• Support PF! Buy your school textbooks, materials and every day products Here!

Smaller and larger resistance in circuit

  • #1

Homework Statement



When resistors 1 and 2 are connected in series, the equivalent resistance is 14.7 Ω. When they are connected in parallel, the equivalent resistance is 2.56 Ω. What are (a) the smaller resistance and (b) the larger resistance of these two resistors?

Homework Equations



1/Req=1/R1+1/R2 parallel
R1+R2+Req series

The Attempt at a Solution


My t.a. said that there would be a substitution and then end up with a quadratic equation. So far i have: R1+R2=14.7ohm
1/R1+1/R2=1/2.56ohm
R1(R2)=2.56(R1+R2) => R1(R2)=37.632ohm
2.56R1+2.56R2-37.632=0 <= my incorrect quadratic which i don't know how to correct.

How can i develop a quadratic from my info? Thanks!

Homework Statement





Homework Equations





The Attempt at a Solution

 

Answers and Replies

  • #2
collinsmark
Homework Helper
Gold Member
2,880
1,181
Hello nothingatall,

The Attempt at a Solution


My t.a. said that there would be a substitution and then end up with a quadratic equation. So far i have: R1+R2=14.7ohm
1/R1+1/R2=1/2.56ohm
R1(R2)=2.56(R1+R2) => R1(R2)=37.632ohm
Okay, I follow you up to here. So far things seem okay. But you've missed one of the substitutions. what you'd like to do is get an equation that is completely in terms of either R1 or R2. (See more below.)

2.56R1+2.56R2-37.632=0 <= my incorrect quadratic which i don't know how to correct.
Nothing particularly bad, but you're failing to make a substitution. You know that

R1+R2 = 14.7 Ohm.

That means you also know that

R2 = 14.7 Ohm - R1.

Using that knowledge, now go back to on of your earlier equations (involving the equivalent parallel resistance) and try to put everything in terms of R1. Then solve for R1. Once you've found R1, you can use the above equation to get R2.
 

Related Threads for: Smaller and larger resistance in circuit

Replies
5
Views
2K
Replies
1
Views
2K
Replies
3
Views
986
  • Last Post
Replies
2
Views
2K
Replies
2
Views
3K
Replies
7
Views
8K
Top