Direction of current flow

[gar]

151107-1309 EST

K8MHZ:

The output polarity of a standard ignition coil can be defined by the winding direction of the primary coil to the secondary coil. Thus, no great need to determine battery polarity for the purpose of controlling spark polarity.

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[gar]

151107-1324 EST

Johnnybob:

In a physical external circuit (meaning outside of an energy source) electron flow is from a more negative potential to a more positive potential. This could be from -100 V to -90 V.

For circuit analysis current flow direction is not defined by electrons, holes, or protons.

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[Carultch]

Every book on electrical theory that I've read states the current flows from negative to positive in a dc circuit.

Why then are the battery terminals in a car hooked up with black (neg) to the chassis/ground and the red (pos) to the loads. In automotive the current, as I understand it, leaves the positive terminal, flows through the loads, to the chassis, then back to the positive terminal.

Isn't this backwards from how were taught?

The current vs electrons issue is an artifact of history.

Before we knew whether it was positive or negative particles that carried the current, we made a guess. And we happened to be wrong. Benjamin Franklin set the sign convention that the charge left on the glass by the silk was the "positive" charge, and the charge left on the silk is the "negative" charge. He thought that the glass accumulated charge from the silk, and the silk lost charge to the glass. He didn't have scales accurate enough to weigh the difference.

JJ Thompson discovered that there was something special about the negative, with the photoelectric effect. Two electrodes in a vacuum with a voltage across them. Shine high enough frequency light on the positive and nothing happens. Shine it on the negative, and the circuit is completed when electrons are ejected to the other electrode.

When we learned that the electrons were carrying current, we didn't revise the signs on every diagram. We left them the way they've always been written.

This is why "current" flows from positive to negative, while the actual electrons are flowing from negative to positive. Current considers the hypothetical case that it is POSITIVE charges flowing, and it will create the same field effects as the negative charges that actually flow.

And remember, it doesn't have to be electrons that flow. In metals, it usually is electrons. Because electrons have A LOT LESS MASS than any other part of the atom, and electrons aren't bound by nuclear forces. So electrons carry current in metals. But could a positive charge in a completely different kind of circuit. In the human brain, it is positive sodium ions.

In Alternate History, Benjamin Franklin could've not made his "mistake", and called the charge left on the glass by the silk negative. JJ Thompson would discover that the positive electrode would be special in the photoelectric effect, and current and electrons would flow in the same direction. However, this would make us complacent to think that current is only carried by electrons. It wouldn't make us confront the problem head-on, of what exactly is current?

Current is indeed the flow of charges. And its direction is defined as the direction that hypothetical positive charges would be flowing, which would create the same electric/magnetic field effects as the charges which actually flow.


In the automotive application, it is ARBITRARY which terminal you connect to the chassis. The reason you do this, is to make a voltage reference. No current should flow on the chassis. It should flow on the wires. From the source and back to the source. The reason we bond one terminal to the chassis at exclusively one point, is such that in the event of a wire faulting to the chassis, the chassis can carry current back to the source.

 

[Curious nerd]

Imagine a capacitor discharging.

In real life, electrons are drifting from the negative plate to the positive plate.

Mathematically, the positive plate is becoming less positive and the negative plate is becoming less negative. Less negative means a number is going up toward zero.

So, in math-land current is flowing from positive to negative to bring both plates toward zero.

 

[K8MHZ]

In the automotive application, it is ARBITRARY which terminal you connect to the chassis. The reason you do this, is to make a voltage reference. No current should flow on the chassis. It should flow on the wires. From the source and back to the source. The reason we bond one terminal to the chassis at exclusively one point, is such that in the event of a wire faulting to the chassis, the chassis can carry current back to the source.

You are incorrect about the chassis. By design, the chassis is meant to carry current. There are several points on the body and chassis meant for ground connections. Current flows from those points back to either the battery or the alternator, whichever one is the supply at the time.

 

[K8MHZ]

151107-1309 EST

K8MHZ:

The output polarity of a standard ignition coil can be defined by the winding direction of the primary coil to the secondary coil. Thus, no great need to determine battery polarity for the purpose of controlling spark polarity.

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Since the coil case and the threaded part of the spark plug are solidly bonded via vehicle metal, that means that the threaded part of the spark plug will always be the same 'polarity' as the metal part of the coil case. Those are all bonded to the grounded terminal of the battery. I don't see how it would be possible to have a positive center terminal in the spark plug, plug wire and center HV terminal with a positive grounded system. You would have both the inside and the outside of the spark plugs being positive.

 

[Johnnybob]

151107-1324 EST

Johnnybob:

In a physical external circuit (meaning outside of an energy source) electron flow is from a more negative potential to a more positive potential. This could be from -100 V to -90 V.

For circuit analysis current flow direction is not defined by electrons, holes, or protons.

.

Thank you, gar. As a maintenance electrician in an industrial setting, I don't do alot of circuit analysis. Load calc.s, yup. On terms of how much load I can add to an existing MCC section, absolutely! Conductor ampacity, de-rating, conduit fill? You bet. As a child, I was fascinated by electricity. I bet I shook hands with screwdrivers (inserted in the bathroom receptacle, of course) more than anyone else my age. Unfortunately, I was born into a predominately logging family, so felt the need to carry on with tradition.
When I decided it was no longer feasible to earn a sufficient living in the woods, I followed my childhood inclination into the electrical field. If I'd had another 100 grand to spend, I probably would have furthered my education another 2 years and got an engineering degree, but I didn't, so here I am in maintenance, and honestly, I love my job. It's a new challenge every day! So electron flow, to me, is just what I learned in college. And I've remembered it because #1, it fascinated me, and #2, it is such fun to baffle the millwrights:angel:

 

[gar]

151107-1929 EST

K8MHZ:

A standard ignition coil is a transformer consisting of a primary and a secondary. This is possibly wound so that the secondary is connected to the non-grounded end of the primary such that the secondary voltage is added to the primary voltage.

How does an ignition coil work? The primary is wound with many fewer turns than the secondary and is the source of energy to produce a high voltage to the spark plug.

The primary is charged with energy by closing the breaker points, and current approaches its steady-state value defined by the series loop resistance of the primary and any external resistance. The L/R time constant of the primary in relation to engine RPM determines the current thru the primary inductance when the breaker points open.

With a negative ground battery a positive voltage is applied to the ungrounded end of the ignition coil primary. In order for the primary inductance to maintain exactly the same current and direction flowing at the instant before and after the switch opens the instantaneous voltage at the primary has to switch from + to -, and it is much larger than the battery voltage. The rate of rise is limited by the capacitor across the breaker points. If the secondary is phased to add to the primary, then in the negative ground battery system the center electrode of the spark plug is driven negative.

The spark voltage rises (negative in this case) to a breakdown voltage (10 to 20 kV) where a spark discharge occurs possibly in the range of 1000 V. The voltage to the spark plug prior to breakdown is a portion of a sine wave before the 90 deg point.

In the mid 1950s I did experiments on a system that used a capacitor discharge into a special transformer to produce a very rapid rate of rise of the ignition voltage, and then we dumped additional energy into the spark after the spark was initiated.

We needed a very fast rate of rise of spark voltage in order to fire fouled plugs in a 12 to 1 compression engine.

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[gar]

151107-2017 EST

Johnnybob:

If I have an electron tube and this includes CRTs, then the entities that participate in current flow are electrons.

If I produce a proton gun, then the particles that allow current flow are positively charged particles.

In a conductive liquid I can have both positive and negative charges floating around at the same time.

In all these cases I can define the direction of current flow by either the "conventional" definition or by the "electron" definition.

I always use the conventional definition, even with vacuum tube circuits.

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[GoldDigger]

There is no particular reason to configure the ignition coil as an autotransformer just so the 12V primary adds to the 10kV+ secondary.
I think that there is no reason to determine the spark plug polarity based on the primary polarity.

 

[gar]

151107-2237 EST

GoldDigger:

The reason the ignition coil is probably wound in an additive fashion is to gain extra voltage without additional secondary turns. Also the winding machine can run in the same direction as a continuation of the primary. However wire size has to change.

Whether a negative voltage to the spark is of great importance I don't know. However, the center electrode is hotter than the spark plug shell.

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[ggunn]

Imagine a capacitor discharging.

In real life, electrons are drifting from the negative plate to the positive plate.

When a capacitor discharges, the electrons flow though the external circuit, not from plate to plate inside the capacitor.

 

[gar]

151107-2348 EST

GoldDigger:

From rereading your post I have to comment that there is good reason to make the ignition coil an autotransformer. The spark voltage is generated upon opening of the breaker points. This generates an inductive kick of multi-hundreds of volts across the primary if not a 1000 V, I don't remember the typical value.

You are not just adding to 12 V.

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[FionaZuppa]

there are no pos ions that can flow in a circuit where the circuit itself is not a fluid or plasma. in other words, amps (the vector) is from neg to pos, no way around it. how you describe it in words by making a reference for the purpose of controlling it is entirely up to you, but when you apply the physics of it all, its electrons that can move, not atoms.

 

[kwired]

For "conventional flow" we don't really care what is flowing or really which way it is actually flowing either. A circuit needs to complete a path for current to flow and with "conventional flow" it doesn't even matter if it is AC or DC most of the time.

With "conventional flow" you can even have "reversing" of certain things by simply changing polarity in relation to other associated items even if you don't have a specific directional reference to begin with. Take simple motors that are reversed by changing polarity of an aux winding - you know it will reverse if you change polarity regardless of whether or not you know which way it was going in the first place.

 

[gar]

151108-1411 EST

I need to make a correction to my post numbered 28.

I found an old ignition coil, Auto-Lite, probably Chrysler, and I don't know if it was 6 or 12 V. Has a part number so it was probably standard production.

This is a three terminal device, labeled BAT, DIST, and no label on the high voltage.

By measurements I can determine that BAT and "high voltage" are the outside terminals of an autotransformer.

In an actual semi-modren ignition sysytem meaning after about 1956 - 1958 the car battery negative terminal was connected to the car chassis (ground), and the battery was changed to 12 V. From a 1952 Mercury chassis manual, page 165, "The Mercury electrical system is POSITIVE ground.". Also it was still 6 V. Small junction transistors became available by 1954, Delco had power transistors by 1955, and low cost power silicon diodes came along about 1958, and SCRs. Silicon power diodes made an alternator (Chrysler) practical for automotive use by about 1958. In our expetrimental ignition system I used the Delco power transistors for DC to DC conversion from 6 V to around 1000 V.

In a conventional ignition system the coil primary is directly connected from the BAT terminal to the battery hot terminal (-6 V in 1952) thru the ignition switch or a relay. For AC signal purposes this is essentially a connection to the chassis.

The DIST terminal connects to one side of the breaker points and the other to the engine block (car chassis). Before the breaker points open the DIST terminal is -6 volts relative to the battery + terminal. Upon the breaker points opening the DIST terminal goes 100s of volts positive relative to the + 6 V terminal of the battery, and in turn the "high voltage" terminal goes 1000s of volts positive relative to the DIST terminal. All these voltages add together,

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[FionaZuppa]

for spark plugs its like the same reasons TIG can do DCEN and DCEP, it makes a diff what way the electrons hit the other side. you can make electrons jump the gap by controlling the mag field of a coil, which way the mag field is pointing prior to its collapse doesnt matter for getting electrons to jump the gap, but it does matter in knowing what direction the electrons are going to, hence the mag field direction prior to collapse matters. for the old car coils its the reason why the coil has a - and + on it, etc.

so when you get into things that rely on mag field, direction of electrons flow is a big deal, and is better explained using vectors.

 

[jumper]

151106-1948 EST

Direction of current flow is based on a definition.

In an external circuit the two different standard definitions are:
Note that the current flow direction is in the opposite direction within an energy source (the internal citrcuit) as compare to the external circuit.

Conventional flow --- current flows from positive to negative.
Electron flow --- current flows from negative to positive.

For any circuit analysis to be successful you must stick with one definition or the other. I believe all US engineering schools will use "conventional flow" as the definition of current flow for circuit analysis.

Direction of current flow has nothing to do with where someting is earthed or connected to a chassis or cabinet.

If I analyze a circuit that contains an electron tube, then the direction of current flow is from the plate to the cathode. Physically inside the tube current is flowing via electrons from the cathode to the plate.

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:thumbsup:

 

[Barbqranch]

A side light on this subject, (those few) times I have taught a class on electricity, I start by putting this number on the board:

6,241,509,300,000,000,000 electrons per second meander past a point to form 1 ampere of current.

 

[Strathead]

Ironically 4 pages and I didn't see where anyone focused on the word theory. Someone correct me if I am wrong, but the last I really paid attention, it was becoming accepted that electrons and protons may not be particles. They may be waves. So everything could be energy, not substance. Then again, every atom could be a universe. I am not pointing this out to be argumentative or weird, but to point out that as far as I know, we can try to put electrical flow in to concepts we can understand, and it can fit on all the levels we need it to for our current uses of energy. That doesn't make any of us right. I think the most important thing in this thread is that electricity is looked and and taught with conventional flow and with electron flow. Regretfully to me, we are taught the very first steps, magnetism, lines of flux, etc. utilizing conventional flow, and then without a lot of focus, everything else we learn is electron flow. I have often wondered why the teachers haven't changed on or the other so both match. It would be a simple matter of calling the right hand rule the left hand rule etc.