# Find the resistance between 2 points

#### UndoubtedCrow

Joined Jun 2, 2020
6
Hi,
I am working on the homework from "https://www.allaboutcircuits.com/worksheets/series-parallel-dc-circuits/" Question number 5. I thought I had a decent understanding of resistance, but since I can't figure these out, maybe not. The question asks me to find the resistance between 2 points. I assumed that the question wanted me to add the resistances of the shortest path from A to B, but that's not the answer provided. Neither is the equivalent resistance of the network. This problem has me stumped. Did I miss something?

#### UndoubtedCrow

Joined Jun 2, 2020
6
hi UC,
Welcome to AAC.
Which one of the questions from #5 are you referring tto.?
E
View attachment 208747
Actually, all of them, but I would be happy if someone just explained the general idea.

#### ericgibbs

Joined Jan 29, 2010
10,018
hi,
OK,
Do you know the Formula for Resistors in Parallel.?
E

#### UndoubtedCrow

Joined Jun 2, 2020
6
hi,
OK,
Do you know the Formula for Resistors in Parallel.?
E
Rt = 1/R1 + 1/R2 + ...

#### ericgibbs

Joined Jan 29, 2010
10,018
[QUOTE]Rt = 1/R1 + 1/R2 + ... [/QUOTE]
hi,
Almost... E
1/Rtotal = 1/R1 + 1/R2

Update:

https://www.electronics-tutorials.ws/resistor/res_4.html

#### UndoubtedCrow

Joined Jun 2, 2020
6

#### ericgibbs

Joined Jan 29, 2010
10,018
hi,
OK,
Post the calculations for the circuit from Fig 5, which is giving you a problem.
E

#### UndoubtedCrow

Joined Jun 2, 2020
6
Combining the two resistors in parallel first,
R = (1/2.2kohm +1/ 2.2kohm )^-1 = 1.1 k ohms
Then adding the resistors in series, Rt = 1.1 k ohm + 2(2.2 k ohms) = 5.5 k ohms

#### WBahn

Joined Mar 31, 2012
25,751
Combining the two resistors in parallel first,
R = (1/2.2kohm +1/ 2.2kohm )^-1 = 1.1 k ohms
Then adding the resistors in series, Rt = 1.1 k ohm + 2(2.2 k ohms) = 5.5 k ohms
In order for the 1.1 kΩ resistor (the parallel equivalent of the two 2.2 kΩ resistors) to be in series with the 4.4 kΩ resistor (the series equivalent of the two 2.2 kΩ resistors), whatever current that flows through the 1.1 kΩ resistor MUST also flow throw the 4.4 kΩ resistor? Is that the situation?

Two resistors are in series ONLY if whatever current flows through one must flow through the other.

The resistors are in parallel ONLY if whatever voltage appears across one must appear across the other.

#### WBahn

Joined Mar 31, 2012
25,751
I assumed that the question wanted me to add the resistances of the shortest path from A to B, but that's not the answer provided.
It's important that you realize why this is completely incorrect reasoning. If I have multiple paths between A and B, current will flow in ALL of them and the equivalent resistance is the ratio of the voltage across ALL of them to the sum of the currents flowing in ALL of them.

Think of the simple case -- two resistors in parallel that are slightly different. Is the total resistance just the value of the smaller of the two since it is the "shortest path", or is it the parallel combination of the two?

Think of a bucket of water with a bunch of different sized holes in the bottom. If you take another bucket the same size and put a hole in the bottom the same size as the biggest hole in the original one, would you expect both buckets to drain water at the same rate? No, because the original bucket also has all those other holes that are also draining water. To a first approximation, you need to put a hole in the second bucket whose area is equal to the sum of the areas of ALL the holes in the original bucket.

Neither is the equivalent resistance of the network. This problem has me stumped. Did I miss something?
How are you determining the equivalent resistance of the network? It needs to be the equivalent resistance of the network AS SEEN between points A and B.

#### UndoubtedCrow

Joined Jun 2, 2020
6
hi,
The 2 off 2k2 in parallel is 1k1 as you say,
also
2 off 2k2 in series is 4k4,

But that 4k4 is in parallel with the 1k1

So what is that Value.???

E
View attachment 208757
Okay, so the equivalent resistance is 880. I think I get it now.