Role of engine in DC "boat ground potential"

[floatsome]

Originally Posted by wind_magic

...if you were to hook the negative lead of your light to EARTH GROUND (instead of your batteries ground) and then you hooked up the positive lead to your batteries positive terminal, electrons are not going to flow from your battery into earth ground - instead what is going to happen is that your batteries potential is going to move relative to earth ground instead of pumping electrons and nothing is going to happen. Of course, if you were to hook the negative terminal of your battery up to earth ground, you've got a circuit, and the light would shine...

I'm annoyed with myself: still not getting it.

I updated my sketch to show electrical continuity from engine block to the ocean via the propshaft. This makes the boat's ground potential equivalent to earth ground, rather than a floating ground where the boat's potential is not connected to earth. This is the usual situation in boats, unless there is a plastic coupling between the transmission and propshaft that breaks electrical continuity.

I am still not getting this critical point: What stops current that flows through components grounded via the block (alternator, senders) from flowing into the ocean, instead of returning to the the boat's battery? (Recognizing that electrons actually flow from negative to positive, but the question remains.)

[wind_magic]

Originally Posted by floatsome

I'm annoyed with myself: still not getting it.

I updated my sketch to show electrical continuity from engine block to the ocean via the propshaft. This makes the boat's ground potential equivalent to earth ground, rather than a floating ground where the boat's potential is not connected to earth. This is the usual situation in boats, unless there is a plastic coupling between the transmission and propshaft that breaks electrical continuity.

I am still not getting this critical point: What stops current that flows through components grounded via the block (alternator, senders) from flowing into the ocean, instead of returning to the the boat's battery? (Recognizing that electrons actually flow from negative to positive, but the question remains.)

The short answer is because there is no circuit.

The long answer ...

The battery wants to do work (for lack of a better word), when it is charged there exists a situation where the chemical reaction that discharges it can only occur if electrons flow from the negative terminal to the positive terminal - if the negative terminal is not connected in some way to the positive terminal the battery will not discharge, it will just sit there, a chemical soup in a state that will not change until it is allowed to move electrons from the negative terminal to the positive terminal.

Connecting the negative terminal to the ocean does not create a situation where the negative terminal can move electrons to the positive terminal, instead all it does is create a REALLY GIGANTIC negative terminal that STILL wants to be connected to the positive terminal, and since it isn't connected the chemical reaction still won't take place.

Instead of connected the negative to the ocean, you could instead connect the positive to the ocean and leave the negative terminal floating, and again you would have a situation where the electrons could NOT move from the negative terminal to the positive terminal and the chemical reaction could not take place. All you would have done is create a REALLY GIGANTIC positive terminal that STILL wants to be connected to the negative terminal.

You are imagining that hooking the negative terminal up to the ocean is going to discharge the battery because now it can release all the charged up electrons it's been wanting to let go of, but it isn't like that. The negative terminal DOES have a different amount of electron density than the earth when it is floating, but that is a static charge and not an ongoing potential, so when you hook the floating ground up to the ocean you may get a static discharge, but then the terminal immediately comes to the same ground potential and it STILL wants to move electrons from its negative terminal to its positive terminal, because it still needs electrons to move in order to work its chemical magic. Hooking the negative terminal up to the ocean is fine, but you can't get the chemistry to work in the battery because the battery actually NEEDS the electrons at the other terminal in order for the reaction to take place.

It is important to understand that there is no such thing as a Universal zero volts or a Universal 12vdc, you can only have 12vdc relative to something else. When you connect the batteries negative terminal to the ocean any static charge on the battery dissipates into the ocean (possibly as a spark) and no matter what potential the batteries negative terminal ends up at, the positive terminal will end up approximately 10-13 volts above that potential, because no matter what the batteries negative terminal is at in terms of electron density, the battery still wants to conduct 10-13 volts of electrons from one terminal to the other.

The above is why you can hook the negative terminal to the ocean and pretend it is zero volts and you will then be able to read 12'ish volts at the positive terminal. It's also why you could hook the negative terminal to the positive terminal of ANOTHER BATTERY and you will still get 12'ish volts at the positive terminal, and 24'ish volts from the negative of the first battery to the positive of the second. The above is also why you could hook the ocean up to either (1) the negative terminal of the first battery, or (2) the negative terminal of the second battery and the positive terminal of the first, or (3) the positive terminal of the second, and the batteries won't discharge, all you will do is create a potential to do work relative to the ocean that is different in the 3 cases. In case (1) your voltage potentials will be 0, 12vdc, and 24vdc relative to earth, in case (2) they will be -12vdc, 0, and 12vdc, and in case (3) you would have -24vdc, -12vdc, and 0.

If that doesn't make sense I can try to explain it in another way.

[floatsome]

wind-magic - that helps, thanks. I plan to read it again when I'm not so beat from a 14 hour day. I'm beginning to see it.

[wind_magic]

Originally Posted by floatsome

wind-magic - that helps, thanks. I plan to read it again when I'm not so beat from a 14 hour day. I'm beginning to see it.

Floatsome, I thought of an easier way to explain all of the above - with some simple questions.

If you put a penny on the negative terminal of your 12vdc battery and it just sits there, will the battery discharge ?

How about a screwdriver ? If you touch a screwdriver to just the negative terminal and don't touch the positive, is it going to discharge the battery ?

How about a wrench ?

What about a 1lb steel weight ? If you hook a 1lb steel weight up to the negative terminal on a battery and it doesn't touch the positive terminal, will that discharge the battery ?

A piece of rebar ?

Something bigger, if you wire up a bowling ball sized piece of steel to the negative terminal, will the battery discharge ?

What about if you hook up a piece of steel like an old car frame ?

Or something bigger, a backhoe ?

Or a steel boat that is sitting on stands ?

Or a WW2 tank ?

Etc, etc ....

So why would wiring the whole earth up to the negative terminal of the battery be any different than the things above - besides being much bigger, is it really any different ? No matter how big the negative terminal is (no matter what you wire it up to) it still has to touch the positive terminal on the battery to discharge the battery.

[floatsome]

Originally Posted by wind_magic

So why would wiring the whole earth up to the negative terminal of the battery be any different than the things above - besides being much bigger, is it really any different ? No matter how big the negative terminal is (no matter what you wire it up to) it still has to touch the positive terminal on the battery to discharge the battery.

A temperature sender hooked up to the positive terminal of the battery completes its circuit through the engine block ground when the engine is running and the circuit to the sender is open. Electrons flow from negative to positive. So, do the electrons flow from the negative terminal of the battery through the black ground cable attached to the block rather than from the ocean and up into the block via the propshaft because the battery has a ready supply of electrons and not the ocean (?) and then flow from the block to the sender and back through the red wire to the positive terminal of the battery?

[wind_magic]

Originally Posted by floatsome

A temperature sender hooked up to the positive terminal of the battery completes its circuit through the engine block ground when the engine is running and the circuit to the sender is open. Electrons flow from negative to positive. So, do the electrons flow from the negative terminal of the battery through the black ground cable attached to the block rather than from the ocean and up into the block via the propshaft because the battery has a ready supply of electrons and not the ocean (?) and then flow from the block to the sender and back through the red wire to the positive terminal of the battery?

Electrons repel each other. Think of electron density like heat - when you touch something that is very hot to something big and cold, the heat from the hot thing dissipates into the cold thing until both things are the exact same temperature. The hot thing won't get colder than the cold thing, and the cold thing can't get hotter than the hot thing, they meet somewhere in the middle. If the hot thing is very big compared to the cold thing (a stove top and a penny) then the hot thing won't cool off very much, and the cold thing will heat up a lot (hot penny). If the hot thing is very small compared to the cold thing (a penny and an ocean of water) then the hot thing will cool off a lot relative to the cold thing and the cold thing will heat up very little (ocean doesn't change temperature if you drop a hot penny into it, but the penny cools off). Whatever happens though, both things share the heat until they both end up at the same temperature (so long as they are not isolated by some kind of insulator like the atmosphere).

Touching the negative terminal of your battery to something works the same way, the negative terminal (and plates) of your battery may have some kind of static charge - that is to say they may have a higher (or lower) electron density than whatever you touch the terminal to, but as soon as you touch the terminal the electrons will either spark out to decrease their density to be the same as whatever it touched, or spark in to increase their density to be the same as whatever it touched, but in the end they will have the same basic density as whatever it touched. Why ? Because like heat (which is really just vibrating atoms) electrons repel one another and "want" to move away from each other, so they "want" to move into areas where the density of electrons is lower. If you touch the engine block of your engine to the ocean, because the ocean is so vast and your engine block is so small your engine block will essentially spark to whatever electron density the ocean has - they will end up with the exact same field strength, the same ground potential, electrically they will become one.

Now, NO MATTER WHAT THE NEGATIVE TERMINALS GROUND POTENTIAL IS, the positive terminal will be approximately 10-14 volts above it. This is important to understand. The battery wants to do work relative to whatever its negative terminal's potential is NOT relative to some Universal zero volt that is a constant. No matter what the negative terminal's electron density the battery will ALWAYS want to do 10-14 volts of work relative to it, the two terminals will ALWAYS want to conduct so long as the battery has a charge (lead sulfate and sulfuric acid in solution).

So ....

Your question is, when the sender on the motor works, how do electrons from the negative terminal know how to go through the sender and not through wire to the ocean.

The answer is - it doesn't matter. The negative terminal hooked to the ocean is like one big giant ground plane, so it doesn't matter what any individual electron does. If an electron goes through the sender from the engine block, in an instant the entire field (engine block, negative terminal of the battery, earth) loses an electron and an electron moves out of the negative terminal of the battery to compensate for the loss, and an electron moves into the positive terminal of the battery and the chemistry in the battery works. By hooking the battery's negative terminal to the ocean all you have done is hook it to the mother of all engine blocks, electrically the whole earth becomes part of your battery's negative terminal. Said another way (in answer to your queston), it's impossible for a stream of electrons to flow out of the negative terminal of your battery "into the ocean" because they are at the same potential the moment you hook the terminal up to the ocean, so why would electrons "flow" there ? If they wanted to flow into the ocean they would have done it (and did as a static discharge) as soon as you hooked the ocean to the negative terminal of the battery.

Said yet another way ...

You could, if you wanted to, hook the negative terminal of your battery up to the ocean and then run a wire over the side of the boat into the ocean and make that wire your entire electrical systems ground wire. Whenever your lights were on electrons would flow out of the ocean through the lights and into the positive terminal of the battery and electrons would move out of the battery's negative terminal into the engine block and out into the ocean to complete the circuit. The entire ocean would then become a single piece of wire in your circuit drawing, a really, really, really big piece of wire, and you would not need another piece of wire hooking the engine block up to the negative wire in your electrical system to "complete the circuit", the ocean would serve the purpose.

When you hook one thing up to another thing electrically they take on the same electron density. That's why you can hang from a 10,000 volt electrical line and not die, and then fall and touch the ground and not die (so long as you survive the impact). When you are hanging from the electrical line your electron density is changing with the wire, you may static discharge to or from it when you first touch it, but then you take on it's same electron density, you become part of the wire you are touching. When you fall to the ground the earth has a different density and you will charge/discharge into/out of it in an instant, but you won't die unless you actually become the wire that completes the circuit between the wire and the earth - leave that trick for the household appliances in people's homes, they are better suited to the task than your body is, you don't want to heat up like a light filament.

[floatsome]

Originally Posted by wind_magic

If an electron goes through the sender from the engine block, in an instant the entire field (engine block, negative terminal of the battery, earth) loses an electron and an electron moves out of the negative terminal of the battery to compensate for the loss, and an electron moves into the positive terminal of the battery and the chemistry in the battery works.

Eureka. Thanks.

So my simple-minded way of thinking now is this: The positive side of a battery has an electron deficit compared to the ground side. Closing a circuit switch in the positive side of the circuit allows electrons to flow from ground to the battery.

[wind_magic]

Originally Posted by floatsome

Eureka. Thanks.

So my simple-minded way of thinking now is this: The positive side of a battery has an electron deficit compared to the ground side. Closing a circuit switch in the positive side of the circuit allows electrons to flow from ground to the battery.

Exactly.

If the battery were a capacitor instead of a battery it would discharge the negative terminal's electrons into the positive terminal and the device would instantly reach equilibrium. Since it is a battery, however, hooking the negative terminal to the positive terminal does not "satisfy" the battery, because the battery's positive terminal continues to have a lower electron density than the battery's negative terminal because the positive terminal is "eating electrons" to work its chemical magic - as electrons move from negative to positive to increase its density the chemistry in the battery removes those electrons to work its chemistry re-creating the deficit, the result is a flow of continuous current. The reason the battery doesn't instantly reach equilibrium is because the chemistry can only "eat" so many electrons at a time, so it takes it a while to discharge completely. Once the battery completely discharges it will have the same field strength (electron density) at both terminals and electrons will no longer flow from the negative terminal to the positive terminal and the battery will stop doing work. That's why batteries can only supply such-and-such amount of current, and also why it takes them such-and-such amount of time to recharge in the presence of an even lower electron density at the positive terminal (sucking electrons back out of the positive terminal i.e. recharging) than the negative terminal, the chemistry takes time to work.

[floatsome]

This helps too: Safe circuit design : ELECTRICAL SAFETY.

Originally Posted by wind_magic

you can float a ground. Airplanes do this all the time, they are not grounded to earth.

That scene in "Hunt for Red October" comes to mind, where the helicopter is hovering over the submarine and the sailor on the sub gets shocked by a spark jumping from the helicopter to the sub as they try to ground the helicopter prior to accepting the hero on the end of a tether. My understanding now is that the helo's floating ground ended up at a different voltage from earth as it picked up electrons during the flight, but within the helo all the electronics were happily working with voltage determined by their own ground. The voltage potential between the helo's ground and earth ground rapidly corrected itself when helo and sub came together. I presume this is the same shock a person gets when they feel a zap as they step out of a car. Our propshaft connection ensures that our boat's ground potential is always the same as earth.

1. So, do aircraft routinely connect to earth ground when they land, especially on a metal aircraft carrier? Just found this: Helicopter Static Charge? [Archive] - PPRuNe Forums

2. Hence also the rationale for connecting a green wire from the grounding bus bar in the AC system to the main DC ground bus bar, so that stray current from a short in an AC appliance can dissipate safely "to earth"?

[tager]

just don't wire any studebakers