Why positive ground?

[JSeery]

I'll take a crack at this, however, remember going in it is a confusing if you are not familiar with electronics!

In the early years of electric theory and before atomic theory was well understood, it was established that current flowed from a positive source to a negative. Based on this concept the theories of electricity, the basic formulas and symbols were developed. As the atomic nature of mater became better understood it was realized that what was called current was actually the movement of free electrons and that electrons have a negative charge. Free electrons move from a negative source to a positive source. There was a period of time when this flow from negative to positive was considered an important factor and there were attempts to account for it. However because electric/electronic theory was already established and the realization that actual electron movement was not that important in most applications it was determined to keep the original convention of current flow as positive to negative. This is now referred to as Conventional Current Flow and it what is used in almost all references to current flow in a circuit. The other term used is Electron Flow which refers to the actual direction electrons flow in a circuit. The true electron flow is normally only used in explaining specific circuit characteristics.

Current is conducted through some materials and not others. One of the characteristic of conductors is the way the outer part (shell) of the atoms of these materials behave. In most conductors, the electrons are loosely connected to any given atoms nucleus and tend to float or drift around between different atoms (something referred to as an electron cloud). This allows materials such as metals to be shaped. When a metal is hit or bent the atoms can fairly easily reposition themselves as opposed to a rigid outer shell with the electrons held tightly in place (this is really referring to the relationship between the atoms in a material and not to the individual atom). This same property make metals good conductor of current as the electrons can easily move from atom to atom along the conductor. These electrons carry a negative charge and move away from a negative source and toward a positive source, thus the current is moving from the negative to the positive.

It was eventually determined that this actual flow from negative to positive was not an important factor in normal day to day electrical practice. The idea was that it could just as easily be looked at as the “holes” left by the missing electrons (leaving a positive charge to the atom) moving from positive to negative. Now almost everything matched with the original theories and all was well with the world. So, if you are referring to the actual movement of electrons in a circuit it is negative to positive. If you are referring to what is now called Conventional Current Flow, it is from positive to negative.

[Binx]

Essentially the old argument about lightning going up from or down to earth.

Lonnie

[rotorwrench]

When voltages get to the amount in a lightning bolt, they can go anywhere they want to. I just learned to stay out of their way because the ones that hit humans don't care which way they come from.

[COE Dan]

Originally Posted by rotorwrench

... The 6-volt system is taxed by any resistance in an electrical system. This is why the average wiring is 16 to 10 wire gauge instead of the smaller gauges used in the 12-volt systems...

In a 12-volt system it's not necessary. Resistance has a much lower effect than with the lower voltages.

6V systems need double the amperage of a 12V system to accomplish the same amount of work. That's why 6V systems require larger wires to carry the increased current load, not to overcome system resistance. However, the statement that 12V systems are less affected by resistance change is correct.

Using Ohms law (I won't do the math here), one will find that a 6V system is 4 times as sensitive to resistance changes compared to a 12V system.

Also, as JSeery suggests, electron flow is irrelevant for our purposes. Conventional Current Flow makes sense in that most folks equate “positive” with a value greater than something “negative”, probably largely based on the fact that “negative ground” has been common for so long that to most people “negative ground” means “zero voltage”. All of our math and measurement conventions reflect this Conventional Current Flow idea.

However, even current flow really doesn't matter as it is a mathematical convention based upon standard assumptions. If you randomly assume current flows in one direction for a particular circuit, conduct an analysis, and find the resultant current value is a negative number, that merely means your initial assumption of current flow direction was opposite of the convention. The current's magnitude (value) will be identical regardless of the mathematical sign.

[rotorwrench]

Its actually for both reasons. You can use an 18 guage wire in a 6-volt system as long as it's real short. Most electrical engineers just played it safe and used wiring that would have both lower resistance and higher current capacity. These old systems were only 35 amp max. A lot of early era cars burned up due to wiring problems back in the day. That and a lot of wood construction didn't help either. In aviation, they went to 24-volt so they can use even smaller wire gauge to save weight. Long wire runs have to use a larger gauge due to the resistance drop.

[terry]

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Something to remember in this thread is that some devices need electricity moving the right way in order to work properly in our old cars. LED lights, transisters?, etc. can be polarity sensitive. Most of these modern day devices are made for negative ground. Terry

[Mike51Merc]

Originally Posted by terry

Something to remember in this thread is that some devices need electricity moving the right way in order to work properly in our old cars. LED lights, transisters?, etc. can be polarity sensitive. Most of these modern day devices are made for negative ground. Terry

This is true, and diodes act as one-way valves based on polarity.

I think all-in-all it doesn't really matter, but once you choose you have to stick with your choice, especially in modern cars.

[Bill S]

The current flow from pos to neg is the flow of holes in the valiance of the electron. Electron flow is neg to pos.
In old days engineers talked about current flow. That was pos to neg. When I started in electronics I worked for an older engineer. Thus he would always talked current flow. I had to readjust my thinking. I Had learned electron flow.

[JSeery]

Originally Posted by Bill S

The current flow from pos to neg is the flow of holes in the valiance of the electron. Electron flow is neg to pos.
In old days engineers talked about current flow. That was pos to neg. When I started in electronics I worked for an older engineer. Thus he would always talked current flow. I had to readjust my thinking. I Had learned electron flow.

Yep, except there is no real flow, it is just a convention used. When you say "current flow" you are referring to "conventional current flow" which for all practical purposes is a made up term. The electrons flow counter to conventional current flow, the positive ions ("holes") don't flow through the wires. For almost all automotive applications and especially the older ones all of this is of very little concern and only confusing to most people. It is best to just go with the conventional current flow, positive to negative and not get to concerned about what is going on at the atomic partial level.