# Homework Help: Req of this series/parallel circuit?

1. Oct 27, 2012

### letsgo

1. The problem statement, all variables and given/known data
What is the Req of this circuit?
The answer should be 150 ohms, but I can't get that.

2. Relevant equations
Req = R1 + R2
1/Req = 1/R1 + 1/R2

3. The attempt at a solution
If I treat R1 and R3 as series, I get R13= 200 ohms.
Then if I treat R13 and R2 and parallel I get R123 = 66.7 ohms.
Then if I treat R123 and R4 as series, I get Req = 266.7 ohms.

Also, since the current is 0 (open switch), then the potential difference between points P and Q would also be 0, right?

Last edited: Oct 27, 2012
2. Oct 27, 2012

### CWatters

A hint..

There appears to be a wire (grey) connected from one end of R3 to the other end. What does that mean for the voltage across R3 or R3 in general?

3. Oct 27, 2012

### letsgo

No clue. It's the same as the source's voltage?

Also, did you manage to get Req?

4. Oct 28, 2012

### CWatters

The grey wire has a very low resistance, lets assume it's zero Ohms. You therefore have R3 in parallel with a zero Ohm resistor..

For that bit of the circuit..

I/R = 1/R3 + 1/zero

but 1/zero = ∞

so R3 in parallel with zero Ohms = zero Ohms.

The circuit reduces to..

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5. Oct 28, 2012

### CWatters

I'll let you have another go at calculating Req.

6. Oct 30, 2012

### letsgo

Smart, thanks.