Understanding Kirchoff's Law and Solving Circuit Problems

[Pochen Liu]

Homework Statement

How do I calculate the current I₃?

Homework Equations

I know that I₁ = I₂ + I₃
and that I₂ = 0.4

The Attempt at a Solution

So we have a voltage source of 3.7V I would've thought that the equation for a clockwise circuit be
0.4R₂ - 3.7 - 200I₃ = 0 which gives me an answer of -0.0183 when it should be 0.0187

How am I using the law wrong and how can I prevent doing so in the future?

 

[gneill]

So we have a voltage source of 3.7V I would've thought that the equation for a clockwise circuit be
0.4R2 - 3.7 - 200I3 = 0 which gives me an answer of -0.0183 when it should be 0.0187

How am I using the law wrong and how can I prevent doing so in the future?

You need to pay attention to the direction of the currents as you do your "KVL walk" around the circuit. For example, $I_3$ is flowing down through $R_3$, so as you "walk over" $R_3$ going clockwise around the circuit you'll see a rise in potential.

 

[Pochen Liu]

You need to pay attention to the direction of the currents as you do your "KVL walk" around the circuit. For example, $I_3$ is flowing down through $R_3$, so as you "walk over" $R_3$ going clockwise around the circuit you'll see a rise in potential.

Is this because if I go 'walk over' instead of using the voltage it will add? since it's coming from the other direction, therefore I need to change it's mathematical sign around right?

 

[gneill]

Is this because if I go 'walk over' instead of using the voltage it will add? since it's coming from the other direction, therefore I need to change it's mathematical sign around right?

Right, you need to account for the current directions. If you "walk" in the same direction of the current you'll see a potential drop across the resistor. If you "walk" against the direction of the current you'll see a potential rise.