How to solve circuit analysis problems with multiple loops?

[Questions999]

I have two problems and as you have seen I've been posting some exercises like this lately because I have 150 exercises to solve and I can't solve only a FEW of them
1. Homework Statement
For the first one :We have the circuit in the figure.We have to find i.If its not clear,the current on the right top says 3i,and the one in the left just i.
http://i.imgur.com/kQdKOnQ.jpg
The second one : This exercise is an alien! Its more difficult than the first one ,can you please give me a hint?http://oi41.tinypic.com/4vsi2p.jpg

Homework Equations

KCL,KVL.

The Attempt at a Solution

For the first one :I don't really know what to do about this one.The current across 10 Ohm is i1,now I write the KCL and I have i=3i+i1 here I have that i1=-2i.I find the voltage across 10 Ohm and it is -20 V.I am thinking about finding the resistance across 3i,so I can find the whole resistance of the whole circuit and then 25/ (Equivalent resistance) but the resistance is negative :/

 

[NascentOxygen]

For the first one :I don't really know what to do about this one.The current across 10 Ohm is i1,now I write the KCL and I have i=3i+i1 here I have that i1=-2i.I find the voltage across 10 Ohm and it is -20 V.

Try again, what is the correct expression for the voltage across the 10Ω?

 

[Questions999]

V across 10 Ohm= -2i * 10 Ohm=-20 V...

 

[NascentOxygen]

V across 10 Ohm= -2i * 10 Ohm=-20 V...

Do you know the value of variable i?

 

[Questions999]

No,I dont...sorry it is -20i Volt

 

[NascentOxygen]

No,I dont...sorry it is -20i Volt

So now you know the current through each resistor (in terms of variable i). Add them up and what do you get?

 

[Questions999]

No,I know the voltage across 10 Ohm in terms of variable i,not the current...

 

[SammyS]

I have two problems and as you have seen I've been posting some exercises like this lately because I have 150 exercises to solve and I can't solve only a FEW of them.

Homework Statement

For the first one :We have the circuit in the figure.We have to find i.If its not clear,the current on the right top says 3i,and the one in the left just i.
http://i.imgur.com/kQdKOnQ.jpg
The second one : This exercise is an alien! Its more difficult than the first one ,can you please give me a hint?http://oi41.tinypic.com/4vsi2p.jpg

Homework Equations

KCL,KVL.

The Attempt at a Solution

For the first one :I don't really know what to do about this one.The current across 10 Ohm is i1,now I write the KCL and I have i=3i+i1 here I have that i1=-2i.I find the voltage across 10 Ohm and it is -20 V.I am thinking about finding the resistance across 3i,so I can find the whole resistance of the whole circuit and then 25/ (Equivalent resistance) but the resistance is negative :/

First of all, terminology:

Voltage is dropped across a resistor or other circuit element.

Current passes through a resistor or other circuit element.​

You have a current of " i " flowing through the branch with the 25V voltage source and 6Ω and 4 Ω resistors .

You have a current of 3i through the branch which has only the current source. That's the diamond shaped object with the arrow in it.

Use KCL to find the current through the 10 Ω resistor. (This will be in terms of " i " at this point.)

Then use KVL for the appropriate loop to give you an equation with only the variable, i .

 

[Questions999]

Omg thanks :D How about the second one?

 

[SammyS]

Omg thanks :D How about the second one?

Similar idea, but a little more complicated.

One fixed current source of 5 Amperes.

One variable current source. For this, the current is v/4, where v is the voltage drop across the 6Ω resistor.


I would let the variable, I, represent the current through the 6Ω resistor.

 

[Questions999]

Yes,but I don't have enough data to apply KCL at the node above 6 ohm..

 

[NascentOxygen]

Yes,but I don't have enough data to apply KCL at the node above 6 ohm..

Yes you do, providing you write each current in terms of variable v.

For starters, how much current flows to ground through the 6Ω resistor (in terms of v)?

 

[Questions999]

I understand it completely now,thanks.For the exercise before this,did you get i=4.5 A?

 

[NascentOxygen]

I understand it completely now,thanks.For the exercise before this,did you get i=4.5 A?

Aren't you to find v?

 

[Questions999]

I have posted two exercises here,I mean the first one,where I have to find i...

 

[NascentOxygen]

I understand it completely now,thanks.For the exercise before this,did you get i=4.5 A?

For the solution to the first question, I did not get i=4.5A

 

[Questions999]

What was the i that you got? I will figure out it again..

 

[NascentOxygen]

You should be able to check your answer. Work your way around the circuit substituting the value you have, and all voltages and currents should tie in with Kirchoff's laws. If they all check out, then your answer is correct!

 

[SammyS]

For the solution to the first question, I did not get i=4.5A

I thought you said you had this figured out.

Anyway, please show how you got this answer.

What was your loop equation?

 

[Questions999]

The final loop equation : -25 V - 4i -20i -6i=0 ..

 

[Questions999]

Oh I am very very sorry,I have been tired from studying these days and I have made many mistakes.I really figured this out now.Thanks again!

 

[SammyS]

The final loop equation : -25 V - 4i -20i -6i=0 ..

I don't agree with that.

The current through the 10Ω resistor is 2i , pointing to the left.

If you take the loop with the voltage source and the two resistors along with the 10Ω resistor, in a clockwise direction, the source gives a positive contribution, so does the voltage across the 10 Ω resistor (The current through the 10 Ω resistor is opposite to the direction taken around the loop.)

Please try again. Give the loop equation and give your numerical result for the current rather than simple stating, "I figured it out".