Hi There,

I am a little confused to how to work out a complex number that needs to be squared. For example

\( y =a+jB = sqrt[(R+jwL) (G+jwC)]\)

where
R=0.2Ω/km
L=1.1mH/km
C=0.01uF/km
G= Assume ≈ Negligible
Freq = 50Hz

\( = sqrt[(0.2+j2*Pi*50*1.1mH) (0+j2*Pi*50*0.01uF)]\)
\( = sqrt[(0.2+j0.3455) (0+j3.141*10^-6)]\)

I know that with complex numbers that to multiply the process is

\((ac-bd)+j(ad+bc) \)
so

\( = sqrt[(0.2*0)- (j0.3455*j3.141*10^-6) + (0.2*j3.141*10^-6)+(j0.3455*0)]\)

\( = sqrt[(1.085*10^-6) + (628*10^-9)]\)

Then If i square roots this the answer that I get is not correct. The answer that is given is

\( = 0.29*10^-3 +j1.082*10^-3\)

I am not sure what I am doing incorrect. So any guidance would be appricated.

Thanks

 

\( = sqrt[(0.2+j0.3455) (0+j3.141*10^-6)]\)

I know that with complex numbers that to multiply the process is

\((ac-bd)+j(ad+bc) \)
so

\( = sqrt[(0.2*0)- (j0.3455*j3.141*10^-6) + (0.2*j3.141*10^-6)+(j0.3455*0)]\)

You have an error here. The expression \((0.2+j0.3455) (0+j3.141*10^-6)\) written in the form (a+jb)(c+jd), you have a=0.2, b=0.3455 (not j0.3455), c=0 and d=3.141E-6.

Therefore the expression equals \(-0.3455 \cdot 3.141 \cdot 10^{-6}+j \cdot (0.2 \cdot 3.141 \cdot 10^{-6})\)

Can you take it from here?

 

So

\( = sqrt[(-1.085*10^{-6})+j(0.628*10^{-6})]\)

then I take out the \(10^{-6}\)

\( = 10^{-6}sqrt[-1.085+j(0.628)]\)

when i sovle this I still get the incorrect answer..... Still unclear sorry

Is there any tutorials on this type of questions, that anyone knows of?

 

Could you post the original question? I noticed that the quantities R, L and C are expressed per kilometer, so the size may differ.

However, in the exercise's defence, for 1km, the correct answer is the one suggested and you mentioned. I, from my part, suggest you take more care of your number crunching.

As an example, you forgot to change \(10^{-6}\ to\ 10^{-3}\) when you took it out of the root.

 

Hi Georacer,

The question I have attached,

I will have another look at the question and work though it again slowly.

I guess what I am after is the steps that need to be taken so I can work this out on paper without using a computer program.

Thanks for your help

 

Georacer,

I have tried to solve this with Matlab but the code I am using is incorrect.

I have tried to input

\(complex sqrt{(R+jwL)(G+jwC)} \) with the corresponding values above and there are errors..

If this what you used to solve the problem? If so how did you enter it?

 

perhaps more like

\(\gamma=sqrt (complex(R,\omega L)*complex(0,\omega C))\)

but using the function 'sqrt(operand)'

The '*' just denotes multiplication

 

Here's an approximately equivalent sequence in Scilab ...

 

Here is an exact Matlab code snippet:

R=0.2;
L=1.1*10^(-3);
C=0.01*10^(-6);
G=0;
f=50;

omega=2*pi*f;
y=sqrt((R+omega*L*i)*(G+omega*C*i))

 

Thanks for that, I will try that code now and see how I go. This seems an easier way to solve these types of questions.

 

It won't help you in an exam, though. Better practice by hand.

 

It might also help if you remembered there is a polar form of complex numbers.

For instance it can be helpful to use the polar form in the square root operation you have been considering. Convert the rectangular form to polar form and then use this realtionship

\(\sqrt{(X\angle \theta)(Y\angle \phi )}=\sqrt{XY}\angle( \frac{\theta + \phi}{2})\)