# Circuit question: Find size and direction of current at P

Tags:
1. Apr 9, 2016

### Faris A

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

2. Relevant equations
I = I1 + I2 ... + In
R = R1 + R2 ... + Rn
1/R = 1/R1 + 1/R2 ... + 1/Rn

3. The attempt at a solution
Let I1 = I2 + I3 where I1 is the current on the top wire, I2 the current on the middle wire and I3 the current on the bottom wire.

4 = 10I2 + 20I1
//This is what I started trying but I can kind of tell that this is wrong

2. Apr 9, 2016

### cnh1995

If you assume I1 to be clockwise and I2 to be from right to left, this equation is correct. Assume a direction for I3 and write another equation. Your directions may be wrong, but that will just give you a negative value for the current.

3. Apr 9, 2016

### Faris A

If 4 = 20I1 + 10I2 is correct then 12 = 10I3 + 10I2 is correct
I1 = I2 + I3
4 = 20(I2 + I3) + 10I2
4 = 20I3 + 30I2
12 = 10I3 + 10I2 can be made into 24 = 20I3 + 20I2
Using simultaneous equations: 20 = -10I2
I2 = -2

But is this correct?

4. Apr 9, 2016

### Faris A

dw I figured out where I was going wrong