Polarity Reversed? (was "Backwards???")

Not exactly that way Jeff, but I do know in the rechargeable NiCad dry cells, they would get a spike that would grow through the insulating plastic and short out the cells, the cure was to nail them with a 12 volt source and the spike would arc itself away, making the battery useable again. Until the spike grew back that is. I would presume the same thing would occur with the lead plates and sponge lead plates in a lead acid battery. The reverse flow of electrons could eat away at the sulphation caused by normal charging and discharging that eats up the cells by depositing the sulphate on the floor of the battery shorting out the cells. I remember there was a product called VX-6 that was supposed to regenerate the batterys. I went even one better, and dumped the electrolyte, used baking soda and water to clean inside the battery, then put in new aircraft battery electrolyte and had a new battery. The specific gravity of aircraft battery electrolyte is significantly higher than automotive battery electrolyte. Of course now, all the aircraft use wet cell NiCads, and Potassium Hydroxide won't work in car batterys. Now if I can just find some, I can regenerate my larger 20 NiCad wet cells and won't need to have the access panel in Willit? for battery replacement through the bottom of the belly pan.

jeff denton said:

As for the length of wire buried in the ground, there is no current flow because you did not make a complete circuit. There has to be a path from negative to positive, the electrons don't just evaporate into thin air, or even wet ground.

Click to expand...

Discussions on Electricity is where a Mechanical Engineer like me has to go back to the Engineering course reference books so he doesn't seem like a complete idiot. Google works too. Here's a good description of "electrical current":

Although it is electrons which are the mobile charge carriers which are responsible for electric current in conductors such as wires, it has long been the convention to take the direction of electric current as if it were the positive charges which are moving. Some texts reverse this convention and take electric current direction as the direction the electrons move, an obviously more physically realistic direction, but the vast majority of references use the conventional current direction and that convention will be followed in most of this material. In common applications such as determining the direction of force on a current carrying wire, treating current as positive charge motion or negative charge motion gives identical results. Besides the advantage of agreeing in direction with most texts, the conventional current direction is the direction from high voltage to low voltage, high energy to low energy, and thus has some appeal in its parallel to the flow of water from high pressure to low.


But as for Jeff's comment, current flows just fine in wet ground. In fact, virtually ALL of the electricity consumed in the world requires that fact. The "neutral" wire in your AC house wiring is actually "grounded" and uses that path to complete the circuit to the generation source.
And underground steel pipelines are protected by corrosion through a process called "Cathodic Protection", where a current (actually a voltage) is "impressed" on a ground bed (called an "Anode Bed") which transfers electrons through the ground over quite large distances (many miles!) to any holes in the pipe's coating (called a "holiday"). This mitigates corrosion on the pipe by forcing electrons onto the pipe, rather than allowing the steel to expel electrons in a "galvanic reaction" (the difference in "Spontaneous Potential" between dissimilar metals, and even between the slightly different materials in the steel or iron itself).
JM2CW

when i first got my 1.9l kadett the battery was connected with a positive ground (the positive battery terminal was grounded to the body)...

it started and charged normally... everything worked except the temp and fuel gauges never moved, and the charging light was always on

i took the battery out and flipped it around so that the black cable was positive (starter) and the red was negative (ground)...
this was the only change i made...

after i flipped the battery nothing changed EXCEPT:
the temperature gauge now functioned correctly (it had been pegged in the cold direction i guess!)
the red charging error light now functioned correctly
the fuel gauge still didn't work, so i swapped it out with another one... now it works fine

STRANGE BUT TRUE

elektrical

"Besides the advantage of agreeing in direction with most texts, the conventional current direction is the direction from high voltage to low voltage, high energy to low energy, and thus has some appeal in its parallel to the flow of water from high pressure to low. "


So its the "potential difference" that makes the current flow one way or the other. The voltage can be likened to pressure of the water in a pipe; While the current is the flow of water.

One last time on the battery. If current is flowing and you feel a tingle then Websters definition of a "shock" is being described as "The effect caused by an electric current passing through the body." Therefore you do receive a shock. It's just that the current flow is low enough that a greater effect isn't felt.

Now a car batterys amp rating is from what 450-650 amps of course you only have 12V of pressure attempting to push the electrons. So here's an interesting experiment stand in a bucket of water then attach a wire to the negative side of the battery and put the other end in the bucket of water. Then take a very low amp rated fuse like a glass fuse around five and not more than ten milliamps and wetting your hand and being careful not to get the fuse wet, might want to use a fuse holder with wires sticking out. Then touch the free side of the fuse to the positive side of the battery while holding the other end. I would be very surprised if you didn't feel something before the fuse blew. If you don't feel anything and you know for certain that you won't feel a shock try a slow acting fuse of the same rating. I wouldn't do this because I think you can get a shock from straight DC. Now from what I've seen on television on one of those reality shows it only takes 20 milliamps coursing through the heart to make it stop. Even if I was confident enough about electrical theory to do it, I surely wouldn't do it unsupervised. If any of you try this I would be very interested to know the results.

No fact is better than a fact you learned from your own experience

dc burns

Bar none, the worst electrical shock I've ever got, worse than 4160V AC even, was from the main storage battery on a submarine. Sure it's only DC, but it's 250+V and has the potential for over 10k Amps. I have scars from the third degree burns I can show people if they want to see them.

The human body has quite a bit of resistance to electrical shock. Typical values of resistance from one hand to the other through the body range in the 200kohm+ range. Wet skin, alchohol, dehydration, being tired, and other things all tend to lower the body's resistance to current flow. In general, the most commonly used # for body resistance is that it's always over 300 ohms, and since the standard for what will stop the heart was traditionaly always 0.1A, a little math with ohms law leads to 30 volts being the safe limit for working with electricity. (This is 9/10 of the time the value of voltage that if exceded in the workplace the worker is required to use precautions to prevent shock, and usually the cut off that seperates "low voltage" systems from "high voltage" systems.)

All that said, most of the time on dry skin you won't feel a shock from 12VDC unless something else is going on. The # most often used for feeling a shock is .001A, and with only 12V to push it you would need a resistance less than 12kohms to feel it, which is rather unusual. Not impossible, but unusual.

Now, back to the discussion at hand, "ground" is an arbitrary point in any electrical system. When you establish ground in a system you are establishing the point with which you intend to reference everything. It does not necessarilly reflect anything with respect to the dirt under your feet. Voltage is a potential energy until the current actually starts to flow, so it's a lot like gravity. If you pour water from a cup into the sink, and the sink is 12" below the cup, the water will hit with a certain ammount of force. That ammount of force is completely independant of your location when you pour the water, and will be equal in every location where you pour the water. Death Valley or Denver, the same splash in the sink. That 12" only has meaning when you set a reerence point, the sink, and measure from there. Voltage works the same way, only with respect to one terminal does the other terminal of a battery have any potential.

My Dodge truck runs 2 alternators, one with the output +12V with respect to the chassis and the other -12V with respect to the chassis. The 2 batteries are then wired so one has it's (+) terminal grounded and the other has it's (-) terminal grounded. All my 12V acessories work fine, as long as they are hooked up from one battery's (+) to the same battery's (-), but if I need 24V for something, like the really big winch I got military surplus really cheap, I can hook it from one battery to the other and get 24V.

Oh, and the starter worked because it's a series wound DC motor, which means that even though the field on the rotor reversed magnetic poles due to the reversed voltage so did the field in the stator, so the effect on rotation was the same. If both south poles turn into north poles at the same time they will still repulse each other the same as when they were magnetic souths.

From the bleacher seats, anyway...

DC also shocks and in the worst way if the current potential is great enough. Because of the nature of Alternating Current a mild AC shock generally causes involuntary muscle contractions and is likely to cause one's heart to fibrillate. After the current source is removed the muscle contractions normally cease and the heart may return to a regular rhythm, but not always. With a Direct Current shock muscle tetanus is likely to occur, which in effect freezes the victim in place. This tetanus effect can also stop involuntary muscle activity, like your heart, not good. The regulated current jolt supplied by a defibrillator unit is DC, which halts fibrillation and gives the heart a chance to recover or if it’s inactive, jumps the heart into an active state.

The human body is actually a fairly good conductor of electricity, however our skin is a relatively poor conductor due to its relatively high resistance factor. Add water or any other conductive substance and obviously the resistance factor decreases. The thickness of the skin also plays a major role. The palm of your hand where the skin is thicker has a higher resistance than the ear lobe where the skin is thinner.

The tingle on your tongue when you lick the terminals of a 9-volt battery is the DC flowing between the terminals through the salvia on your tongue. The tingle is the reaction of the nerve cells in your tongue to the DC current as the DC interrupts their normal activity. The 9-volt battery trick works because of the combination of the number of nerve endings, their proximity to the surface and the saliva on your tongue. If you lick your palm and touch the battery to it you probably won't feel the current flow.

Most people that work on low voltage, low current or high voltage, low current equipment normally don’t get hurt directly from an electrical shock. The injury is usually sustained during the rapid withdrawal from the electrical source, like smacking your head on the underside of the hood when the coil zaps you. Remember it’s not the voltage that is dangerous it’s the current, and the sharp metal inside a TV chassis, OUCH!

Brian (EE)