MWBC= more heat or Less heat?

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mivey

Senior Member
...if a take a circuit with a 12v battery, a 12 ohm resistor and two 0.5 ohm impedance conductors, with 0.923 amps flowing through the circuit, and I decide to take out one of the 0.5 ohm conductors and replace it with a 1 ohm conductor, leaving the rest of the circuit the same, then I will get 0.888 Amps flowing through the new conductor from "the rest of the circuit" without increasing the voltage in the circuit.
But of course. That has never been disputed. But the conductor has now become parasitic to "the rest of the circuit".
 

mivey

Senior Member
Regarding David's answers to my questions:

Example 1: 12 volt source with 0 ohm circuit, current = infinity

Example 2: 480 volt source with 0 ohm circuit, current = infinity

Looks like ohm's law has gone out the window. Raising the voltage saw no corresponding increase in current.

Ohm's law at the extreme edge of theory fails.
Unlimited power corrupts unlimitedly.:grin:

Example 1: I = P/12
Example 2: I = P/480
:grin:
 

Rick Christopherson

Senior Member
The electrons moving in these neutrals are NOT moving BECAUSE of the potential difference from one end to the other. The electrons are moving BECAUSE we have applied a POTENTIAL SOURCE to the circuit. The potential source and the potential source alone is what moves the electrons in the circuit.
Ohm's law: "The current through a conductor between two points is directly proportional to the potential difference or voltage across the two points."
You are contradicting yourself because you are trying to make a "chicken or the egg" argument. Did the current create the voltage, or did the voltage create the current?
The potential difference that you measure from one end to the other of the neutral wire is a RESULT of the current that flows in the circuit.
So do the electrons discriminate? Do they say, "Hey, this isn't a real voltage because the force we are feeling does not have a Radio Shack trademark stamped on it." Of course not. The electrons don't know what or where the source of the voltage is, but they move nonetheless. It doesn't matter if that voltage is caused by a Radio Shack power supply, voltage division across loads, or rubbing the cat's fur.

Do electrons move because there is a potential difference, or because their neighbors are pushing on them? Is the voltage drop across a wire causing the electrons to move, or is the voltage drop the result of the electron movement? It actually doesn't matter, because it is just semantics, and everything is all interrelated. If anything, you would have a harder time arguing that the current through a wire is causing the voltage drop, because you cannot define the mechanism creating the voltage, aside from restating Ohm's Law.
 

david luchini

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Engineer
You are contradicting yourself because you are trying to make a "chicken or the egg" argument. Did the current create the voltage, or did the voltage create the current? So do the electrons discriminate? Do they say, "Hey, this isn't a real voltage because the force we are feeling does not have a Radio Shack trademark stamped on it." Of course not. The electrons don't know what or where the source of the voltage is, but they move nonetheless. It doesn't matter if that voltage is caused by a Radio Shack power supply, voltage division across loads, or rubbing the cat's fur.
Rick, it is clear from your earlier posts with the "Mystery Minus Signs" that you are having trouble grasping basic electrical concepts, but I will endeavor to answer your points here. I have not contradicted myself, I have said all along that you must have a potential source to create current flow in the circuit. "Do electrons discriminate?" I have no idea what point you are trying to make with this thought. After rambling about Radio Shack and discriminating electrons, you come up with the same answer as I have repeated over and over. There most be a potential source in the circuit for the current to flow.

Do electrons move because there is a potential difference, or because their neighbors are pushing on them? Is the voltage drop across a wire causing the electrons to move, or is the voltage drop the result of the electron movement?
The electrons move because there is a potential difference AND a completed circuit path. Their neighbors pushing on them has nothing to do with current flow. The voltage drop across the wire does NOT cause the electrons to move. I have already demonstrated that. Yes, the voltage drop is the result of electron movement (or current flow.)

It actually doesn't matter, because it is just semantics, and everything is all interrelated. If anything, you would have a harder time arguing that the current through a wire is causing the voltage drop, because you cannot define the mechanism creating the voltage, aside from restating Ohm's Law.
No, it is NOT just semantics. If we have a 120V potential source, and run circuit conductors out to a load location, and connect the line side of the load, but leave the other end of the load disconnect, we will have a potential for current to flow in the circuit, but we have no current flow. We can measure 120V from A-N at the source, we can measure 120V from the end of the A conductor to N at the source, or 120V from the end of the A conductor to the end of the N conductor, and we can measure 120V from the "load" end of the resistor to N at the source. Conversely, we will measure 0V across conductor A, 0V across conductor N, and 0V from the "load" end of the resistor to A at the source.

If we then connect the open circuit at the N conductor, the potential that already exists will cause current to flow in the circuit. There will be a voltage drop along each conductor and the load because the current flowing in the circuit.

Now consider if we remove the potential source from the circuit and replace it with another conductor. We still have a closed circuit, rather than an open circuit, so an open circuit is not preventing current from flowing. However, we no longer have a potential source in the circuit, so current is not flowing. It is the current flowing in the circuit that CREATES the voltage drop. The current is created by the potential source in the circuit. This is so elemental, I don't know why you are arguing the point.
 

david luchini

Moderator
Staff member
Location
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Engineer
Regarding David's answers to my questions:

Example 1: 12 volt source with 0 ohm circuit, current = infinity

Example 2: 480 volt source with 0 ohm circuit, current = infinity

Looks like ohm's law has gone out the window. Raising the voltage saw no corresponding increase in current.

Ohm's law at the extreme edge of theory fails.
Crossman gary,

No, you are still not grasping Ohm's law. Ohm's law does NOT say that increasing the voltage in the circuit will increase the current in the circuit. What Ohm's law says is "the current through a conductor between two points is directly proportional to the potential difference or voltage across the two points, and inversely proportional to the resistance between them." In other words, both the potential and resistance are acting on the current at the same time.

If the potential is zero, then current cannot flow in the circuit, and it does not matter what the resistance is. The resistance can be anywhere from zero (short circuit) and infinity (open circuit) and the current in the circuit will always be zero.

If the potential is not zero, but is constant, then a change of the impedance in the circuit will change the current in the circuit. The impedance can anywhere from infinity (open circuit,) where zero current will flow, to zero (short circuit) where infinite current will flow.

Lets take an example of an "ideal" adjustable potential source (with no internal impedance,) an adjustable impedance source, and "ideal" conductors completing the circuit. Lets say we start the source at 480V, and the load at 1 Ohm. The current will be 480 Amps. Now lets reduce the voltage to 120V, the current will be 120 Amps. Now, reduce the load from 1 Ohm, to 0.25 Ohm - the load current will go up to 480 Amps, and if we then return the voltage to 480V, the current will go up to 1920 Amps.

Now with the voltage still at 480V, lets drop the load to 0.001 Ohms. The current will become 480,000 Amps. If the load drops to 0.0000023 Ohms, then the current will become approx. 208,695,000 Amps. If at this point, we then reduce the voltage to 120V, the current becomes 52,173,913 Amps. If from this point, we keep lower the impedance, the current will continue to rise, until we reach 0 ohms (short circuit) and the current flowing in the circuit will be infinite. If we then change the voltage again, changing if from 120V to 75V, the current will not change, because ohm's law says the current will be proportional to the voltage AND inversely proportional to the resistance. Since the 0 ohms resistance has not changed, the current has not changed. This is in keeping with Ohms law, not in contradiction to Ohms law.

And finally, if we take our circuit in its current state, 75Volts and 0 ohms impedance, with infinite current flowing, and turn the voltage down to zero, the current in the circuit will be zero. Both parts of Ohm's law must be applied to any circuit - the proportionality to the voltage and the inverse proportionality to the impedance.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Regarding David's answers to my questions:

Example 1: 12 volt source with 0 ohm circuit, current = infinity

Example 2: 480 volt source with 0 ohm circuit, current = infinity

Looks like ohm's law has gone out the window. Raising the voltage saw no corresponding increase in current.

Ohm's law at the extreme edge of theory fails.
No, we just don't have the ability to see the numbers. ;)

1. 12v source with 0 ohm circuit: current = infinity.

2. 480v source with 0 ohm circuit: current = 40 x infinity. :grin:

Of course, there's no circuit with zero impedance, nor a source with infinite current.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Another way to state it is to say the added force required (voltage) to overcome the resistance to the flow of moving charges (resistance) is proportional to the product of the current and resistance.
Of course, in the real world, we consider the source voltage and load impedance to be constants. There are very few constant-current sources.
 

crossman gary

Senior Member
Just to make sure my conjectures are understood:

1. To have current flow in a conductor, there must be a potential difference from one end of the conductor to the other. It is my understanding that David has contradicted this by saying that current can flow in a conductor without a potential difference between the ends.

2. Ohm's Law cannot be relied upon to give proper answers when dealing with extreme conditions such as zero resistance.

Now, David, please consider the following:

Starting with two 12 volt batteries which are ideal and infinite with no internal resistance as follows:



Now connect a "load" consisting of a conductor with zero resistance across the batteries as shown below:



With respect to the centerpoint between the batteries labeled as 0v, please assign voltages/polarities to the other points A through G. In other words, take a theoretically perfect voltmeter, attach one lead to the 0v point between the batteries, then use the other lead to measure voltage at each of the other points. Can David or anyone else tell us what we would read, and if you can, please do so.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
This is in keeping with Ohms law, not in contradiction to Ohms law.
I agree with that. Sometimes, people point out what they think are inconsistancies, such as the incandescent bulb with the varying filament impedance, but even then, at any given moment, Ohm's law still applies.

Nobody says that a lineaear relatiuonship over time must apply. Ohm's Law is an instantaneous phenomenon. If there could be zero impedance and infinite current, Ohm's Law still works, it's just unmeasurable.
 

crossman gary

Senior Member
No, you are still not grasping Ohm's law. Ohm's law does NOT say that increasing the voltage in the circuit will increase the current in the circuit. What Ohm's law says is "the current through a conductor between two points is directly proportional to the potential difference or voltage across the two points, and inversely proportional to the resistance between them." In other words, both the potential and resistance are acting on the current at the same time.
If the resistance is the same for both voltages, then the higher voltage must produce a higher current. In this case it doesn't. Ohm's Law doesn't accurately predict the outcome of extreme cases.

There are also issues with power. You had given an example of a set voltage, and then slowly lowering the resistance of the circuit. As the resistance gets smaller, the current increases. So far so good.

Since P = I x I x R, then as the resistance is lowered and current is increased, the power also increases with lower resistance. And the closer we get to zero, the higher the power is. But then, as soon as we hit zero resistance, we have infinity x infinity x zero and what does math say about that? Is that infinity, or is that zero?
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
1. To have current flow in a conductor, there must be a potential difference from one end of the conductor to the other. It is my understanding that David has contradicted this by saying that current can flow in a conductor without a potential difference between the ends.
I'd modify that statement to reflect that, if the conductor in question were to be opened and a voltage difference would occur, then connecting that conductor would result in a current.

2. Ohm's Law cannot be relied upon to give proper answers when dealing with extreme conditions such as zero resistance.
It still applies, it's just that the readings are beyond our capabilities. No ammeter has an infinite full-scale capability.



Can David or anyone else tell us what we would read, and if you can, please do so.
I'd say it can't be done. But, if I had to give an equally-impossible answer it's that you'd read zero volts at every point.
 

crossman gary

Senior Member
If there could be zero impedance and infinite current, Ohm's Law still works, it's just unmeasurable.
If there could be a zero impedance, and an infinite current, then why can't there be an ideal perfect theoretical meter which can measure it? Assuming these things exist and then assuming they can't be measured is pointless.
 

crossman gary

Senior Member
I'd say it can't be done. But, if I had to give an equally-impossible answer it's that you'd read zero volts at every point.
Thanks Larry. My point exactly. So here is a case with infinite current but there is no potential difference in the circuit? I'm just pointing out what I see as inconsistencies with zero resistances.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Thanks Larry. My point exactly. So here is a case with infinite current but there is no potential difference in the circuit? I'm just pointing out what I see as inconsistencies with zero resistances.
That's why my point is that infintes are pointless to discuss.
 

crossman gary

Senior Member
If the potential is zero, then current cannot flow in the circuit, and it does not matter what the resistance is.
Man, we seem to be arguing all over the place.

Originally, you stated that current can flow in a neutral even though no potential existed across that neutral. Now you are saying that current cannot flow under zero potential. What gives?

Of course, your conjecture above cannot be proven. When we get to the point of zero resistance, it may very well be that quantum effects cause the electrons to go rocketing all over the place. Saying that a potential is required to move an electron through a zero impedance is equivalent to saying that there is some sort of impedance in that zero impedance.

You are saying that a zero impedance contains some quality which prevents electrons from moving unless a voltage is applied? Again, if this is what you are thinking, then a zero resistance is not really a zero resistance.... zero resistance actually has resistance? Maybe the electrons have inertia which must be overcome? But does Ohm's law say anything about that?

Zero resistance is getting into the realm of quantum mechanics where electrons are not little balls being kicked around. They are waves which behave by different laws than Ohm's.
 

Rick Christopherson

Senior Member
Rick, it is clear from your earlier posts with the "Mystery Minus Signs" that you are having trouble grasping basic electrical concepts, but I will endeavor to answer your points here.
At ease there little buckaroo, this forum is not populated with hacks, hicks, and idiots. Instead of addressing issues that quite knowledgeable people bring up, you have a tendency of sidestepping them with belittling comments. You might want to be a little more careful with what you say and how you say it.
Crossman gary,

No, you are still not grasping Ohm's law. Ohm's law does NOT say that increasing the voltage in the circuit will increase the current in the circuit.
For example, here you tell the poster that he doesn't understand Ohm's Law, and then you go on to make a complete misstatement of Ohm's Law. Please explain how Ohm's Law does not predict an increase in current when the voltage is increased for a given circuit.


I have not contradicted myself, I have said all along that you must have a potential source to create current flow in the circuit. "Do electrons discriminate?" I have no idea what point you are trying to make with this thought. After rambling about Radio Shack and discriminating electrons, you come up with the same answer as I have repeated over and over. There most be a potential source in the circuit for the current to flow.
Those "Radio Shack Ramblings," as you put it, were to point out that you are suggesting that there is some sort of difference between a "potential source" and any other type of "potential difference." The electrons don't care where or why a difference in potential exists. When they encounter a potential difference, they will want to move (so long as they are permitted to move--i.e. a complete circuit).

So contrary to your assertion, I beileve it is you that has lost sight of the basic concepts behind electrical systems, specifically, electron movemement. An electron will not move unless there is a force acting on it (ignoring drift and inertia, as those are separate topics).
The electrons move because there is a potential difference AND a completed circuit path. Their neighbors pushing on them has nothing to do with current flow. The voltage drop across the wire does NOT cause the electrons to move. I have already demonstrated that. Yes, the voltage drop is the result of electron movement (or current flow.)
You don't see the contradictions in these statements? :confused: If the voltage drop across a conductor doesn't cause electrons to move, nor do they move because their neighbors push them, then how do the electrons know that a voltage differential exists in order to move? Do they pass the word along from electron-to-electron that somewhere way up the circuit someone has connected a power supply? (oops, just in case you didn't realize it, like the Radio Shack comments, that was a contemptuous joke, not intended to be percieved as scientific intercourse.)
No, it is NOT just semantics.
Yes, it is just semantics, but unfortuantely, you can't see this.
 

Rick Christopherson

Senior Member
Man, we seem to be arguing all over the place.
Gary, I mean no disrespect, but this is way off topic from this thread. It is nevertheless an interesting topic, but it is completely departed from the other aspects of this thread.

I would enjoy discussing this topic, but right now this thread is branching off in two completely different directions that are not immediately related. To properly address this topic, I would suggest that a new thread be started regarding how Ohm's Law behaves at extreme conditions. That's the only reason why I have not commented on your previous postings.
 
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