# Series RLC circuits help with formula

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#### Electricalapprentice1

Joined Feb 17, 2024
6
Hi I’ve got this question. Doing RLC series circuit. I’ve got a formula sheet that says for capacitor reactance I use formula xc=10^6/2pieFC) when capacitance is in microfarads. The problem I have is the working out in the answers is a different formula and I have no idea where they get 6.28 from? Can someone please explain why I cant use the formula I’m using and what I should do. View attachment IMG_9074.jpegView attachment IMG_9069.jpegView attachment IMG_9072.jpeg

#### Irving

Joined Jan 30, 2016
3,868
Errr 6.28 is 2π to 2 DP.

Your formula is exactly the same,but for μF rather than Farads

The difference in results is they've used 6.28 rather than the PI key....

You need to stop, take a deep breath and just think it through a while...

#### Papabravo

Joined Feb 24, 2006
21,225
$$2\pi\;=\;6.283185307...$$

$$X_C\;=\;\cfrac{1}{2\pi fC_F},\text{ Capacitance in farads, frequency in Hz.}$$

$$X_C\;=\;\cfrac{1\times\;10^6}{2\pi fC_{\mu F}}, \text{ Capacitance in }\mu\text{Farads, frequency in Hz.}$$

#### Irving

Joined Jan 30, 2016
3,868
BTW, welcome to AAC

#### Electricalapprentice1

Joined Feb 17, 2024
6
Errr 6.28 is 2π to 2 DP.

Your formula is exactly the same,but for μF rather than Farads
Ok. Thanks I get 6.28 is the formula. But how is my formula the same? In my working I got 795.77 ohms and their working out they got 796.17ohms?

#### Electricalapprentice1

Joined Feb 17, 2024
6
BTW, welcome to AAC
Thankyou! I’m an electrical apprentice (can probably tell by user name) and trying to wrap my head around all this. Appreciate your replies. Thanks

#### WBahn

Joined Mar 31, 2012
30,034
Ok. Thanks I get 6.28 is the formula. But how is my formula the same? In my working I got 795.77 ohms and their working out they got 796.17ohms?
So let's look at the difference between what you got and what they got.

The difference is 0.4 Ω. which is just 0.05%

Now consider the difference between 6.28 and 2π, which carried out a few more places is 6.28319. That a difference of 0.00319, which is 0.05%.

So the difference is due to the difference in the estimate of π that was used.

Another way of looking at it is that 6.28 is 2π represented to 3 significant figures. That means that your answer is only going to be good to, at best, three significant figures. So round both answers to three sig figs and, in both cases, you get 796 Ω.

#### WBahn

Joined Mar 31, 2012
30,034
Don’t tell me to stop or take a breath buddy. Im just trying to understand the question. Nothing wrong with that. By the looks of thing you need to take a breath more than me..
You came here asking for free help from strangers. If you are unappreciative when those strangers take the time out of their lives to provide that free help, then don't be surprised when they, and everyone else seeing your response, elect to spend their time elsewhere instead of providing free help to someone that isn't going to appreciate it.

#### Electricalapprentice1

Joined Feb 17, 2024
6
You came here asking for free help from strangers. If you are unappreciative when those strangers take the time out of their lives to provide that free help, then don't be surprised when they, and everyone else seeing your response, elect to spend their time elsewhere instead of providing free help to someone that isn't going to appreciate it.

If you want to answer for someone else then you are more than welcome.but before you go calling me unappreciative go up and read earlier in the thread. I literally said thankyou and that I did appreciate your replies.

#### Electricalapprentice1

Joined Feb 17, 2024
6
If you want to answer for someone else then you are more than welcome.but before you go calling me unappreciative go up and read earlier in the thread. I literally said thankyou and that I did appreciate your replies.
And I couldn’t care less who replies. Delete your reply to my question and move on!

#### Irving

Joined Jan 30, 2016
3,868
OK, I don't normally reply to those that leap down my throat, but in my view of your question you seemed to be panicking, so I gave you the same advice I'd give any of my students. Maybe it wasn't obvious to you, but putting the two formulae side by side it should have been immediately obvious that 2π and 6.28 were synonymous. And if you'd tried the calculation with both formulae you'd have seen where the difference in result was.

I'm guessing you're pretty young and inexperienced. Understood you're trying to get your head around this. It isn't easy but its going to get a lot harder if you're serious about getting into electronics.

#### Electricalapprentice1

Joined Feb 17, 2024
6
OK, I don't normally reply to those that leap down my throat, but in my view of your question you seemed to be panicking, so I gave you the same advice I'd give any of my students. Maybe it wasn't obvious to you, but putting the two formulae side by side it should have been immediately obvious that 2π and 6.28 were synonymous. And if you'd tried the calculation with both formulae you'd have seen where the difference in result was.

I'm guessing you're pretty young and inexperienced. Understood you're trying to get your head around this. It isn't easy but its going to get a lot harder if you're serious about getting into electronics.

Where did you get the idea that I was panicking?

And it wasn’t obvious that’s why I was asking the questions…

Why don’t you just move on if you think I’m jumping down your throat. Seems like your the sort of guy that can dish it out but cant take it.[/QUOTE]

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#### MrAl

Joined Jun 17, 2014
11,453
Where did you get the idea that I was panicking?

And it wasn’t obvious that’s why I was asking the questions…

Why don’t you just move on if you think I’m jumping down your throat. Seems like your the sort of guy that can dish it out but cant take it.
[/QUOTE]

Hello there,

Wow, that's some attitude you got there I can't help but mention that it won't help you learn more. Even if someone is overtly rude to you the best way to handle it is to reply in a sensible way not to fan the fire. I know it is not easy to do sometimes though.
Part of the learning process is to learn the best way to socially interact with other professionals, unless of course you want to become a politician
<insert really big chuckle here>

I think all he meant by 'take a breath' was to look the problem over again and see if there is anything logical that pops out at you.

What I have to do sometimes is take a break and come back to the problem later. I find this helps a lot. I can work on something for hours trying to figure it out not realizing that I am too tired at the moment. After even a short rest sometimes, I can come back and solve it within a few minutes.

Also, when your result comes out very close to the correct answer it usually means there is something just slightly off, so you start to look at differences and ratios. If you have a checking account and it does not balance, you can subtract the bank balance from your balance and almost immediately figure out that one item was left off the list accidentally, and everything works out. Thus, just looking a difference helps solve the problem easily.
Sometimes a ratio helps. looking at the ratio of the given answer and your answer, you can compare that to the ratio of 2*pi over 6.28 and see if that makes enough sense to solve the problem.

#### DickCappels

Joined Aug 21, 2008
10,168
Time out to cool off.

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