• Support PF! Buy your school textbooks, materials and every day products Here!

Help New to the circuit

  • Engineering
  • Thread starter anthony0118
  • Start date
  • #1
Help!!!!New to the circuit

1. Homework Statement
Question 1
Use the Mesh analysis to find v
0 and power absorbed by the 20 resistor in the circuit
Question 2
a) Find the Thevenin equivalent circuit external to the resistor R
b) Find the condition under which maximum power is transferred to R
c) Determine the maximum power transfer to R

Does anyone know how to solve them I tried them but it didnt seen right and the dependent source is so annoying is there any way to get rid of them? thanks

p.s untitiled is question 1 and 2 for question 2 thankz
 

Attachments

Answers and Replies

  • #2
i got Vo + 24v dont know if thats right and quesrion 2 im bit confused i got Rth 10ohms sound right?
Need great help thankz
 
  • #3
CEL
655
0
There is no way to cancel dependent sources. Show what you have done in question 1 in order to be helped.
For question 2. Va is the difference between the 10V source and the open circuit voltage Vab. Develop your analysis and you will have one equation with one unknown: Vab.
In order to calculate the equivalent resistance, the best approach with dependent sources is to use Rth = Voc/Isc. You already have Voc. Short circuit terminals a and b and calculate Isc.
 
  • #4
i get number one now i have I1 as -2.585A ,I2=-3.732A ,I3=-5A
AND V0= (I1-I2)R= 23V AND POWER = 26.45W
But i still dont know question2 where is the 10v u talking about here is and is Vab = Vth?
 
  • #5
CEL
655
0
i get number one now i have I1 as -2.585A ,I2=-3.732A ,I3=-5A
AND V0= (I1-I2)R= 23V AND POWER = 26.45W
But i still dont know question2 where is the 10v u talking about here is and is Vab = Vth?
Sorry! I meant 12V, not 10V. And Vab = Vth.
 
  • #6
so is Rth = 4+2+4=10?
and Vth =
(Vth-2Va)/4 +Vth -12/4 -4=0
5Vth-4Va=84

what's Va?
12V/4=3A
3+4=7A
7*4=28v=Va????
Sorry im trying my best to understand it
 
  • #7
CEL
655
0
so is Rth = 4+2+4=10?
and Vth =
(Vth-2Va)/4 +Vth -12/4 -4=0
5Vth-4Va=84

what's Va?
12V/4=3A
3+4=7A
7*4=28v=Va????
Sorry im trying my best to understand it
Write the equation for the outside mesh, using Va as unknown. Remember that the current through the resistors 2 ohm and 4 ohm after the controlled source is the sum of the current through the firsr 4 ohm resistor with 4A of the current source. From this I found Va = -24V and Vab = Vth = 36V.
Next, short circuit the points a and b and calculate the current through them, Isc.

Rth = Vth/Isc.
 
  • #8
CEL can you write out the equation coz i cant get anywhere close to 36V for Vth!!!!!
I did node a (Vth-12)/4 -4+ 2Va+Vth/6=0
Vth equals 14 sth and not even -24 or 36 I am really really lost and i spend any hours on this still cant finish part A how do u do part B and C
Max power is = Rth as well as Rn but where do i go from there

damn i feel stupid with this circuits stuff!!!!
 
  • #9
CEL
655
0
CEL can you write out the equation coz i cant get anywhere close to 36V for Vth!!!!!
I did node a (Vth-12)/4 -4+ 2Va+Vth/6=0
Vth equals 14 sth and not even -24 or 36 I am really really lost and i spend any hours on this still cant finish part A how do u do part B and C
Max power is = Rth as well as Rn but where do i go from there

damn i feel stupid with this circuits stuff!!!!
Calling Ia the current trough the 4ohm resistor:
Ia = Va/4
The outermost mesh:
-12 + Va - 2Va +(2+4)(Ia + 4) = 0
or
-12 -Va + 6Va/4 + 24 = 0
12 + Va/2 = 0
Va = -24V.

Vth = 12 - Va = 12 + 24 = 36V.
 
  • #10
Wow are u a teacher? That was quick so u did a supper mesh and find out Va and Vth is an open circuit ya? and Now how do i find R's resistor Rth is a short circuits right? only way i know is Rth=Vth/Isc any other way to solve other then the one i menotion any hint to start with finding Isc?

sorry my head is just not working atm and been on this question like 9 or 10 hrs already and havnt sleep for longlong time


I alomst finishing my self study!!! YOU should teach me I'm kind of doing it as self study so hard without a teacher
 
  • #11
CEL
655
0
Wow are u a teacher? That was quick so u did a supper mesh and find out Va and Vth is an open circuit ya? and Now how do i find R's resistor Rth is a short circuits right? only way i know is Rth=Vth/Isc any other way to solve other then the one i menotion any hint to start with finding Isc?

sorry my head is just not working atm and been on this question like 9 or 10 hrs already and havnt sleep for longlong time


I alomst finishing my self study!!! YOU should teach me I'm kind of doing it as self study so hard without a teacher
When you short circuit points a and b you get two independent meshes: one with the two independent sources an one resistor and the other with the dependent source ant two resistors. Work from there.
 
  • #12
k i have loop 1 as -12 +4I1+16=0 and I1 =-1A

loop2 as -48+6I2=0 then I2=8A

is this what u mean in the last post and if so Isc= (I1-I2) = -9A
is Rth become -4?

max power would be???
 
  • #13
CEL
655
0
k i have loop 1 as -12 +4I1+16=0 and I1 =-1A

loop2 as -48+6I2=0 then I2=8A

is this what u mean in the last post and if so Isc= (I1-I2) = -9A
is Rth become -4?

max power would be???
Where did you get the value 16 in the first equation? Loop 1 comprises the 12V source, the 4 ohm resistor and the short circuit ab.
 
  • #14
so u mean loop1 as -12+4I1=0 SINCE SHORT CIRCUIT AB IS THERE?
I1=3 BUT IS LOOP 2 RIGHT?

THEN ISC = -5A, Rth = -36/5

somehow i hva a feeling of thats wrong
 
  • #15
CEL
655
0
so u mean loop1 as -12+4I1=0 SINCE SHORT CIRCUIT AB IS THERE?
I1=3 BUT IS LOOP 2 RIGHT?

THEN ISC = -5A, Rth = -36/5

somehow i hva a feeling of thats wrong
It is wrong. Don't forget that the current source also flows through the short circuit and that Va has now a new value that must be substituted in the dependent source.
 
  • #16
so what do u do with the current source then


ya VA was sub and loop2 = 2*-24+2i2+4i2=0
-48+6i2=0
i2=8A isnt that it?

and is ISC=(I1-I2)???????? IM KIND OF CONFUSED
 
  • #17
CEL
655
0
so what do u do with the current source then


ya VA was sub and loop2 = 2*-24+2i2+4i2=0
-48+6i2=0
i2=8A isnt that it?

and is ISC=(I1-I2)???????? IM KIND OF CONFUSED
Where did you get the -24 value? Va = 4i1!!!
You have 3 currents through the short circuit: i1, i2 and 4A from the current source.
 
  • #18
im really confused
so how many loop is it

is it one on the lefthand side and second on the right of short circuit
 
  • #19
-12+4i1+16=0 loop 1
what do i do with the 4A in this loop do u do source converction? and change it to voltage? then thats 16V 4A*4OHMS


coz VA =-24V AND 2VA=-48V for the dependent source
2 and 4 ohms are added up
-48+6i2=0 and this is loop2


and one thing is Isc=(i1- i2)?????? if so how that make sense i have a negative number for current
that is how i arrive at that point and im sorry i cant get it any further
 
  • #20
are u saying 4i1+6i2=0 for a second equation?
 
  • #21
demonstrate ur methods please master

i really need a proper teacher to teach me all this

thankz for ur time really
i helps me to think a bit faster :)
 
  • #22
CEL
655
0
im really confused
so how many loop is it

is it one on the lefthand side and second on the right of short circuit
There are 3 loops:
1. The independent voltage source plus the 4 ohm resistor.
2. The current source.
3. The dependent voltage source and two resistors in series,
 
  • #23
CEL
655
0
-12+4i1+16=0 loop 1
Where did you get the 16 V value?
what do i do with the 4A in this loop do u do source converction? and change it to voltage? then thats 16V 4A*4OHMS
The source and the short circuit form a loop.

coz VA =-24V AND 2VA=-48V for the dependent source
2 and 4 ohms are added up
-48+6i2=0 and this is loop2
No, Va was -24V when there was an open circuit between points a and b. Now there is a short circuit there, so you must write new equations.
and one thing is Isc=(i1- i2)?????? if so how that make sense i have a negative number for current
that is how i arrive at that point and im sorry i cant get it any further
No, there are 3 currents through the short circuit.
 
  • #24
CEL
655
0
are u saying 4i1+6i2=0 for a second equation?
No, i1 and i2 belong to independent loops.
 
  • #25
CEL
655
0
demonstrate ur methods please master

i really need a proper teacher to teach me all this

thankz for ur time really
i helps me to think a bit faster :)
According to the rules of the forum, you must show some work before being helped.
 
Top