# Calculate Missing Resistors in a parallel circuit when given the Rtotal

#### DrewStupid

Joined Nov 28, 2018
64
Sorry ! Don't know what happened now !

#### rdb1

Joined Feb 6, 2019
54
Right ok I think I’ve got that. If for example you are given two resistors and the total resistance would it just be the same calculation?

For example R1 is 48 R2 is 80 and Rt is 15.

#### djsfantasi

Joined Apr 11, 2010
8,075
@rdb1 You have been given very specific information. You’re very lucky. This sub forum is titled “Homework Help”. It’s not titled “Homework Done for You”.

Part of that information was the equation to calculate the resistor values for two OR MORE resistors in parallel. Re-read post #19.

#### LesJones

Joined Jan 8, 2017
3,655
Hi DrewStupid,
Your answer of both resistors having a value of 160 ohms is only one of an infinite number of valid answers . (It has not been said that the two resistors have to be the same value.) values of 320 ohms and 106.67 ohms would be another valid answer. Another would be one was 80 ohms and the other infinity.

Les

#### rdb1

Joined Feb 6, 2019
54
Thanks for the help. I think I’ve got the hang of this. Been a while since I’ve done some maths.

#### WBahn

Joined Mar 31, 2012
26,398
That’s great, so that would work then as long as you know one resistor and the total resistance you can use the calculation to find the other resistors. I’m not sure why that’s not covered in the text book I have. But thank you I’ve made a note of that formula.
A big part of your problem is that the approach you are taking is to just memorize formulas instead of understanding what those formulas mean, where they came from, or how to manipulate them to solve types of problems you haven't seen before. This thread is a shining example of that. You were asked by a few folks to post some attempt so that we could help you along the path of discovering how to arrive at what you want starting from what you know. But you wouldn't do that. Instead, you just waited and hoped that eventually someone would just spoon feed you an answer so that you could "make a note" of their final formula. Not a recipe for learning or long-term success.

#### rdb1

Joined Feb 6, 2019
54
A big part of your problem is that the approach you are taking is to just memorize formulas instead of understanding what those formulas mean, where they came from, or how to manipulate them to solve types of problems you haven't seen before. This thread is a shining example of that. You were asked by a few folks to post some attempt so that we could help you along the path of discovering how to arrive at what you want starting from what you know. But you wouldn't do that. Instead, you just waited and hoped that eventually someone would just spoon feed you an answer so that you could "make a note" of their final formula. Not a recipe for learning or long-term success.
I see your point, but I’ve never come across the formula that was presented from other users. So I did make a note of it. I then on my own worked step by step to then understand how they had done it, and got the numbers. I then worked out a number of problems on my own.

#### rdb1

Joined Feb 6, 2019
54
A big part of your problem is that the approach you are taking is to just memorize formulas instead of understanding what those formulas mean, where they came from, or how to manipulate them to solve types of problems you haven't seen before. This thread is a shining example of that. You were asked by a few folks to post some attempt so that we could help you along the path of discovering how to arrive at what you want starting from what you know. But you wouldn't do that. Instead, you just waited and hoped that eventually someone would just spoon feed you an answer so that you could "make a note" of their final formula. Not a recipe for learning or long-term success.
If you want to remove the thread if it makes you feel better. Even when you do ohms law your given a calculation to use. It’s the same with this. I wasn’t expecting the answer rather an explication of how you would work or out. I then took note of how it was done then broke the equasion down and then I was able to do a similar question. Sometimes it takes someone to say have you tried this or that. As it’s something I’ve not come across, so I was greatful for the help and advice.

#### crutschow

Joined Mar 14, 2008
28,542
The previous calculations are all correct, but tend to be more complicated than required.
The only formula you need to remember for parallel resistors is 1/Rtot = 1/R1 +1/R2 + 1/R3 + --- + 1/Rx.
To solve for the unknown you just do simple algebra and transfer the unknown to the left of the equation.
Thus, if you know the parallel value of R1 and R2, and also the value R1, the equation would be 1/R2 = 1/Rtot - 1/R1.

So for your example it would be 1/R2 = 1/48 - 1/120 = 120/5760 - 48/5760 = 72/5760 (where 5760 is the common denominator, 48*120).
Inverting that to get R2 gives 5760/72 = 80Ω.

Edit: Just realized this is basically a rehash of LesJones post #19.

Last edited:

#### DrewStupid

Joined Nov 28, 2018
64
Hi DrewStupid,
Your answer of both resistors having a value of 160 ohms is only one of an infinite number of valid answers . (It has not been said that the two resistors have to be the same value.) values of 320 ohms and 106.67 ohms would be another valid answer. Another would be one was 80 ohms and the other infinity.

Les
Hi DrewStupid,
Your answer of both resistors having a value of 160 ohms is only one of an infinite number of valid answers . (It has not been said that the two resistors have to be the same value.) values of 320 ohms and 106.67 ohms would be another valid answer. Another would be one was 80 ohms and the other infinity.

Les
Look at my user Name ! Only participating.
RSA.

#### WBahn

Joined Mar 31, 2012
26,398
I see your point, but I’ve never come across the formula that was presented from other users. So I did make a note of it. I then on my own worked step by step to then understand how they had done it, and got the numbers. I then worked out a number of problems on my own.
I think the thing that you're missing is that it's not a matter of whether you've ever seen a particular formula before, but whether you have learned the concepts well enough to be able to take what you HAVE seen before and DERIVE a formula that you've never seen before. If, instead, you have to wait for someone else to show you a new formula, then you are not operating at the level of comprehension that you should be (in a case like this -- certainly not always the case).

Presumably you know how to calculate the equivalent resistance of two resistors in parallel given the values of the two resistors. It shouldn't be a completely new problem to then be asked how to calculate the value of one of the resistors given the equivalent resistors and the value of the other resistor. If it is, then you've either missed a major conceptual understanding or have particularly weak algebra skills (both are pretty common). So THAT's the area(s) you really need to try to bolster up, and just adding a new formula to you sheet of formulas is not going to help you get there.

#### DrewStupid

Joined Nov 28, 2018
64
Look at my user Name ! Only participating.
RSA.
Why does this interest me. If you wire a star/delta starter your ohm readings should corrulate between 3-phases. L1 L2 L3 before energizing .

#### rdb1

Joined Feb 6, 2019
54
I think the thing that you're missing is that it's not a matter of whether you've ever seen a particular formula before, but whether you have learned the concepts well enough to be able to take what you HAVE seen before and DERIVE a formula that you've never seen before. If, instead, you have to wait for someone else to show you a new formula, then you are not operating at the level of comprehension that you should be (in a case like this -- certainly not always the case).

Presumably you know how to calculate the equivalent resistance of two resistors in parallel given the values of the two resistors. It shouldn't be a completely new problem to then be asked how to calculate the value of one of the resistors given the equivalent resistors and the value of the other resistor. If it is, then you've either missed a major conceptual understanding or have particularly weak algebra skills (both are pretty common). So THAT's the area(s) you really need to try to bolster up, and just adding a new formula to you sheet of formulas is not going to help you get there.
I may have missed a major concept and I will be the first to admit my algebra skills are not the best. I did attempt the question before posting it on here. As I said you can remove the thread of you want.

#### DrewStupid

Joined Nov 28, 2018
64
I think the thing that you're missing is that it's not a matter of whether you've ever seen a particular formula before, but whether you have learned the concepts well enough to be able to take what you HAVE seen before and DERIVE a formula that you've never seen before. If, instead, you have to wait for someone else to show you a new formula, then you are not operating at the level of comprehension that you should be (in a case like this -- certainly not always the case).

Presumably you know how to calculate the equivalent resistance of two resistors in parallel given the values of the two resistors. It shouldn't be a completely new problem to then be asked how to calculate the value of one of the resistors given the equivalent resistors and the value of the other resistor. If it is, then you've either missed a major conceptual understanding or have particularly weak algebra skills (both are pretty common). So THAT's the area(s) you really need to try to bolster up, and just adding a new formula to you sheet of formulas is not going to help you get there.
It all bores down to basic education. .....remember there is a contenent called Africa ! I am just trying to help.

#### DrewStupid

Joined Nov 28, 2018
64
I think the thing that you're missing is that it's not a matter of whether you've ever seen a particular formula before, but whether you have learned the concepts well enough to be able to take what you HAVE seen before and DERIVE a formula that you've never seen before. If, instead, you have to wait for someone else to show you a new formula, then you are not operating at the level of comprehension that you should be (in a case like this -- certainly not always the case).

Presumably you know how to calculate the equivalent resistance of two resistors in parallel given the values of the two resistors. It shouldn't be a completely new problem to then be asked how to calculate the value of one of the resistors given the equivalent resistors and the value of the other resistor. If it is, then you've either missed a major conceptual understanding or have particularly weak algebra skills (both are pretty common). So THAT's the area(s) you really need to try to bolster up, and just adding a new formula to you sheet of formulas is not going to help you get there.
On the lighter side. Is this forum only for rocet scientests ?

#### DrewStupid

Joined Nov 28, 2018
64
On the lighter side. Is this forum only for rocet scientests ?
(rocke
@rdb1 You have been given very specific information. You’re very lucky. This sub forum is titled “Homework Help”. It’s not titled “Homework Done for You”.

Part of that information was the equation to calculate the resistor values for two OR MORE resistors in parallel. Re-read post #19.

You must have been a strict parent or still are !

#### WBahn

Joined Mar 31, 2012
26,398
On the lighter side. Is this forum only for rocet scientests ?
Who said it was only for rocet scientests?

If it was only for rocet scientests, they'd have been told to go figure it out on their own. That's not what they were told. They were told to show their best attempt so that we had something to work from to help them make the connections from where they currently are to where they need to be.

Consider that a large fraction of questions fall into a category in which the person is struggling with something that they are at a point in their education where they are expected to be able to solve that problem. Something is tripping them up. They've seen it presented in their text. They've seen examples worked in the text. They've seen it presented in class. They've seen examples worked in class. Now they are trying to do it on the homework and something hasn't clicked. It is unlikely that seeing a stranger on the Internet show them yet one more worked example is going to change that. The far more likely outcome is that the person will THINK they've learned something, turn their homework in THINKING that they know how to work it, and then the next time they see a similar problem it will be on the exam when they discover that whatever didn't click after all of the previous examples they've see still hasn't clicked and they tank the exam.

Instead, it's usually far better to help the person struggle through the problem while trying to spot the specific points where they are getting stuck and help them just enough to get past that point and then let them see how far they can get on their own before getting stuck again. That way that are much more likely to have their "Aha!" moment and actually learn the concept.

#### WBahn

Joined Mar 31, 2012
26,398
I may have missed a major concept and I will be the first to admit my algebra skills are not the best. I did attempt the question before posting it on here. As I said you can remove the thread of you want.
No need to remove the thread. The hope is that you not only get help on this particular problem, but also that you perhaps learn something about learning. That's a skill that few people innately know enough about and that nearly everyone never knows enough (most definitely myself included). Similarly, we all have weakness in one area or another. When such an area is identified, we need to see it for what it is -- a weakness that now we have an opportunity to address. Similarly with missing major concepts -- that happens to everyone as they learn things. But we can't learn that major concept until we can at least identify what it is.

That's why just attempting to work the problem on your own and then posting here without showing that work isn't what we are looking for. You need to post your best attempt -- it doesn't matter how wrong or how incomplete it happens to be. That way we can help you identify the specific points where you are going astray and then you can either go fill in the gap from your text or your notes or you can ask more specific questions about it here and we can focus on that point until you get past it.

#### djsfantasi

Joined Apr 11, 2010
8,075
You must have been a strict parent or still are !
My children, now grown up, may have thought I was strict. But as an educator, I may be strict but you’ll know your stuff, if you want to.