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## Homework Statement

http://session.masteringphysics.com/problemAsset/1003291/40/15092_a.jpg

In previous parts of this problem, I solved that the current travels counterclockwise, and that I_A = \frac{2{\cal{E}}}{R_{2}+2R_{1}} .

## Homework Equations

V= IR

R[series,total] = (1/R1 + 1/R2 + 1/R3 ... ) ^-1

KCL: sum of currents at a junction point equals zero

KVL: sum of voltages at any point equals zero

## The Attempt at a Solution

I'm not entirely sure how to tell if I'm supposed to be using Kirchoff's Laws, or how for that matter. I think I can use KCL, so that the current that passes through R2 is the sum of the currents passing through the two "R1" resistors, which I will identify as R1a and R1b. Because R1a and R1b are in parallel, they should have the same voltage yet different currents. The voltage that goes through R1a and R1b is the sum of the two EMFs in their parallel connection: 2ε. (?)

The current that goes through R2 is I=V/R. The total voltage is 2ε. The total resistance is: (1/R1 + 1/R1)^-1 + R2 = R1/2 + R2.

So: I = 2ε / (R1/2 + R2)

This can be simplified: I = 4ε / (R1 + 2R2).

The answer in the back of the book: I = 2ε / (R1 + 2R2).

So my answer is twice the correct current. I think this has to do with the EMF I calculated above, marked with a (?). In addition to the mistake I made, it would be nice if someone could also clarify how I can tell by looking at a problem if I need to use KVL, KCL... I'm not comfortable with these topics, so I'm not sure if I even applied KCL to this one.

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