Finding resistor values in parallel-series circuit

[cseet]

Hi there,

I've another problem... as you can see I'm new to all these... pls forgive!

senario:

point A to B... across A and B there're 2 resistors R(1) and R(2) connected in parallel and R(3) in series to (R(1) and R(2))...

Questions:
1) assume R(1)=5ohm, R(3)=2ohm and find R(2) that will make R(AB)=R(3)...

I thought there's a mistake in the question given that R(AB)= cannot be 2ohm...? correctly me if I', wrong...

2) assume R(2)=4ohm, R(3)=2ohm and find the value of R(1) that will make R(AB)=R(1)...

pls advise how do I go about it.

thanks heaps!
Cseet

 

[turin]

cseet,
I agree with your assessment of # 1) based on what's given. For # 2), just use your series parallel stuff. Be careful with your order of combination.

Originally posted by cseet
pls advise how do I go about it.

Step 1: post in the HW forum (for these kinds of q's). It's way the hell down at the bottom, though, so it is understandable that you haven't noticed it.

 

[Cliff_J]

nothing wrong with question #1, and yes, homework section seems appropriate.

Cliff

 

[Chen]

$$R_{AB} = R_1||R_2 + R_3 = \frac{R_1R_2}{R_1 + R_2} + R_3$$
Both questions require you to solve one equation with one unknown.

 

[turin]

After reading the two subsequent responses, I would like to change mine for #1). We were probably both thinking too physically, not idealistically enough (that's a hint: what resistance value cannot be obtained for an actual (non-superconducting) resistor?).