# Homework Help: Very simple series circuit question

1. Jun 3, 2009

### crazybandit

1. The problem statement, all variables and given/known data

http://img2.imageshack.us/img2/6536/physics1.png [Broken]

2. Relevant equations

I = V/R, R = R1+R2+R3+...

3. The attempt at a solution

2R+R=3R

V/3R

this is an SAT II physics practice question, i just want to grasp this concept so im asking for an explanation, i.e. why this is a series circuit (or not?)

oh and how do i know to treat the section with the 1/2 R's as a parallel circuit and not the entire right side? dont they both have parallel resistors? how do i differentiate for circuits in general?

Last edited by a moderator: May 4, 2017
2. Jun 3, 2009

### rock.freak667

Because if S1 is left open then only a closed series circuit with V1R,S2 and 2R is formed.

3. Jun 3, 2009

### crazybandit

so that means i add the 2R and the R to get 3R, hence V/3R, and that the right side is a series circuit? why is it a series circuit if they are parallel to each other? yet the 1/2 R's on the left side are treated as parallel?

4. Jun 3, 2009

### rock.freak667

It is only because S1 is left open that you can only consider the right side of the circuit only.

Sure the two R/2 resistors are in parallel but since S1 is left open, no current flows through it, so you don't need to consider it.

Only consider where the current is flowing through. Which happens to be through R and 2R since S2 is closed.

5. Jun 3, 2009

### crazybandit

i know this, but never mind the question, how do i differentiate between a series and a parallel circuit? going back to the question, all the resistors are "parallel" to each other so how can i tell?

6. Jun 3, 2009

### rock.freak667

If the same current passes through two resistors, then they are in series. If the current splits, then they are in parallel. I believe that is how it works, though I am not the best at circuit questions.

7. Jun 3, 2009

### crazybandit

oh ok, sidenote have you taken the SAT II physics?

8. Jun 3, 2009

### rock.freak667

Nope. Only GCE A level physics a few years ago.