# Homework Help: Find external resistance

1. Oct 31, 2011

### auk411

1. The problem statement, all variables and given/known data
Two identical batteries of emf ε = 12 V and internal resistance r = 0.200 Ω are to be connected to an external resistance R, either in parallel or in series. If R = 2.00r Ω, what is the current i in the external resistance in the (a) parallel and (b) in the series arrangements?

2. Relevant equations

3. The attempt at a solution

By symmetry, when the two batteries are connected in parallel the current i going
through either one is the same. So from ε = ir + (2i)R with r = 0.200 Ω and R = 2.00r, we
get ... etc.

I have no idea what the above means. What is going on?

While I appreciate the help, can you please give more than a sentence or two. I have found shortness = unhelpful response (not necessarily true, but there does seem to be a correlation).

2. Nov 1, 2011

### SammyS

Staff Emeritus
Sorry, but this will be rather short.

Where did you get the following information, if you don't understand it? (I think that's what you indicated.)
"By symmetry, when the two batteries are connected in parallel the current i going
through either one is the same. So from ε = ir + (2i)R with r = 0.200 Ω and R = 2.00r, we
get ... etc."​
It looks like you could plug-in the values for r, R, and ε, to find the current, i .

3. Nov 1, 2011

### Staff: Mentor

The information would appear to be a quote from a solution, perhaps from a textbook example problem.

It might help if auk411 were to draw the circuit in question and label the currents.