Kirchhoff Law Problem: Find Current and Potential Difference | Solution Included

[WeiLoong]

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 kirchhoff 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]

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-V_40Ω potential. Go on with this along the loop and find potential of point y. Difference between them is your Vxy.

 

[WeiLoong]

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 kirchhoff 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]

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.

 

[WeiLoong]

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]

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( V_B=0V), you can find Vab like you did in the last problem.

 

[WeiLoong]

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

 

[cnh1995]

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

-6I1+3V=Va..

 

[WeiLoong]

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

 

[WeiLoong]

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]

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.