Find Current I3 in Circuit - Homework Statement

[Gauss M.D.]

Homework Statement

Find the current marked I3 in the circuit.

Homework Equations

The Attempt at a Solution

The voltage over R2 and R1 is the same. The current I0 (12mA) is the sum of I2 and I1.

I1*R1 = I2*R2:
4*I1 = 1.25*I2

I1 + I2 = 12 <=> I2 = 12 - I1

4*I1 = 1.25*(12 - I1) = 15 - 1.25I1
5.25*I1 = 15

I1 = 2.86

I think I've messed up already here. Can anyone spot what I'm doing wrong??

 

[ehild]

Homework Statement

Find the current marked I3 in the circuit.

Homework Equations

The Attempt at a Solution

The voltage over R2 and R1 is the same. The current I0 (12mA) is the sum of I2 and I1.

I1*R1 = I2*R2:
4*I1 = 1.25*I2

It is wrong. The voltage over R3 and R4 is the same .

I1 + I2 = 12 <=> I2 = 12 - I1

4*I1 = 1.25*(12 - I1) = 15 - 1.25I1
5.25*I1 = 15

I1 = 2.86

I think I've messed up already here. Can anyone spot what I'm doing wrong??

 

[Gauss M.D.]

Thanks! Is I2 still the sum of I3+I4 though? :S

 

[gneill]

Thanks! Is I2 still the sum of I3+I4 though? :S

Sure.