# Find emf and the inner resistor of the battery

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1. Jan 7, 2016

### prishila

1. The problem statement, all variables and given/known data
When the resistor is R1, in the circle there is the intensity I1 and when the resistor is R2, the intensity becomes I2.Find emf and the inner resistance.

2. Relevant equations
I=emf/R+r

3. The attempt at a solution
I1=efm/R1+r
I2=efm/R2+r
efm is equal
I1*(R1+r)=I2(R2+r)
r=(I2R2-I1R1)/(I1-I2)
And how can I find emf?

2. Jan 7, 2016

### BvU

Can I rephrase the problem statement as :
With a resistor R1 connected to a battery, the current is I1.
With a resistor R2 connected to a battery, the current is I2.
Find emf and the internal resistance.
Your equation requires brackets: $I_i = emf / (R_i + r_{internal})$

So you have two equations with two unknowns: $emf$ and $r_{internal}$. Can you write them down and post ?

And: the mentor will move this thread to introductory physics homework.

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3. Jan 20, 2016

### prishila

Here's what I did
efm=I1*(R1+r)=I2*(R2+r)
I1*(R1+r)=I2*(R2+r)
I1/I2=(R2+r)/(R1+r)
I1R1+I1r=R2I2+I2r
r(I1-I2)=R2I2-I1R1
r=(R2I2-I1R1)/(I1-I2)
I replaced R with what I found and it results
efm=I1I2(R2-R1)/(I1-I2)
Did I find efm and r correctly?

Last edited: Jan 20, 2016
4. Jan 20, 2016

### cnh1995

V=IR and not I/R.

5. Jan 20, 2016

### prishila

You're right. i corrected it

6. Jan 20, 2016

### cnh1995

Check the 3rd equation.

7. Jan 20, 2016

### prishila

Corrected

8. Jan 20, 2016

### cnh1995

Looks correct!