Problem solving for current in parallel resistors

[Kavorka]

Homework Statement

I am able to solve most of the DC circuit problems we're given, but I seem to be missing something conceptually when it comes to finding current that passes through individual resistors when they are in parallel. An example problem would be #10 at this link:

http://www.ahsd.org/science/stroyan/APPhysics/ch17/apc1718wq/APC17WQ4.HTM

Where the correct answer is 1.2 A.

2. Homework Equations

I know that I = V/R, and that in parallel resistors the voltage is divided equally between branches and the voltage in each branch is equal to the original, which is 15 V in this case.

The Attempt at a Solution

What I'm not sure is what resistance to plug in. According to the problem the resistance to use would be 12.5 ohms (R = V/I = 15 V / 1.2 A). I have no idea where this equivalent resistance came from and why it's used for the 2.7 ohm resistor. Any pointers would be appreciated!

 

[gneill]

I know that I = V/R, and that in parallel resistors the voltage is divided equally between branches and the voltage in each branch is equal to the original, which is 15 V in this case.

Ah. Well the problem is that is not true (the 15 V statement). Current passing from A to B passes through the 3.2 and 3.6 Ohm resistors, too, and they will both cause a potential drop. So your parallel paths will not have a 15 V potential across them but something less depending upon the drops on those resistors.

One way to proceed would be to (temporarily) combine the parallel resistances into a single resistance and determine the current in the circuit. Then you can either use that current to find the potential drop across this new resistor, hence the potential difference for the original parallel connection, or you can use the current divider rule.