KCL: Understanding Current Directions

[mike41]

i have been trouble with this. iunderstand the concept but i don't know how to set up the current directions onto a circuit when i apply KCL. I am not sure which way the arrows should be going when doing kcl. for example like in this problem.

 

[Okefenokee]

Current flows from the "+" side to the "-" side. So, for the dependent current source you have +V3 - V2 = 5i.

You can draw the current arrows through a resistor anyway you like. The arrow tells us to take the voltage at the start of the arrow and minus the voltage at the end of the arrow. Sources have an explicit polarity so you should choose an arrow that goes from it's positive to negative.

 

[mike41]

ok but let's say for Node v1 how do i draw the currents

would this be correct. this is the part that is holding me back

 

[Okefenokee]

The 10V source doesn't need a current arrow. V1 will always be exactly 10V higher than V2. The equation for that branch will by +V1 - V2 = 10V. The rest looks fine.

Look at the direction of the arrow that you drew for the 3 Ohm resistor. It implies this formula: i3 = (0V - V3)/(3Ohm)= -V3/(3Ohm). The math will still work out just fine.

 

[mike41]

ok so i can choose what ever i want for my reference at first, but i must stick to the current flow for the whole circuit and the math will work then?

 

[Okefenokee]

exactly. You can flip the arrows and repeat the problem. You'll get negative currents where you had positive currents the first time and vice versa. A negative current implies that it flows in the opposite direction of the reference arrow that you drew.

 

[mike41]

how about choosing your reference nodes. is there something special to that? choosing the node above the 60v wouldn't be the best choice would it?