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.

 

[Nickie]

Your approach to finding the current I3 in the circuit is correct. However, there are a few errors in your calculations. Firstly, the voltage over R2 and R1 is not necessarily the same, as there is a voltage drop across each resistor. Secondly, the equation I1*R1 = I2*R2 is not applicable in this circuit. Instead, the correct equation to use is I1*R1 = I2*R2 + I3*R3. Additionally, the equation I1 + I2 = 12 is incorrect as it does not take into account the current through R3. The correct equation is I1 + I2 + I3 = 12. By rearranging and substituting for I2 and I3, you should get the correct value for I3 as 2.86 mA. Remember to always consider all components in the circuit when solving for current.