# Confused about signs of currents in circuit

#### WBahn

Joined Mar 31, 2012
28,486
I still say:

You can use these interchangeably:
The sum of currents into a node is zero.
The sum of currents out of a node is zero.
The currents in to a node equals the current out of a node...
Sure. Those are all equivalent and, depending on the problem, one of them might be easier to apply than the others. All I'm saying is that formal Nodal Analysis is nothing more than the middle one applied systematically.

#### studiot

Joined Nov 9, 2007
4,998
This is homework help and the OP did ask about Kirchoff's Current Law.

tjohnson, I don't know what you are being taught about node currents and I don't want to confuse you.
All of Mike's versions are correct, it was just a question of the best one for beginners I was commenting on.

Kirchoff's two laws, his current law and his voltage law, both lead to a set of simultaneous equations.
The current law is also called nodal analysis.
The voltage law is called loop analysis.
Since these laws have common variables ( some unknown) they can be used in combination to produce yet more equations.

For most circuits there are more equations available than there are unknowns and the analysis skill looks to pick out the set that makes for the easiest solution.

#### WBahn

Joined Mar 31, 2012
28,486
For most circuits there are more equations available than there are unknowns and the analysis skill looks to pick out the set that makes for the easiest solution.
And picking up on this point, because there are more equations available than unknowns (often well over twice as many) you not only need to pick a set that makes it easy, but also a set that makes it possible. You can't just pick a set at random and figure that you can slog through the algebra because if you have N unknowns then you must pick a set of N equations that are all linearly independent, which may not be at all obvious. That's perhaps the single greatest value of the formalized methods (MCA/NVA) is that they ensure that the N equations you get satisfy this requirement.

#### tjohnson

Joined Dec 23, 2014
611
I'm confused again. I had another problem with a circuit like this:

I thought that I2=I1+I3, but the correct answer is obtained by using the equation I1=I2-I3. And whereas I originally thought all three values were positive, I1 is actually negative. What am I doing wrong here?

#### kubeek

Joined Sep 20, 2005
5,785
As far as I know I2=I1+I3 is the exact same equation as I1=I2-I3
A negative value of the current simply means that it is flowing against the arrow you initially drew. Nothing wrong there.

#### tjohnson

Joined Dec 23, 2014
611
As far as I know I2=I1+I3 is the exact same equation as I1=I2-I3
A negative value of the current simply means that it is flowing against the arrow you initially drew. Nothing wrong there.
Oops, I made that mistake before. Somehow when I did the problem before I wasn't getting the right answers though. I'll try to work it again, and see if I can figure out where I went wrong.

#### tjohnson

Joined Dec 23, 2014
611
I just tried the problem again, and got the correct answer the first time I tried it! I guess I must have been too tired to think well anymore the day I originally did the problem. Sorry for my foolish question.

#### studiot

Joined Nov 9, 2007
4,998
Sorry for my foolish question.
don't worry

Confucious he say

#### tjohnson

Joined Dec 23, 2014
611
I guess I must have been too tired to think well anymore the day I originally did the problem.
It appears that I was. I checked my original calculations for the problem, and found that I had treated the 4V battery as a 4Ω resistor in one of my KVL equations.

#### WBahn

Joined Mar 31, 2012
28,486
It appears that I was. I checked my original calculations for the problem, and found that I had treated the 4V battery as a 4Ω resistor in one of my KVL equations.
I can't be sure, of course, but I wouldn't be surprised if you didn't track your units throughout your work and that, if you had, you would have caught this mistake (a simple and easy one to make) almost immediately.

#### tjohnson

Joined Dec 23, 2014
611
I can't be sure, of course, but I wouldn't be surprised if you didn't track your units throughout your work and that, if you had, you would have caught this mistake (a simple and easy one to make) almost immediately.
You're right that I didn't. I suppose even if someone is familiar enough with how to solve a certain type of problem that they don't need to write down the units, it's still necessary to keep track of them meticulously in one's head.

#### WBahn

Joined Mar 31, 2012
28,486
You're right that I didn't. I suppose even if someone is familiar enough with how to solve a certain type of problem that they don't need to write down the units, it's still necessary to keep track of them meticulously in one's head.
It is my position and belief that there is never anyone that is so familiar with how to solve a particular type of problem that they can ignore units. The only time that you are justified in keeping track of them in your head is if you are doing ALL the math in your head (or if you are plugging the numerical values straight into a calculator). If you write a quantity down, then you write down the units since the units ARE part of the quantity.

We ALL make mistakes and we do so on a pretty regular basis. It doesn't matter how familiar we are with a type of problem or how simple the problem is. If you fail to track your units then you are choosing not to use what is very possibly THE most effective error detection and correction tool available to the engineer. As far as I'm concerned, that constitutes gross negligence and failure to exercise due professional diligence. On occasion, courts have agreed.

#### tjohnson

Joined Dec 23, 2014
611
It is my position and belief that there is never anyone that is so familiar with how to solve a particular type of problem that they can ignore units. The only time that you are justified in keeping track of them in your head is if you are doing ALL the math in your head (or if you are plugging the numerical values straight into a calculator). If you write a quantity down, then you write down the units since the units ARE part of the quantity.

We ALL make mistakes and we do so on a pretty regular basis. It doesn't matter how familiar we are with a type of problem or how simple the problem is. If you fail to track your units then you are choosing not to use what is very possibly THE most effective error detection and correction tool available to the engineer. As far as I'm concerned, that constitutes gross negligence and failure to exercise due professional diligence. On occasion, courts have agreed.
I agree that units are extremely important. I've heard someone emphasize this by mentioning how millions of dollars were wasted because of the failure of the Mars Climate Orbiter (which shows that NASA makes mistakes as well, but they can be quite costly).

#### WBahn

Joined Mar 31, 2012
28,486
I agree that units are extremely important. I've heard someone emphasize this by mentioning how millions of dollars were wasted because of the failure of the Mars Climate Orbiter (which shows that NASA makes mistakes as well, but they can be quite costly).
That's the classic example that is thrown out -- and it was hundreds of millions of dollars! That particular incident was a lot more complex than it seems at the surface, but I still believe that the mistake would not have been made if the engineering culture took units as seriously as they should.

But there are numerous other examples, most of which of course never get reported in such a way that they can be tracked down. There's the case of an airliner full of passengers that ran out of fuel because the people that fueled it couldn't be bothered with units and so just took a number and put in that much fuel in the units they usually worked with since they weren't given the units that the number was computed with, since those people couldn't be bothered to track their units, either. There have been numerous instances in which significant property damage, injury, and death have resulted from engineers being too lazy to track their units or ask if the answer makes sense.

Since academia doesn't care about units (since the overwhelming majority of academicians have little or no real-world work experience), the situation will not improve until the courts routinely hold the failure to track units as inexcusable negligence in both criminal and civil matters. THEN the academicians and the textbook writers will start addressing it.