# Conductance and resistance in parallel circuits

Discussion in 'Homework Help' started by msn56, Apr 9, 2010.

1. ### msn56 Thread Starter Member

Apr 9, 2010
15
0
HI; First off let me say this is one of the best sites I have ever seen - I am just amazed at how everyone helps each other out. I am 52 and dont know anything about electronics but have always wanted to learn. I found some great lectures on you tube and that coupled with your site is giving me a great introduction

Heres a simple question that just isnt clicking with me. Conductance in a parallel series just adds up . OK so picture three resistors in parrallel . Give them values of 4, 2 and 3 ohms.

since G = 1/R then G (total) = 1/4 + 1/2 + 1/3 = 1.08

R (Total ) = 4*2*3/ 4 + 2 +3 = 24/9 = 2.66

then since G = 1/R --> 1/2.66 = 0.375 which is not 1.08

what am I doing wrong?

Mike

2. ### Supervisor Active Member

Jan 17, 2009
32
0
If they are in parallel, then you have the right formula for the resistance:
1/R = 1/R1 + 1/R2 + ...
So...
1 / R = 1/4 + 1/2 + 1/3 = 13/12
That's 1/R , though, so to get your total R, you have to take the reciprocal = 12 / 13 or around 0.923.

EDIT: I think you may have just not added right:
1/4 + 1/2 + 1/3 = 3/12 + 6/12 + 4/12 = 13/12

3. ### The Electrician AAC Fanatic!

Oct 9, 2007
2,703
493
Right here is where you've gone wrong. The well-known formula for parallel resistors, the product over the sum formula, only works for 2 resistors. It doesn't work for 3 or more.

There is a formula for 3 resistors; it is:

Rtotal = R1*R2*R3/(R1*R2+R1*R3+R2*R3)

For more resistors, the formula begins to become even more complicated. Then it's best to use the "reciprocal of the sum of the reciprocals" formula.

msn56 likes this.
4. ### Supervisor Active Member

Jan 17, 2009
32
0
According to this - http://www.physics.uoguelph.ca/tutorials/ohm/Q.ohm.intro.parallel.html
Of course, I need to actually see your circuit to see how the resistors are lined up because all I'm assuming they are like those in Figure 2.

Post a picture or draw one on Paint of the circuit if you can.

EDIT: Oh, it's not you that's the OP.
Post is to OP - please show us a picture of the circuit so we can better assist.

5. ### JoeJester AAC Fanatic!

Apr 26, 2005
4,070
1,774
Paralle is parallel. There is no need to "see" how they are wired up.

There are three types of circuits ... series, parallel, and series-parallel.

The third one, series-parallel, definately requires a drawing as there are many possibilities for that one, as opposed to the simple series or simple parallel circuits.

I use the "reciprocal of the sum of the reciprocals" formula for all parallel circuits. But for those who choose the other method, that's fine ... for them ... I have my preferences, they have theirs.

6. ### msn56 Thread Starter Member

Apr 9, 2010
15
0
Hi Everyone: THanks for the great response. A couple of general questions. Will this site automatically notify me when someone posts to the thread? If so how do I do that?

I was not aware that the parallel resistance formula only worked for two resistors! SO that clearly explains it - thanks

also I tried to put a pic in and it wont let me paste any reason why?

Mike  7. ### msn56 Thread Starter Member

Apr 9, 2010
15
0
HI "The Electrician"

by the reciprocal of the sum of the reciprocals you mean 1/R(total) = 1/R1 + 1/R2 + 1/R3 ...... ?

Thus for parallel circuits forget the two resistor equation (R1*R2) / (R1 + R2) as it only works for two and just remember that reciprocal equation. Then if you want R(total) just do the inverse?

THanks for the answers - really a great group of people!!