Circuit with 2 batteries is confusing me

1. Nov 7, 2007

1. The problem statement, all variables and given/known data
The circuit in the figure is composed of two batteries (E1 = 6 V and E2 = 4 V) and three resistors (R1 = 4 , R2 = 4 , R3 = 5 ) as shown.

(a) What is the current I3 flowing through the batteries?

I3 = .286A (no problem here)

(b) What is the voltage Vb - Va between the points a and b in the circuit?

Vb - Va = V

So i combined the parallel Rs to get R1 + R2 = 2ohms
So voltage in is 2V right?
Then voltage on other side of Req should be .579V right? (because I=.286A)
so then 2V - .579V = 1.43 = wrong answer

thanks

2. Relevant equations

3. The attempt at a solution
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

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2. Nov 7, 2007

Staff: Mentor

Good.
How did you deduce this? You have the resistance and the current.
Do you mean the voltage across R3? What's the current through R3?

3. Nov 7, 2007

because picture says current is going cw, so 1st batt is -4V and 2nd batt is 6V = 2V
so 2V enters Req.
Req = 2ohms
and total current in the circuit is .286A
so V=IR = .286 X 2ohm = .579V
2V - .579V = 1.421V

4. Nov 8, 2007

Staff: Mentor

Much better. (Unless the reference point is very clear, it's always a good idea to specify the voltage difference between two points or across something.)

5. Nov 8, 2007

1.421V is wrong answer though, so can u please tell me what im doing wrong?
thanks

6. Nov 9, 2007

Staff: Mentor

You did the work correctly, but you're getting confused about which voltage drop is the answer to the question.
This is the equivalent resistance between points A and B.
This is the total current going through both R3 and between points A and B.
That's the voltage drop across A and B. That's the answer to the question.
That's the voltage across R3.