter readings and the resistances involved, with some participants expressing uncertainty about the relationships between the currents and resistances.

Discussion Status

The discussion is ongoing, with participants offering various equations and relationships to consider. Some guidance has been provided regarding naming unknowns and writing simultaneous equations, but there is no explicit consensus on the correct approach or resolution of the problem.

Contextual Notes

Participants are grappling with the implications of different resistances in the circuit and how they affect the measurements taken by the voltmeters. There is mention of the battery's internal resistance, which adds complexity to the problem.

palkia
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Homework Statement


We have to measure emf of a battery. We have to voltmeters. Each of them when connected alone across the battery, they read V1 = 0.9V and V2 = 0.6V and when both of them are connected to the battery simultaneously, they both read V3 = 0.45V . What is the emf of battery

Homework Equations


E-ir=V

The Attempt at a Solution


I don't know how to proceed...I thought the voltage across the battery will be it's emf when connceted without a load but it is showing different readings for the voltmeter
 
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Find out the difference between an "ideal" voltmeter and a "real" voltmeter. Likewise ideal and real batteries.
 
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ideal voltmeter-Which doesn't influence the circuit

Real voltmeter-Which has resistance that is not infinitely largeSame for battery
 
okay so it makes sense that it shows different readings now...but still How can I proceed if they have different resistance
 
Ok so you will need to decide how many unknowns there are and give them names (eg R1 is the resistance of Voltmeter 1).

Then write some simultaneous equations and if you have enough then you can solve them.
 
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V1=E- I1R1
V2=E-I2R2
V3=E-I3R3

It's getting bigger to solve as I tried
 
In the last case,when we connect the voltmeters ,my book states that I3=I1+I2 but I am not sure How it came
 
palkia said:
V1=E- I1R1
V2=E-I2R2
V3=E-I3R3

I don't think those are correct. What about the battery resistance? With just one voltmeter connected the circuit looks like this...

Battery Circuit.jpg


So the equations would be

V1 = EMF - (I1*RB)
you also know
I1 = V1/R1

V2 = EMF - (I2*RB)
you also know
I2 = V2/R2

With both meters connected...

V3 = EMF - (I3*RB)
you also know
I3 = V3/R3
and
R3 = R1 in parallel with R2
 

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  • Battery Circuit.jpg
    Battery Circuit.jpg
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