Current flow

swty_todd,

I was wondering why the direction of the current is from left to right and not right to left as the current should flow from a higher potential to a lower potential.

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You are succumbing to the intuitive notion that charge movement direction should be determined by the movement of the charge carriers. Trouble is, that is the wrong way to do it. To see what I mean, read #26 of this link. http://forum.allaboutcircuits.com/showthread.php?t=11410&highlight=conventional&page=3. Then we can discuss it.

Ratch

Ratch said:

swty_todd,



You are succumbing to the intuitive notion that charge movement direction should be determined by the movement of the charge carriers. Trouble is, that is the wrong way to do it. To see what I mean, read #26 of this link. http://forum.allaboutcircuits.com/showthread.php?t=11410&highlight=conventional&page=3. Then we can discuss it.

Ratch

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and another thread
http://forum.allaboutcircuits.com/showthread.php?p=125063

309. Is there any definite direction to the flow of an electric current?

The magnetic and chemical effects of the current show a directionality.. Scientists have agreed to say that a current flows in one direction when certain effects appear, and say it flows in the opposite direction when these results are reversed; but it is not certain that what is called positive may not actually be negative. By common consent, a current acting in a certain manner is said to flow in a given direction, the point from which it flows being called positive and that to which it flows being called negative. This is somewhat analogous to what we call up and down in relation to gravity, ......


Electrical catechism By George Defrees Shepardson 1901

So settled over 100 years ago

http://books.google.com/books?id=Tg...=Electrical+Catechism&ei=0vnLSbqEDJHGzATlt9V1

As the discussion is diverging from the question in the thread - http://forum.allaboutcircuits.com/showthread.php?t=21254 - I have moved it to another location.

It's still quite difficult to have the charge carriers in a wire be anything other than electrons, and the charge they carry is uniformly negative. If I place a positive potential at one end of a wire connected to ground at the other end, it will attract electrons. Making the charge negative will repel electrons.

The nomenclature of "negative" may be entirely arbitrary, but it got pretty firmly settled in the early 1900's when Dr. Millikan did his oil drop experiments. I believe his work is definitive.

russ_hensel,

309. Is there any definite direction to the flow of an electric current?

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Current has existence and direction, but not flow. Current is charge flow. Current flow is charge flow flow, which does not make sense.

...a current acting in a certain manner is said to flow in a given direction,...

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No, current has an existence in a direction. Charge has a flow in a direction.

As I pointed out in my previous thread, using the real physical movement of the charge carriers to define current can be confusing. That is because current can also consist of positive charges in some non-metalic mediums. That would make it seem like two currents moving in opposite directions when they really have the equivalency of moving in the same direction.

The conventional current model disregards what the charge carriers are. It is a mathematical model that assumes that the positive side of a voltage source outputs a positive curent direction. After all the currents are computed and measured, then if necessary, the polarity of the charge carriers can be determined and the "real" flow of the charge carriers determined. The conventional current model is taught in most schools and is an industry standard. If you connect a diode to a battery with the positive side on the diode arrow, then charge will flow and current will exist in the diode. If you connect a battery with the positive post to the positive end of a current meter, then the meter will show current exists in the circuit in a "positive" direction. If you connect a battery with the positive post to the P side of a PN juction diode, then again current will exist. If you connect a battery with the positive post to the arrow of a PNP or NPN junction transistor, then current will exist. So use the conventional current model for calculations and evaluate the real flow if necessary by looking at the polarity of the charge carriers.

Ratch

beenthere,

It's still quite difficult to have the charge carriers in a wire be anything other than electrons, and the charge they carry is uniformly negative. If I place a positive potential at one end of a wire connected to ground at the other end, it will attract electrons. Making the charge negative will repel electrons.

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Yes, holes don't stand a chance of existing in the overwhelming sea of electrons within a conducting metal. Only in a semiconductor where negative charge carriers like electrons are scarce are they able to exist. Again, whether the carriers are negative or positive, the conventional model treats them mathematically the same. That is the strength of conventional charge flow.

Ratch

I guess I'm simply AMAZED that we were able to design and detonate dozens of nuclear warheads, build effective RADAR, develop long distance radio telemetry, AND put a man on the moon, while doing math with electricity flowing in the "wrong direction" the ENTIRE TIME!

Current is charge flow

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Whilst in principle I agree there is an inconsistency, I think the real situation is more complicated than such a simplistic statement.

You might like to review it in the light of this observation.

I can pass microwaves down a waveguide, inputting power (and current) at one end and extracting power (and current) at the other. I could say that I am using the guide to transmit electrical power or current from A to B. Yet no charges move or flow within the guide. In fact the best guides contain no charges whatsoever, only vacuum.

studiot,

I can pass microwaves down a waveguide, inputting power (and current) at one end and extracting power (and current) at the other. I could say that I am using the guide to transmit electrical power or current from A to B. Yet no charges move or flow within the guide. In fact the best guides contain no charges whatsoever, only vacuum.

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Focused electromagnetic radiation is not the same as sending a current through a wire. Both ways can transfer energy, but they do it in different ways.

Ratch

Yes I agree wholeheartedly that charge flow constitutes current so the 'phrase currrent' flow is inconsistent.

What I am asking isthe converse.

Is there any current which is not charge flow?

eg displacement current.

Whilst there is charge flow associated with the displacement current in a capacitor, there is none associated with the displacement current associated with an electromagnetic wave.
So is there still an inconsistency describing current flow in this case?

Ratch said:

Focused electromagnetic radiation is not the same as sending a current through a wire. Both ways can transfer energy, but they do it in different ways.

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Forcing hydraulic fluid through a tube is not the same as sending current through a wire either. All three ways can transfer energy, but do so in different ways. The term "current" applies to all three.

The point here, Ratch, is the definition. You prefer one definition, for the reasons you have listed ad nausium. Other folk prefer other definitions, for reasons others have listed ad nausium. You have demonstrated the ability to understand the other definitions. You have demonstrated math skill far above average. Why do you so object to a little thing like keeping track of a numeric polarity?

Ratch said:

Current has existence and direction, but not flow. Current is charge flow. Current flow is charge flow flow, which does not make sense.

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Current is rate of charge flow.

studiot,

What I am asking isthe converse.

Is there any current which is not charge flow?

eg displacement current.

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Yes, in particle physics, a neuton current does not any charge flow. Just a thought, a proton current would follow the conventional current model, wouldn't it?

Whilst there is charge flow associated with the displacement current in a capacitor, there is none associated with the displacement current associated with an electromagnetic wave.
So is there still an inconsistency describing current flow in this case?

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A electromagnetic wave can cause charge to flow, but not necessary while it is propagating. A common example is that there is no current between a transmitter and a receiver. At the receiver, however, the energy of the EM is used to produce voltage and thereby current.

Ratch

thingmaker3,

...The point here, Ratch, is the definition. You prefer one definition, for the reasons you have listed ad nausium....Why do you so object to a little thing like keeping track of a numeric polarity?

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Actually, I agree with the definition. Electrons really do travel from neg to pos, etc. But I advocate a particular and popular method of implementing that definition, for reasons I listed ad nauseam.

Ratch

recca02

Current is rate of charge flow.

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Correct, but not profound. Anything that flows has a rate. That is implicit.

By the way, whatever happened to recca01?

Ratch

Things are so much simpler in a vacuum tube, where the electrons actually DO flow at near the speed of light. At least inside the vacuum the charge carriers and current flow are one and the same. It's definitely not the case in a wire.

A free-space plasma is similar...the motion of the ions is the same as the current...though generally MUCH slower than the speed of light. (This is why you have things like long-delayed echoes generated in the ionosphere).

It's an ironic twist of history that the electron tube was developed before the transistor...when in fact the field effect "transistor" was discovered BEFORE the vacuum tube triode. The triode could be explained, while the field effect could not...so all the research funds went into the vacuum tube. If anyone had a clue as to why the field effect worked, the history of electronics would have been VERY different!

Eric

DO flow at near the speed of light.

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Of course at near light speeds fields can appear as magnetic to one observer and electric to another due to relativistic effects.

An interesting idea for your next SF novel, Eric