[SOLVED] Pls help me to solve some beginner problem !!!

[darrenbkl]

Hi all,help me solve this problem pls,I m a begginer.

1) Using Thevenin's Theorem,find the voltage across R4.



2)Simplify the circuit external to R3 to its Thevenin Equivalent.

 

[diegobb]

Hi, i'm not going to solve it for you, but i can help you.
(I'm at work right now )

In this link you will find the Thevenin theorem, and and example of how to use it:
https://en.wikipedia.org/wiki/Thevenin's_theorem

In this link you will find how to use complex variables to solve the circuit:
https://en.wikipedia.org/wiki/Electrical_impedance

To calculate the thevenin equivalent, you must calculate the open circuit voltaje, and the short circuit current, take a look at the fist link.

For the circuit in the figure 1:
First: remember you are working with inductors and capacitors, they have complex values, where j is the imaginary unit. j=sqrt(-1).
The values of these components are:
XC=1/(j*5k ohm)
XL=j*3k ohm
And the source is Vs=50
Procedure:
Calculate the parallel betwen XC and R2, it is X1=(XC*R2)/(XC+R2)=(j*9.9k)/(3.3+3*j)
After that calculate the series between what i call X1 and R1, it is X2=(XL||R2)+R1=1k+((j*9.9k)/(3.3+3*j))=(3.3k+12.9k*j)/(3.3+3*j)
After that you must calculate the parallel between i call X2 and R3 givin X3, after that you can calculate the voltaje Vab as Vab=Vs*X3/(X3+XC).

To solve the circuit in figure 2:
First, the complex values of the components:
XC=1/(120*j)
XL=90*j
Vs=75
Procedure:
In this case the procedure (Not the procedure to find the thevenin equivalent, i mean the procedure you must follow to solve the circuit and find Vab and Rab) is a bit different, because you must calculate the currents in the series circuits (XC&R3) and (R1&XL), after that you can calculate the voltaje in A and B, by the voltaje divisor, for example if we call A the node between XC and R3, Va=Vs*R3/(R3+XC).
To calculate the Rth (Thevenin equivalent resistance, or impedance in this case) you must eliminate the source Vs, after that you calculate the impedance the circuit between the points A and B sees, ie, (XC+R1) in parallel with (XL+R3), so you get: Rab=((XC+R1)*(XL+R3))/((XL+R3)+(XC+R1))

Please take a look to the links i've posted, i'm sorry but my english is not so good.
Please make the calculus by yourself, i can be wrong (Not in the general procedure, i hope).

I hope this can help you.
(If someone see this and find some error, please post the correction)

Regards,
Diego Bouvier