# KCl and node voltages, need help

1. Mar 4, 2015

### TheRedDevil18

1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

I'm not getting the correct answers, here are my equations

Node A:
80 = ix + i2
80 = Va/0.143 + (Va-(Vb+10))/0.2..........................1

Node B:
20 = iy-i2
20 = Vb/0.125 - (Va-(Vb+10))/0.2..........................2

Are these equations correct ?

2. Mar 4, 2015

### donpacino

equation 1... i see a sign error
equation 2....same problem

3. Mar 4, 2015

### TheRedDevil18

I don't see where the sign error is

4. Mar 4, 2015

### donpacino

There is an error with your I2 term. Look at the relationship between VB and the voltage source

5. Mar 5, 2015

### TheRedDevil18

I don't know, Is it -10V ?, I'm confused with the signs

6. Mar 5, 2015

### Staff: Mentor

You drop down from Vb by 10v to get to the 0.2 ohm resistor. Should be ....(Vb-10)....

If you draw an arrow from (-) to (+) on the battery, which you should do and label it +10V, you can see the drop in potential in going from Vb towards the 0.2 ohm. The bottom of the battery is 10v less than the top.

7. Mar 5, 2015

### TheRedDevil18

I still don't get the correct answers. I get Va = 5.65V and Vb = 7.56V
These are my equations
80 = Va/0.143 + (Va-(Vb-10))/0.2

20 = Vb/0.125 - (Va-(Vb+10))/0.2

8. Mar 5, 2015

### donpacino

that is what I got too. either we both made the same mistake, or the 'solution' is incorrect.

9. Mar 5, 2015

### Staff: Mentor

The answers accompanying the question in post #1 are consistent with the 20A source being directed downwards, so are not appropriate for the circuit as drawn.

10. Mar 6, 2015

### TheRedDevil18

Ok, maybe the solutions are incorrect. I'm a bit confused with the second equation though, why is it Vb+10, because the 0.2 ohm resistor is still connected to the negative end of the battery. Shouldn't it be Vb-10 ?

11. Mar 6, 2015

### Staff: Mentor

It should be Vb - 10 for the reason I gave.

12. Mar 6, 2015

### TheRedDevil18

For both the equations ?

13. Mar 6, 2015

### Staff: Mentor

Yes, it defines the voltage at one end of that 0.2 Ω resistor.

14. Mar 6, 2015

### TheRedDevil18

Ok thanks guys