Kirchhoff current law question

[princejan7]

http://postimg.org/image/mk8vo32in/

Just wondering whether the 3A from the current source should be included when writing a KCL equation for nodes a and c,
and if yes, why the full 3A? Wouldn't it split up along the way


thanks

 

[BvU]

KCl is not potassium chloride but some relevant relationship (yes, I know). If you use the template (required by PF rules), you make it easier for helpers to help you. 1. Introduces us to what is asked, 2. helps us help you if you miss some relationship or use the wrong ones and 3. Helps us help you by indicating what you can and can not do and where you get stuck.

Your direct question has a direct answer: No. the 3A isn't entering or exiting point a nor point b. If you smack KCL on the points just above a and b, then it shows up!

 

[princejan7]

KCl is not potassium chloride but some relevant relationship (yes, I know). If you use the template (required by PF rules), you make it easier for helpers to help you. 1. Introduces us to what is asked, 2. helps us help you if you miss some relationship or use the wrong ones and 3. Helps us help you by indicating what you can and can not do and where you get stuck.

Your direct question has a direct answer: No. the 3A isn't entering or exiting point a nor point b. If you smack KCL on the points just above a and b, then it shows up!

http://postimg.org/image/j6jddk8if/

But the solution to the problem treats the 3A as entering both node a and node c? Is it wrong?

 

[BvU]

It's not 'legally' wrong, as long as you take the current through "2" into account. And the j6jddk8if link seems to do so Ok (I didn't try to untangle what V1,2,3, stand for (lazy me, because it's pretty obvious :-); in general answers supplied are pretty decent).

My general impression is that you understand what's going on and how it is dealt with. Am I far off? If so (or if not so) I'm glad to answer further questions.

 

[gneill]

A node consists of any contiguous network of wiring. So in your diagram, reproduced here, all of the blue highlighted wiring comprises node a, while all of the green highlighted wiring comprises node c. So yes, when writing KCL for either of those nodes you need to include all branches connecting to them, which of course includes the 3 A current source.

 

[BvU]

Fully agree. Makes life easier too. Conductors and chunks of wire are ideal conductors in homework assignments. I stand (actually, I sit) corrected.