# Emf of a battery

1. Mar 31, 2011

### pb23me

1. The problem statement, all variables and given/known data
determine the terminal voltage in each battery.
Emf1=18V
r1=1 ohm
EMF2=12V
r2=2 ohm
R=6.6 ohms
also the batterys currents are opposing each other.
2. Relevant equations Vab=emf-Ir

3. The attempt at a solution i used kirchnoffs rule to find the current I , 18-I6.6-12-I2-I=0 and got I= .625 then i thought i would just use Vab=emf-Ir to find the terminal voltage of each battery however im not getting the correct answer... could someone please explain this to me i have a test tomorrow morning and i really need to figure this out! the answers i got were 17.375 for the 18V batt and 10.75 for the 12V batt

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2. Mar 31, 2011

### Staff: Mentor

Watch out for the current direction and the thus the polarity of the voltage drop it creates on the internal resistors. If current is being forced into a battery's positive terminal it's possible for it to have a higher terminal voltage than its internal EMF.

3. Mar 31, 2011

### pb23me

so on the larger voltage batt would i use equation Vab=Emf-Ir = 18-(.625*1) and on the batt with voltage being forced across it use Vab=Emf+... idk

4. Mar 31, 2011

### pb23me

what are the equations you would use in this situation?

5. Mar 31, 2011

### Staff: Mentor

Yes. It can be helpful to sketch in your current and the polarities of the voltage drops across the resistors. You did fine writing the KVL loop equation.

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