DC Analysis Question

Thread Starter

magicalengineer

Joined Dec 8, 2022
2
I tried superposition method, shorted the voltage source first and then disconnect the current source but then I stuck. Secondly I tried loop equations but don't understand what the 2amps means, the resistors should determine the amps right? But then what does the 2amp current source is doing I am confused.
 

DickCappels

Joined Aug 21, 2008
9,513
The symbol with an arrow pointing down represents a current source. It is injecting 2 amp into the circuit.

I wish I could tell you whether the arrow points the direction of electron flow or hole flow. Maybe somebody with more smarts will answer that part.
 

WBahn

Joined Mar 31, 2012
27,894
I tried superposition method, shorted the voltage source first and then disconnect the current source but then I stuck. Secondly I tried loop equations but don't understand what the 2amps means, the resistors should determine the amps right? But then what does the 2amp current source is doing I am confused.
You need to SHOW your work. How else can we see what you've done right and where you've gone wrong so that we can help you get back on track towards solving it?

It sounds like you need to take a bit of a step back and review your basic components.

An ideal voltage source is a device that will produce whatever current is needed, be it positive, negative, or zero, in order to maintain a specific voltage across it.

An ideal current source is a device that will produce whatever voltage is needed, be it positive, negative, or zero, in order to maintain a specific current through it.
 

Papabravo

Joined Feb 24, 2006
19,595
You also need to know the answers to the following two questions:
  1. Do you know how voltage drops and voltage rises behave going around a closed loop?
  2. Do you know how currents behave going into and out of a node?
 

WBahn

Joined Mar 31, 2012
27,894
Makes no difference if you assume the appropriate current/electron flow direction from the voltage sources, for whichever assumption you make..
Huh?

So let's say that we have two pieces of paper, one of which says to show a current source with the arrow pointed in the direction of conventional current flow and the other says to draw it in the direction of electron flow.

Two people are given the two pieces of paper. One of them draws the circuit as shown (arrow pointing down) and the other draws the circuit with the arrow pointing up. Those are the only two differences between the two drawings.

They now give them to you and ask you to analyze the circuit. You are claiming that you are going to get the same answer for both of them, since it does matter what convention the person drawing it used as long as the person analyzing it is consistent.

Let's assume that's true.

Now, tomorrow, those same two people draw schematics for two different circuits where the only difference is the polarity of the current source. They both use the same convention as far as the direction of the arrow.

They now give them to you and ask you to analyze them. The two drawing they give you are identical to the two drawings you were given the day before and to which you claim that you would have gotten the same answer. That must still apply to these two drawings, which means that the conclusion is that the polarity of a current source has no impact on the behavior of the circuit.

Since this is obviously not the case, the premise that the convention used by the person that drew the schematic makes no difference must be wrong.
 

RBR1317

Joined Nov 13, 2010
706
My go-to analysis technique is always nodal analysis. But before writing the node equations, the schematic must be marked-up to identify the known and unknown node voltages. I use grey for known voltages (power & ground) and separate colors for unknown voltages. However, if you don't use nodal analysis then my only advice is: Use Nodal Analysis, always.
 

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WBahn

Joined Mar 31, 2012
27,894
I tried superposition method, shorted the voltage source first and then disconnect the current source but then I stuck. Secondly I tried loop equations but don't understand what the 2amps means, the resistors should determine the amps right? But then what does the 2amp current source is doing I am confused.
Still waiting to see your best attempt (or AN attempt). You say you tried superposition. That will work just fine. So show what you did to try to use superposition.
 

crutschow

Joined Mar 14, 2008
31,126
Huh?

So let's say that we have two pieces of paper, one of which says to show a current source with the arrow pointed in the direction of conventional current flow and the other says to draw it in the direction of electron flow.
Apparently I wasn't clear in my short explanation.

The drawing does not change (why would it?).

If you assume the arrow points in the direction of conventional current-flow then, to be consistent, you also must assume that current flows out of the positive terminal of the voltage source.

If you assume the arrow points in the direction of electron flow then, again to be consistent, you also must assume that the electrons flow out of the negative terminal of the voltage source.

Either way will give you the correct answer.
 

WBahn

Joined Mar 31, 2012
27,894
Apparently I wasn't clear in my short explanation.

The drawing does not change (why would it?).

If you assume the arrow points in the direction of conventional current-flow then, to be consistent, you also must assume that current flows out of the positive terminal of the voltage source.

If you assume the arrow points in the direction of electron flow then, again to be consistent, you also must assume that the electrons flow out of the negative terminal of the voltage source.

Either way will give you the correct answer.
Again, this leads directly to that same contradiction.

Would you agree that if we build that circuit and measure the voltage across the 10 Ω resistor as indicated, that we would get a different result than if we then turn that 2 A current source around?

So now let's say that Fred, whose is an adherent to conventional current, constructs that circuit such that conventional current is going from top to bottom through that source. He then draws the schematic up and sends it to you for you to analyze, wanting you to send him back the expected result to compare to his measurement.

Next door, Sue, whose is an adhere to electron-current, constructs that circuit such that electron current is going from top to bottom through that source, meaning that her current source is connected exactly the opposite of Fred's. She then draws the schematic up and sends it to you for you to analyze, again wanting you to send her back the expected result to compare to her measurement.

The two schematics you receive are identical and you have know idea which current convention either Fred or Sue prefer.

Please show how you are going to get the two different correct answers from the same schematic and how you will know which answer to send to whom.

There are only three possible solutions:
First, the premise that Fred and Sue will obtain different measurement results is wrong, which would require that the polarity of a current source has no effect.
Second, the claim that the schematics provided by Fred and Sue will be the same is wrong, which requires an explanation of how and why they would differ.
Three, the information about which convention that polarity arrow adheres to matters and you can't properly analyze the circuit without knowing that information.
 

crutschow

Joined Mar 14, 2008
31,126
Again, this leads directly to that same contradiction.

Would you agree that if we build that circuit and measure the voltage across the 10 Ω resistor as indicated, that we would get a different result than if we then turn that 2 A current source around?
Of course.
But why do you persist in wanting to turn the current source around, which obviously changes the circuit, and leads to the supposed contradiction?

The arrow can be assumed to point in the direction of either current flow, or electron flow.
It makes no difference as long as the proper direction is also assumed for the flow through the voltage sources.

It's a basic premise that all circuits can be correctly analyzed with either current flow or electron flow, as long as you are everywhere consistent in the assumption.

Do you not agree with that?
 

WBahn

Joined Mar 31, 2012
27,894
Of course.
But why do you persist in wanting to turn the current source around, which obviously changes the circuit, and leads to the supposed contradiction?
Because that SAME schematic can represent EITHER circuit depending on the convention used by the person DRAWING the schematic.

The arrow can be assumed to point in the direction of either current flow, or electron flow.
It makes no difference as long as the proper direction is also assumed for the flow through the voltage sources.

It's a basic premise that all circuits can be correctly analyzed with either current flow or electron flow, as long as you are everywhere consistent in the assumption.

Do you not agree with that?
I would agree with it IF people that used electron flow used it correctly, but they universally don't. When they say that a current is 1 A to the right, they invariably mean that the electrons are flowing to the right. Even though they acknowledge that electrons are negatively charged, the insist that there is somehow a positive amount of charge flowing to the right and then patch things up with magical mystery minus signs sprinkled through their work.

So, based on how virtually everyone uses electron flow, I do not agree with your basic premise. That would only be the case as long as the schematic contains no information that is dependent upon which convention was used. As soon as the schematic includes such information, BOTH the person drawing the schematic AND the person reading the schematic must be consistent with each other.

Let's look at about the simplest possible example -- the voltage across a resistor.

1670701266185.png

What is the value of V_R?

You are claiming that, in order to answer that question, you do not need to know whether that 2 A current was drawn by someone using conventional current or using electron flow current.

So is V_R equal to +8 V or -8 V?
 

crutschow

Joined Mar 14, 2008
31,126
Okay, after relooking at things I realize I had my head on backwards, and the current direction of the current source arrow would have to change if you go from current flow to electron flow.
Sorry for that, I think my old timer's disease is showing. :oops:
 

WBahn

Joined Mar 31, 2012
27,894
Okay, after relooking at things I realize I had my head on backwards, and the current direction of the current source arrow would have to change if you go from current flow to electron flow.
Sorry for that, I think my old timer's disease is showing. :oops:
Yeah, I'm increasingly showing symptoms of that malady, myself. Glad we got it resolved.

To answer Dick's query, while I agree that it would make a difference if the current source was defined using electron flow (as universally applied), I think it is a reasonable assumption to make that the schematic is based on conventional current unless something strongly implies otherwise.
 
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