resistance calculation in parallel help

shane_nd

Joined Nov 20, 2006
4
Hi I Have A Question On An Assignment For Parallel Resistors
Its As Follows...

There Is Two Resistors In Parallel.. R1 And R2. (r2 Is Unknown Value)
R1=3.3kohm
R=1.42kohm.

Can Somebody Help I Need The Formula For Finding The Unknown Value
As I Need To Show My Workings Out In The Assignment.

All Help Will Be Very Much Appreciated
Cheers Shane

EEMajor

Joined Aug 9, 2006
67
Total resistance in parallel is: Rtotal = 1/((1/R1)+(1/R2)) (and so on if you had more then two resistors...) So solve this equation for R2. You know Rtotal and you know R1.

OR you could use this one: (NOTE: This equation ONLY WORKS FOR TWO RESISTORS...If you have more than two resistors in parallel this WILL NOT WORK)

Rtotal = (R1 * R2) / (R1 + R2)
Again, just solve for R2.

Do your best to solve these equations, if you need help just post what you have done, then we will help point you in the right direction if need be. We do not simply do homework for you here, but we will be happy to give you a gentle nudge here and there.

GOOD LUCK!

shane_nd

Joined Nov 20, 2006
4
i dont quite follow.... havin a bit of a block..
i know the values of rt and r1
but need to find r2
i know that r1*r2/ r1+r2=rt
but i dont know the way to get answer to =r2

EEMajor

Joined Aug 9, 2006
67
Think of it as fractions. Here is another way to look at the formula:

(1/R1) + (1/R2) = (1/Rtotal)

Think of it as A + B = C Where A = 1/R1 B = 1/R2 and C = 1/Rtotal.

Solve the A + B = C for B. (In other words, make the equation say B = ?
When you have it solved for B = something, then plug in what A, B and C equal.

Does any of that make sense? I hope so. Glad to help!

EEMajor

Joined Aug 9, 2006
67
Another hint: Your final answer is 2492 ohms....So if you solve one of the equations for R2, check yourself by plugging in your numbers. If you get about 2492 ohms, you got it!

ALSO: REMEMBER when adding fractions, you need a COMMON DENOMINATOR!