How does current flow in a circuit?

[mivey]

This discussion is getting way beyond what I believe to be the intended scope. First, we are examining currents at a node (or junction, if you prefer). It does not matter whether the branch conductors are connected to the load or the source, or whether the currents are going source to load or load to source. All that matters is the currents going through the node under examination and their measures.

Doesn't it always? :grin: I guess we need to get back to picking on Mayimbe for saying current never flows towards the source.

 

[Smart $]

If it makes it easier for you to understand, by all means stick to what you know. I prefer to define my currents in whatever manner is prudent for me at the time (and thanks for your permission to do so ).

Kirchhoff's current law can be stated both ways (actually 3 ways) and to say otherwise is just nonsense. We could site sources all day that show you are wrong to imply those that do not formulate KCL the way you do are making a mistake, but here are a few:

Slone's Standard Electrical Dictionary (1892):
When a steady current branches, the quantity of electricity arriving by the single wire is equal to the quantity leaving the junction by the branches. The algebraical sum of the intensities of the currents passing towards (or passing from) the junction is equal to zero; Summation(C) = 0 (Daniell.) In the last sentence currents flowing towards the point are considered of one sign and those flowing away from it of the other.

James Clerk Maxwell
An Elementary Treatise On Electricity (1881):
(re-stating Kirchhoff's Condition of 'continuity') At any point of the system the sum of all the currents which flow towards that point is zero.

Kendall L. Su - Georgia Tech
Fundamentals of Circuits, Electronics, and Signal Analysis-1978
Kirchhoff's current law (KCL): The sum of all currents entering any ambit at any instant must be zero.

Carter/Richardson - Univ. of Leeds
Techniques of Circuit Analysis - 1972
Kirchhoff's current law or Kirchhoff's node law, states that the algebraic sum of all the currents entering a junction point (i.e. a node) in a circuit is equal to zero

Handbook for Electricity Metering - EEI
Kirchhoff?s Current Law (KCL) can be stated in three ways:
1. The sum of the currents leaving a junction of conductors is zero at all times.
2. The sum of the currents entering a junction of conductors is zero at all times.
3. The sum of the currents entering a junction of conductors is equal to the sum of the currents leaving the junction of conductors.

Of the ways cited, only these are the accurate ones. The olive colored one depends upon how you interpret the word "ambit". I interpret it to be synonomous with "circuit". As such, the statement is in error in that current neither enters nor leaves a circuit... it flows within the circuit. The statement would be accurate regarding a sub-circuit... but it doesn't make that distinction, IMO, or perhaps it is cited out of context.

Denigrate my assertion(s) all you want... it won't change my position.

en?ter
Pronunciation: 'en-tər
Function: verb
Inflected Form: en?tered ; en?ter?ing \'en-t(ə-)riŋ\
Etymology: Middle English entren, from Anglo-French entrer, from Latin intrare, from intra within; akin to Latin inter between ― more at INTER-
Date: 13th century


intransitive verb
1 : to go or come in
2 : to come or gain admission into a group : JOIN ― often used with into
3 a : to make a beginning <entering upon a career> b : to begin to consider a subject ― usually used with into or upon
4 : to go upon land for the purpose of taking possession
5 a : to come onstage ― usually used in the subjunctive as a stage direction <enter Hamlet reading> b : to come into a preestablished situation or context like an actor coming onstage ― usually used in the subjunctive <enter the new principal with her radical ideas>
6 : to play a part : be a factor <other considerations enter when money is involved>
transitive verb
1 : to come or go into <enter a room>
2 : INSCRIBE, REGISTER <enter the names of qualified voters>
3 : to cause to be received or admitted <enter a child at a school>
4 : to put in : INSERT <enter the new data into the computer>
5 a : to make a beginning in <enter politics> b : to go into (a particular period of time) <enter middle age>
6 : to become a member of or an active participant in <enter the university> <enter a race>
7 : to make report of (a ship or its cargo) to customs authorities
8 : to place in proper form before a court of law or upon record <enter a writ>
9 : to go into or upon and take actual possession of (as land)
10 : to put formally on record <entering a complaint>
?en?ter?able \'en-t(ə-)rə-bəl\ adjective
?enter into
1 : to make oneself a party to or in <enter into an agreement>
2 : to form or be part of <your prejudices shouldn't enter into it>
3 : to participate or share in <enter into the spirit of the occasion>
?enter the lists : to engage in a fight or struggle
synonyms ENTER, PENETRATE, PIERCE, PROBE mean to make way into something. ENTER is the most general of these and may imply either going in or forcing a way in <entered the city in triumph>. PENETRATE carries a strong implication of an impelling force or compelling power that achieves entrance <the enemy penetrated the fortress>. PIERCE means an entering or cutting through with a sharp pointed instrument <pierced the boil with a lancet>. PROBE implies penetration to investigate or explore something hidden from sight or knowledge <probed the depths of the sea>.

x
enter-
Variant: or entero-
Function: combining form
Etymology: Greek, from enteron ― more at INTER-
: intestine <enteritis>

x
enter (as used in expressions)
enter the lists
enter into
breaking and entering




? 2005 Merriam-Webster, Incorporated

1leave
Pronunciation: 'lēv
Function: verb
Inflected Form: left \'left\ ; leav?ing
Etymology: Middle English leven, from Old English lǣfan; akin to Old High German verleiben to leave, Old English belīfan to be left over, and perhaps to Lithuanian lipti to adhere, Greek lipos grease, fat
Date: before 12th century


transitive verb
1 a (1) : BEQUEATH, DEVISE <left a fortune to his son> (2) : to have remaining after one's death <leaves a widow and two children> b : to cause to remain as a trace or aftereffect <oil leaves a stain> <the wound left an ugly scar>
2 a : to cause or allow to be or remain in a specified condition <leave the door open> <his manner left me cold> b : to fail to include or take along <left the notes at home> <the movie leaves a lot out> c : to have as a remainder <4 from 7 leaves 3> d : to permit to be or remain subject to another's action or control <just leave everything to me> e : LET f : to cause or allow to be or remain available <leave room for expansion> <left myself an out>
3 a : to go away from : DEPART <leave the room> b : DESERT, ABANDON <left his wife> c : to terminate association with : withdraw from <left school before graduation>
4 : to put, deposit, or deliver before or in the process of departing <I left a package for you> <leave a message>
intransitive verb : SET OUT, DEPART
?leav?er noun
?leave alone : to refrain from bothering, disturbing, or using
usage Leave (sense 2e) with the infinitive but without to <leave it be> is a mostly spoken idiom used in writing especially for humorous effect. It is not often criticized in British English, but American commentators, adhering to an opinion first expressed in 1881, still dislike it.

? 2005 Merriam-Webster, Incorporated

 

[Smart $]

It's killing me but I think you have me convinced.

Wow! Such a rare occurence I think I'll celebrate

 

[mivey]

Of the ways cited, only these are the accurate ones.

I'm reasonably sure that Maxwell knew what he was talking about. I am also sure that most of the world also thinks he knew what he was talking about. After all, he developed or laid the groundwork for much of what we engineers use today.

The olive colored one depends upon how you interpret the word "ambit". I interpret it to be synonomous with "circuit". As such, the statement is in error in that current neither enters nor leaves a circuit... it flows within the circuit. The statement would be accurate regarding a sub-circuit... but it doesn't make that distinction, IMO, or perhaps it is cited out of context.

While the larger ambit is called a "super-node" by some scholars, the smaller ambit can just be a simple node.

From Dr. Su's textbook:
"An ambit is a closed imaginary curve (in a two-dimensional drawing of a circuit) or surface (in a three dimensional construction of a circuit) that intersects a number of branches of a network. (It is understood that each branch can be intersected only once by an ambit)...the simplest ambit is one that can be drawn to enclose only a node."

Denigrate my assertion(s) all you want... it won't change my position.

And you would not be the first to try to assert that they alone have the true interpretation and that the rest of the world is in error. You heap the defamations upon yourself by claiming the rest of the engineering world is lacking the enlightenment that you alone are privy to.

Wow! Such a rare occurence I think I'll celebrate

I'm also surprised that iwire is considering buying into your nonsense after seeing another one of our members take a similar ridiculous stance based on one-sided definitions and an extremely narrow point of view.

 

[LarryFine]

No... but I would say that implies, of the women stepping into the bus, none are stepping out of, off, or from the bus.

If the bus is filled to capacity (as a conductor is "filled" with electrons), for each new passenger to enter, one must leave.

To me, current flows from the source to the load in one conductor, and from the load to the source in the other conductor.

Added: The same would apply to a node: current cannot enter a node without the same amount of current leaving it.

 

[mivey]

If the bus is filled to capacity (as a conductor is "filled" with electrons), for each new passenger to enter, one must leave. Added: The same would apply to a node: current cannot enter a node without the same amount of current leaving it.

Exactly. It is the sign convention that makes the different statements of KCL mean the same thing to those that understand it. Now using the concept of a negative number (which most of the calculating world is familiar with), let's look at the bus math:
Use the premise that for each person that gets on, one person must get off (our electrons).

You can #1: Let the people getting on the bus be a positive number of people added to the bus. Let the people getting off the bus be a negative number of people added to the bus. This is our positive and negative current flow.

You can also #2: Let the people getting off the bus be a positive number of people subtracted from the bus. Let the people getting on the bus be a negative number of people subtracted from the bus. Again, positive and negative current flow but kind of like on the other 1/2 cycle.

You can say the number of people added to the bus is zero: Johnny has 100 people on the bus. One person got on and one person got off. How many people were added to the number of passengers? Zero. The number of people added to the passengers was zero is the same as saying the sum of the currents into the node is zero.

You can say the number of people subtracted from the bus is zero: Johnny has 100 people on the bus. One person got off and one person got on. How many people were subtracted from the number of passengers? Zero. The number of people subtracted from the passengers was zero is the same as saying the sum of the currents out of the node is zero.

You can also say one person was added and one person was subtracted to yield a net change in the number of passengers equal to zero. Saying the number of people added and subtracted from the passengers must be the equal is the same as saying the sum of the currents into and out of the node must be equal.

From a mathematical standpoint, they are all equally valid ways to look at the change in the number of passengers.

Our world is rich in concepts of positive & negative currents, voltages, power flow, etc. and to limit the analysis to one method by calling the others inferior is being short-sighted and narrow-minded.

 

[Smart $]

If the bus is filled to capacity (as a conductor is "filled" with electrons), for each new passenger to enter, one must leave.

To me, current flows from the source to the load in one conductor, and from the load to the source in the other conductor.

Added: The same would apply to a node: current cannot enter a node without the same amount of current leaving it.

Quite true under the condition stated... but I must point out that I am being told that the sum of current entering OR leaving a node equals zero. Change that to AND and I have no complaints

 

[Smart $]

I'm reasonably sure that Maxwell knew what he was talking about. I am also sure that most of the world also thinks he knew what he was talking about. After all, he developed or laid the groundwork for much of what we engineers use today.While the larger ambit is called a "super-node" by some scholars, the smaller ambit can just be a simple node.

From Dr. Su's textbook:
"An ambit is a closed imaginary curve (in a two-dimensional drawing of a circuit) or surface (in a three dimensional construction of a circuit) that intersects a number of branches of a network. (It is understood that each branch can be intersected only once by an ambit)...the simplest ambit is one that can be drawn to enclose only a node."And you would not be the first to try to assert that they alone have the true interpretation and that the rest of the world is in error. You heap the defamations upon yourself by claiming the rest of the engineering world is lacking the enlightenment that you alone are privy to.
I'm also surprised that iwire is considering buying into your nonsense after seeing another one of our members take a similar ridiculous stance based on one-sided definitions and an extremely narrow point of view.

Exactly. It is the sign convention that makes the different statements of KCL mean the same thing to those that understand it. Now using the concept of a negative number (which most of the calculating world is familiar with), let's look at the bus math:
Use the premise that for each person that gets on, one person must get off (our electrons).

You can #1: Let the people getting on the bus be a positive number of people added to the bus. Let the people getting off the bus be a negative number of people added to the bus. This is our positive and negative current flow.

You can also #2: Let the people getting off the bus be a positive number of people subtracted from the bus. Let the people getting on the bus be a negative number of people subtracted from the bus. Again, positive and negative current flow but kind of like on the other 1/2 cycle.

You can say the number of people added to the bus is zero: Johnny has 100 people on the bus. One person got on and one person got off. How many people were added to the number of passengers? Zero. The number of people added to the passengers was zero is the same as saying the sum of the currents into the node is zero.

You can say the number of people subtracted from the bus is zero: Johnny has 100 people on the bus. One person got off and one person got on. How many people were subtracted from the number of passengers? Zero. The number of people subtracted from the passengers was zero is the same as saying the sum of the currents out of the node is zero.

You can also say one person was added and one person was subtracted to yield a net change in the number of passengers equal to zero. Saying the number of people added and subtracted from the passengers must be the equal is the same as saying the sum of the currents into and out of the node must be equal.

From a mathematical standpoint, they are all equally valid ways to look at the change in the number of passengers.

Our world is rich in concepts of positive & negative currents, voltages, power flow, etc. and to limit the analysis to one method by calling the others inferior is being short-sighted and narrow-minded.

I don't have a problem with your positve, negative association to currents if that is how you want to envision it. All I'm saying it is an unnecessary convention adoption in using static phasor analysis of currents associated with a single node.

Maxwell quite likely knew what he was talking about, but I think he and perhaps the others must have skipped English 101 and perhaps, if offered, Logic 101.

How many people do you know that can enter a doorway once they are already in it? How many people do you knoe that can leave a doorway without entering it first?

 

[mivey]

I don't have a problem with your positve, negative association to currents if that is how you want to envision it. All I'm saying it is an unnecessary convention adoption in using static phasor analysis of currents associated with a single node.

Maxwell quite likely knew what he was talking about, but I think he and perhaps the others must have skipped English 101 and perhaps, if offered, Logic 101.

How many people do you know that can enter a doorway once they are already in it? How many people do you knoe that can leave a doorway without entering it first?

I think I see the trap you have fallen into. We are not counting objects, but rather the rate of flow of those objects.

I agree that the individual physical object is not entering and leaving at the same time (quantum physics aside), but the net rate of flow can be positive or negative or you can sum the individual positive & negative flows.

We are not counting electrons (actually holes given the standard sign conventions for current) but the rate of flow. We can measure the negative flow of elections or the positive flow of holes to measure how many electrons (or holes) are passing each second, or measure the reverse flow.

If we are measuring a lake level, we know for a constant level one water molecule must flow out for every water molecule that flows in.

We can measure the rise of the lake level, the fall of the lake level, or we can measure the flow of all the channels, rivers, tidal waterways, etc. connected to the lake.

 

[Smart $]

I think I see the trap you have fallen into. We are not counting objects, but rather the rate of flow of those objects.

I agree that the individual physical object is not entering and leaving at the same time (quantum physics aside), but the net rate of flow can be positive or negative or you can sum the individual positive & negative flows.

We are not counting electrons (actually holes given the standard sign conventions for current) but the rate of flow. We can measure the negative flow of elections or the positive flow of holes to measure how many electrons (or holes) are passing each second, or measure the reverse flow.

If we are measuring a lake level, we know for a constant level one water molecule must flow out for every water molecule that flows in.

We can measure the rise of the lake level, the fall of the lake level, or we can measure the flow of all the channels, rivers, tidal waterways, etc. connected to the lake.

I had a feeling you were going to change the topic to rates... so perhaps I used a bad example... but it doesn't matter. You're still adopting the positve-negative convention to make your point. Yes you make your point... but your point does not counter mine. It makes no difference how many ways you try to rehash it.

As I said, for static phasor analysis, adopting a positive-negative convention is unnecessary. Each branch current associated with a node has both current flowing into and out of the node (just not at the same time ...and except when the branch current is zero). A [positive-value-only] magnitude coupled with a phase angle describes this current [waveform] without using a positive-negative convention. You have yet to convince me otherwise.

 

[LarryFine]

A [positive-value-only] magnitude coupled with a phase angle describes this current [waveform] without using a positive-negative convention.

This sounds just like the phase-vs-timing discussion about the center-tapped transformer secondary.

 

[neutral]

Seems like this thread has a few folks that believe that if you can?t dazzle them with brilliance then baffle them with BS....:roll: I'm Baffled

 

[cschmid]

man now this is good and we all most went to quantum physics as well. I think we could swing the pendulum so quantum physics slide into this as well since we talking timing.

 

[ohmhead]

How current flows

How current flows

Well here how a simple electrician thinks it works .

Each phase of a wye transformer has a common connection when a load is connected to transformer coil A phase current flows from A phase thur the load to A phase coil opposite end by the common neutral center tap connection. Think polarity wise in the cycle of the wave

Think polarity when A phase is at its peak positive cycle B &C phases are at 50 % going negative current flows into load and to the same phased coil . this is at 120 volts out A and returns to A .

Now when a 208 volt load is attached from A phase to B phase current flows from A coil to B coil thur the load at any point in time these are still 120 degs apart so the voltage is not to me fully at peak to peak in the cycles but at some level like 208 volts . The voltage can be any voltage it depends on the turns & windings used but we need 120 volts so we get 208 volts phase to phase .


Current flows from source A to return A at 120 volts



Current flows from A to B phase when 208 volts .


I will not mention unbalanced loads only a purely resistive load will give no neutral current not in this world today on each leg .

 

[Smart $]

This sounds just like the phase-vs-timing discussion about the center-tapped transformer secondary.

Similarities exist, in both the discussions and the subjects.

 

[Smart $]

Seems like this thread has a few folks that believe that if you can?t dazzle them with brilliance then baffle them with BS....:roll: I'm Baffled

Good to see your not pointing a finger... or is it just that we can't see it

 

[Mayimbe]

The negative cycle is when the waveform is negative relative to the reference point (on the opposite side of the zero axis from the positive side). You should know this.

Maybe it is a translation thing. Are you blending current/energy/power?

Maybe it is, but let me say that in spanish with call it "the negative semicycle" and I think its more accurate, since semi is a prefix that denotes the half of something. And a negative cycle is always negative at all times at any milisecond.

In a pure reactive load, power is flowing in for 1/2 cycle, then flowing out the other 1/2 cycle. In a pure resistive load, power is absorbed in both cycles. If the voltage and current have the same sign, then the load is absorbing energy, if the signs are different, then the load is delivering energy.

If you think current never changes direction, you are thoroughly confused. I find that hard to believe, so there must be something that is getting lost in the translation.

when you say that in a pure reactive load, power is flowing in for 1/2 cycle, what kind of power do you mean?? Reactive Power?? If it is, I will agree with you, if it is not I will not. And I believe you are wrong. You have to specify what kind of reactive load are we talking, is it inductive o capacitive?? it matters. In both cases for me, these pure reactive loads are consuming current.

The current does change the direction, but it "flows from the source to the load". The first time that I have heard the opposite was here.

This discussion is getting way beyond what I believe to be the intended scope. First, we are examining currents at a node (or junction, if you prefer). It does not matter whether the branch conductors are connected to the load or the source, or whether the currents are going source to load or load to source. All that matters is the currents going through the node under examination and their measures.

Doesn't it always? :grin: I guess we need to get back to picking on Mayimbe for saying current never flows towards the source.

A source, its an energy active entity, it CAN deliver energy.

A load, its an energy pasive entity, it CAN NOT deliver energy. For long periods of time, eternity.

since that in electricity, the only way to deliver energy to others its through the current, how can the current flow from the load to the source???

If the bus is filled to capacity (as a conductor is "filled" with electrons), for each new passenger to enter, one must leave.

To me, current flows from the source to the load in one conductor, and from the load to the source in the other conductor.

If there is not such thing as the other conductor?? will the current flow?? In some sort of circuits it does flows. The other conductor as you called its an imperative condition for current to flow.

 

[LarryFine]

If the bus is filled to capacity (as a conductor is "filled" with electrons), for each new passenger to enter, one must leave.

To me, current flows from the source to the load in one conductor, and from the load to the source in the other conductor.

If there is not such thing as the other conductor?? will the current flow?? In some sort of circuits it does flows. The other conductor as you called its an imperative condition for current to flow.

You cannot have current without a complete circuit; thus the "other conductor" must exist for this discussion. If I'm not mistaken, you agree with this. Just to elaborate:

Picture a closed loop of pipe, with a pump (the source) on one side, and a driven impeller (the load) on the other side. Just as much water flows into the pump as leaves it.

The pump imparts the energy into the flow, and the driven impeller impedes it. That defines the source and the load. But, the water flows in a complete circuit, both from and to the source.



Just to throw in reactive current flow:

Put an expansion tank on each side of the loop, one near the source, and one near the load, excess water may flow from tank to tank, oscillating at a "resonant" frequency, and "modulating" the main flow.

The pump doesn't have to supply the energy for the oscillating flow, but the piping between the two tanks must carry both all of the main flow and all of the oscillating flow.

 

[Mayimbe]

You cannot have current without a complete circuit; thus the "other conductor" must exist for this discussion. If I'm not mistaken, you agree with this. Just to elaborate:

Picture a closed loop of pipe, with a pump (the source) on one side, and a driven impeller (the load) on the other side. Just as much water flows into the pump as leaves it.

The pump imparts the energy into the flow, and the driven impeller impedes it. That defines the source and the load. But, the water flows in a complete circuit, both from and to the source.

Just to throw in reactive current flow:

Put an expansion tank on each side of the loop, one near the source, and one near the load, excess water may flow from tank to tank, oscillating at a "resonant" frequency, and "modulating" the main flow.

The pump doesn't have to supply the energy for the oscillating flow, but the piping between the two tanks must carry both all of the main flow and all of the oscillating flow.

I agree but not a 100 %, three phase circuits can survive without this "other conductor"

Im not understanding this analogy hidraulic systems quite well... sorry

 

[LarryFine]

I agree but not a 100 %, three phase circuits can survive without this "other conductor"

Not really, because the other two phases act together to comprise the "other conductor".

Im not understanding this analogy hidraulic systems quite well... sorry

Do you understand how reactive power bounces back and forth between the reactive load component and the source (or PF corrective device, if used)?

The source doesn't have to generate the reactive current, but the system has to carry it, and it does contribute to conductor heating through I^2R losses.