# Kirchhoff law problem

## Homework Statement

https://fbcdn-photos-c-a.akamaihd.net/hphotos-ak-xft1/v/t1.0-0/s480x480/12802893_10205707400875867_4855727067132215897_n.jpg?oh=b486598e6f2016a814eb52bf48870334&oe=57648CD9&__gda__=1464637082_3cae40328f43f09eaa2cf9cf5b2f0b0f

## Homework Equations

Kirchhoff Law
1. Find the current in the circuit
2. Find the potential diferent between X and Y

## The Attempt at a Solution

1 by using the kirchoff law. V=IR
I=0.025

question 2 I=0.025
but how to find Vx and Vy?
Vx =IR
I =0.025 but which R is supposed to use?

cnh1995
Homework Helper
Gold Member
Your current direction is correct. One way to solve this problem would be by assigning potentials and calculating voltage drops. Assume potential of point W to be 0V. That way, +ve terminal of 20V source will be at 20V. Then calculate drop across 40 ohm resistor. Point x will be at 20-V40Ω potential. Go on with this along the loop and find potential of point y. Difference between them is your Vxy.

Thanks!! I got it
https://scontent-kul1-1.xx.fbcdn.net/hphotos-xlt1/v/t1.0-9/12814145_10205707617681287_7591958914593573074_n.jpg?oh=b9163aaa1c77737cb533a124e4292d6a&oe=574F8189

By using kirchoff law i get all the current
the question ask to find the PD between A and B, the question is the voltage drop i should begin in which point? Because there is 2 sources

cnh1995
Homework Helper
Gold Member
You can start by either assuming A at 3V or assuming B at 0V. Since you know i1, assuming B at 0V will be easier.

But point A is supplied by 2 battery of 3V.If i use voltage drop, I only need to look at the top loop?

cnh1995
Homework Helper
Gold Member
But point A is supplied by 2 battery of 3V.If i use voltage drop, I only need to look at the top loop?
You need to find the currents using the two loops (which will include both the sources) and then by assuming a suitable reference( VB=0V), you can find Vab like you did in the last problem.

if i assume Vb =0
-6(I2)+3V=Va?

cnh1995
Homework Helper
Gold Member
if i assume Vb =0
-6(I2)+3V=Va?
-6I1+3V=Va..

Ops mistake haaha. thank you , have a good day!

https://scontent-kul1-1.xx.fbcdn.net/hphotos-xft1/v/t1.0-9/12814383_10205707793045671_2997254212403029818_n.jpg?oh=0a9ed6d9fbec4f8eed32f88ffdfba9b1&oe=57574AC1

If Vx is 0 , what is the V in point Y
-12(I1)-8(I2)=Vy
which i will gef -12V in Y, is this possible for me to get 12V in negative value?

cnh1995
Homework Helper
Gold Member
https://scontent-kul1-1.xx.fbcdn.net/hphotos-xft1/v/t1.0-9/12814383_10205707793045671_2997254212403029818_n.jpg?oh=0a9ed6d9fbec4f8eed32f88ffdfba9b1&oe=57574AC1

If Vx is 0 , what is the V in point Y
-12(I1)-8(I2)=Vy
which i will gef -12V in Y, is this possible for me to get 12V in negative value?
Yes. In fact you don't even need to know the current for that. Moving from x to y along the bottom wire, you can see a voltage drop of 12V.