Electrons flow from the negative post of a battery, through a load, then into the positive post.

When you jump another car, you clamp the positive of the runner to the positive of the dead. Then you clamp the frame or something of the running car to the negative terminal of the deceased. It works, I just did it.

How do the electrons flow if the running car's negative post is not connected?

If you look at the negative terminal, you might be able to see where it's attached to the frame of the far. (Even if you can't see it, it should still be connected.) So the entire frame is part of that ground connection.

Notice the order;



https://en.wikipedia.org/wiki/Jump_start_(vehicle)

"Operation of a lead-acid battery may, in case of overcharge, produce flammable hydrogen gas by electrolysis of water inside the battery. Jump start procedures are usually found in the vehicle owner's manual.[3] The recommended sequence[4] of connections is intended to reduce the chance of accidentally shorting the good battery or igniting hydrogen gas. Owner's manuals will show the preferred locations for connection of jumper cables; for example, some vehicles have the battery mounted under a seat, or may have a jumper terminal in the engine compartment.

Jumper cables should not be used to interconnect between different voltage systems. Connecting 6 V and 12 V systems together may cause damage.[5]

If the dead battery is physically damaged, has a low electrolyte level, is decayed or frozen, a jump start will not repair the battery. A vehicle with a frozen battery should not be jump started, as the battery may explode.[5]"


"Motorists can be severely injured by a battery explosion. In the United States in 1994, a research note by the National Highway Traffic Safety Association estimated that about 442 persons were injured by exploding batteries while attempting a jump-start"

https://www.allaboutcircuits.com/tex...electron-flow/

I think this article explains OP's confusion. Evidently, car batteries use "conventional flow notation" rather than "electron flow notation". So the + battery terminal is the one where there is a surplus of electrons. Thus the electrons flow from the + battery terminal through the starter motor and back to the - terminal of the battery. In the jump start the circuit is completed to ground rather than back to the Dead negative terminal to avoid problems when the Dead battery is damaged.



PairTheBoard

But no flow in either direction can happen if the circuit is open.

I’m pretty sure AaronW is right, the negative terminal is usually connected to ground even if it isn’t obvious.

I've never really understood "grounding" very well. Whenever I jumped cars I always connected both terminals rather than grounding one. Regardless, I don't see how anyone answered the real problem OP presented which centered on the understanding that electrons flowed from the negative terminal to the positive terminal as would seem to make sense. The article I linked explained it goes the other way because conventional notation has had it ass backwards since Ben Franklin's hiccup.



PairTheBoard

OP wanted to know how the electrons can flow if the negative terminal isn't connected to anything. As can be seen from the diagram, it's an open circuit unless there's a connection between point 4 and the negative terminal of the dead battery.

Where are the 2 terminals of a car's starter motor connected? Doesn't one go to + and the other to - on the battery? Is the - battery connected to both the starter motor and the car's frame? Or is the - on the starter motor connected to the frame and from the frame back to the - battery terminal? I don't get it.


Also, am I correct that electrons flow away from the + battery terminal and not from the - terminal as OP was saying?



PairTheBoard

You're overthinking this. Imagine that the car battery and starter motor are hooked up to each other in the way you think:

It turns out that the negative terminal of the battery is also connected to the frame of the car.

When you connect the jumper cables, you're basically just bypassing the original battery with the secondary battery. Connecting to the positive terminal and the car frame creates a closed circuit just as before.

Actually, electrons flow from the negative terminal to the positive terminal, which is super confusing because most diagrams will have arrows that point the opposite direction. The reason is that Ben Franklin originally guessed that positive charges flow from plus to minus. It turns out that actually it's actually negative charges flowing from minus to plus, but when people figured that out it was too late to change the figures and notations, so we call it "conventional current." "Conventional current" is a polite way of saying "completely backwards current." I know this sounds crazy but I'm not making it up. To be fair, it took over a hundred years before anyone realized Ben Franklin had it backwards. Either way that's not the key here.


OK, anyway, think of it this: electricity always flows in a loop, a circle, a circuit. If there isn't a complete circuit, the electrons can't flow. It doesn't matter which direction we say charge is flowing, there has to be a complete circuit. Imagine a race track: you can run in either direction but you have to be able to come back to where you started.

So, to the OP's question:

The answer is no, nothing can flow unless there is a closed circuit and a complete loop.

Thanks, that helps a lot. So, why is a car's (-) battery connected to both the car frame and the car's (+) starter motor? Why the extra connection to the car frame?

Also, when you connect the (-) Jump to the Dead car frame you are connecting to both the (-) Dead battery and the (+) Dead Starter Motor. So why not connect the (-) Jump battery to the (-) Dead battery (like I've always done)? That makes those same two connections. Why go through the Dead car frame?


PairTheBoard

So is the terminal labeled (+) on a car's battery really the positive terminal or is it conventional notation for what's really the negative terminal?



PairTheBoard

It makes it easier to ground all the other electronic things in the car because the frame goes all over the place. Imagine the horrible mess of wires that would be under the hood if every single electronic component had to have a wire running directly to the negative terminal. In essence, this connection just makes a really, really big negative terminal.

It's a safety thing. When you make the last connection, there's a chance of a spark. A spark near the battery can be bad. But a spark at some random point along the frame isn't a concern.

Edit: Here's an quote from a short automotive Q&A article:

http://www.chicagotribune.com/classi...01-column.html

What are "positive" and "negative" terminals anyway? They're just labels that we've applied. So positive terminal means positive terminal.

The current flows from + to -, but the electrons flow from - to +.

The way i see it the purpose of the jump starting connection is to use the good battery to start your dead battery car's engine and then that car's alternator will recover the dead battery if given enough time working to reverse the chemical reactions that will allow it to work properly again as a battery. If you let it connected for a while while the good car is working before you try to turn on the engine of the dead battery car it will improve the condition of the dead battery. The purpose of using another location for the dead battery car negative terminal connection is to avoid sparks that may trigger an explosion by quick highly exothermic burning of Hydrogen gas that is released near the dead battery cathode. All the metal parts are connected to the engine anyway.


The negative charge carriers ie electrons flow from a lower to a higher potential. Conventional historically wrong current does the opposite because it would be what positive charges do, they go from higher to lower potential.




or

My understanding of this stuff is evidently at a pre-Ben Franklin level. So what is the current? How does the current release energy, for example, in the starter motor. I thought the current was the flow of energized electrons which gave up energy in the starter motor. From what I gather now it looks like electrons flow from the battery's (-) to (+) through the battery rather than through the cable-motor-cable connection. While it's something called "current" that flows through the cable-motor-cable connection. What is this "current" and how does it work to carry energy?


This is why I studied mathematics. It's so much easier than physics.



PairTheBoard

Current can be thought of as the flow of electrons. (Ben Franklin thought that it was a flow of positively charged things that created current, which is why there's confusion about the + to - and - to + thing.)

How simplistic do you want to get? Maybe the "release" of energy is the wrong framework. Air molecules cause windmills to turn. It's like that, but electrons... ??

I don't believe that the electrons are "energized." Usually when people talk about energized electrons, they're talking electrons that are tied to a particular nucleus and are not in their ground state (higher energy electron orbitals).

So here's probably the best simplistic (ie, not entirely accurate) picture:

The battery can be thought of as having an excess of electrons on the - side of the battery and a dearth of electrons on the + side. So the electrons want to get from the + side to the - side to create balance. But there's a wall in the way. So in order for them to get there, they need to be connected together externally, which can be done by putting a wire across the terminals.

If you did that, you would get a "short" and the rush of electrons will happen very quickly (causing things to heat up and possibly explode). Instead, if you put something in the way, like a motor, the electrons will still have a pathway to the other side, but will need to "push past/through" the motor to get there, and the energy of the electrons pushing is what causes the starter motor to turn, which causes the rest of the car to turn on.

Crap. Terrible typo is fixed in bold below.

So it looks like the thing that "Conventional notation" and "Electron flow notation" actually apply to is the term "Current". In your first quote above you're using the term "current" according to conventional notation (which has no correct physical interpretation) while in your second quote you're using the term "current" according to Electron Flow Notation. If that's the case then I think I'm getting it.


One more thing if you don't mind. You mentioned that the (+) terminal has the higher potential than the (-) terminal. "Potential" for what? Attracting electrons? Also, I believe there's something called the "Voltage Differential". How does that relate to "potential". It seems to me the voltage drop would be from the (-) terminal through the motor to the (+) terminal.


Great explanations btw.


PairTheBoard

Close enough. If you want an equation to ponder, imagine you have a moving charged particle with charge q and velocity \vec{v}. Now think about the quantity q*\vec{v}.

-- Benjamin Franklin imagined he had positively charged particles flowing from + to -.
-- The reality is that we have negatively charged particles flowing from - to +.

But if you calculate q*\vec{v}, then you get the same result either way because you've changed the sign on both the direction and the charge.

Potential in this context means "electric potential difference" but is also "voltage differential/difference." They all mean the same thing.

If there's no voltage differential, there's nothing pushing the electrons to move around. The abundance of electrons in one place and dearth of electrons in another creates that differential, and then connecting them allows the current to flow to create balance. (Basically...)

Yes.

If you move from A to B, you get a +V (think of it as an energy bump) and from B to C you get a -V (an energy sink). And then if you make a closed loop by going back to A, you will get zero total voltage, which is Kirchhoff's law. (If you could complete a loop and have a positive voltage, you would have a perpetual motion machine because the electrons would just push themselves around in circles.)

Thanks. I'm sure some chemists out there would disagree because I'm probably cheating on the chemistry side. The pictures here are more about intuition. I don't pretend to remember much of the actual mechanisms involved with batteries. It's not actually an abundance/dearth of electrons, but some sort of electro-chemical thing. But it's easier to just think of the electrons.

Yes, think of it like gravitational potential. If you lift a stone up, you've increased its potential. It "wants" to fall to a lower place in the same way electrons "want" to flow to the positive terminal. Imagine a water tower is a battery, if you connect a hose to it, water current will flow to ground.

So it seems to me the higher potential should be at the (-) terminal where the battery has pushed the electrons up the hill, just like the water has been pumped up to the water tower where it has the greater potential energy. So that's where the higher voltage should also be, and the voltage drop should happen as the electrons move from the (-) terminal through the starter motor and on to the (+) terminal, like the water falling to the ground through a water turbine. So in Aaron's diagram the "energy bump" should be from B to A and the voltage drop should be from C to B. But Aaron has it the other way around.


Is this once again conventional notation putting the higher potential and higher voltage backwardly at the (+) terminal when they should be at the (-) terminal according to the true electron flow model?



PairTheBoard

Sure. If you want to think of an electron river, that would be the high point, but that's probably not the best way to think about it.

Yeah. It has the appearance of backwardness because convention says so.

Going back to the q\vec{v} equation, you can see that the negative-ness of the charge introduces sign changes to all sorts of things. In some semi-technical sense, you should either be thinking of

1) + particles rolling downhill (Ben Franklin's original concept (sort of))
2) - particles rolling uphill (How we can understand reality to make sense with Ben Franklin's original concept)

It's not that the conventional understanding is wrong in any mathematical sense. It gives mathematically equivalent results. It's just that the underlying reality is wrong.

Maybe think of it like those oil-water toys. Is the water falling down or is oil floating up? It's kind of equivalent.

They have little worms inside them that are hopping drunk on tequila.

The math works the same whether you consider conventional current flow or electron flow. There is a potential difference between the positive and negative terminals. You have to get bogged down on atomic theory, pretty much never to engineer or design or comprehend electronic circuits.