How can three currents leave a node with no current flowing in??

Mojo Pin__

Joined Apr 13, 2019
83
I thought KCL was "everything in equals everything out" or "the sum of all currents entering and leaving a node equals zero".

When we were doing this example in our lecture today the professor showed this slide:

But to me this looks like the node with id and iout leaving it doesn't have current going in to it, or does it?

I thought what he was trying to say was something like this:

But it just doesn't make sense to me,

Any explanaition would be greatly appreciated

neonstrobe

Joined May 15, 2009
190
Since all resistors in the equivalent circuit are in parallel, and the current source is pulling current down, the voltage on the output node will be negative. So Vout is negative. You can then work out all of the other currents. If the result is negative to the direction shown it means current actually flows in the other direction.

BobTPH

Joined Jun 5, 2013
9,113
Some of the currents will be negative, which simply says the arrow was drawn on the wrong direction. They will then sum to zero.

Nob

Mojo Pin__

Joined Apr 13, 2019
83
Since all resistors in the equivalent circuit are in parallel, and the current source is pulling current down, the voltage on the output node will be negative. So Vout is negative. You can then work out all of the other currents. If the result is negative to the direction shown it means current actually flows in the other direction.
i see what you mean, thanks for the explanaition!

Mojo Pin__

Joined Apr 13, 2019
83
Some of the currents will be negative, which simply says the arrow was drawn on the wrong direction. They will then sum to zero.

Nob
I get you, it makes more sense to me now I've looked over it again, thank you!

wayneh

Joined Sep 9, 2010
17,498
I get you, it makes more sense to me now I've looked over it again, thank you!
Another way to think about it: Arrows (vectors) show which direction is defined as positive. A negative (scalar) value indicates flow in the other direction.

Mojo Pin__

Joined Apr 13, 2019
83
Another way to think about it: Arrows (vectors) show which direction is defined as positive. A negative (scalar) value indicates flow in the other direction.
yes, nice one, but if all arrows pointing out of the node are defining them as positive that makes it confusing again. But... the AC signal part is the Vector and the DC part is the Scalar? Am i understanding you here?

dl324

Joined Mar 30, 2015
16,988
but if all arrows pointing out of the node are defining them as positive that makes it confusing again.
If you're familiar with the circuit, you can assign proper directions for the currents. Otherwise, you guess and sometimes you guess wrong. If you assign all currents as leaving a node, you're not even trying to guess and at least one current direction is wrong.

wayneh

Joined Sep 9, 2010
17,498
yes, nice one, but if all arrows pointing out of the node are defining them as positive that makes it confusing again.
No confusion, it just requires at least one to have a negative value.
But... the AC signal part is the Vector and the DC part is the Scalar? Am i understanding you here?
No, I don't (can't?) think in AC and my comments are limited to the DC world.

Mojo Pin__

Joined Apr 13, 2019
83
If you're familiar with the circuit, you can assign proper directions for the currents. Otherwise, you guess and sometimes you guess wrong. If you assign all currents as leaving a node, you're not even trying to guess and at least one current direction is wrong.
that's why I was confused initially, I see what you are saying, I just don't understand why the professor chooses to make them all leave the node if that's impossible.

I think it is because he has chosen to make the voltage drop in a certain direction such that the current would have to be flowing in a corresponding direction. So in terms of how it is drawn the fact that the voltage drop would be negative means that 2 of those currents would be negative despite the arrows leaving the node.

Does that sound like a good explanation?

dl324

Joined Mar 30, 2015
16,988
I just don't understand why the professor chooses to make them all leave the node if that's impossible.
If I give him the benefit of the doubt, he knew that one or more of the directions he assigned were "wrong" and wanted you to be able to handle a wrong direction assumption that you might make in the future.

A wrong direction assignment will give you a negative number for the current. It all works out in the end.