# Why is the negative side of a battery grounded when it’s the source of electrons?

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I’m trying to understand the reasoning behind grounding the negative side of a battery in vehicle electrical systems, given that the negative terminal has an abundance of electrons. In electron flow theory, electrons move from the negative terminal (where there is an excess of electrons) to the positive terminal (where there is a deficit of electrons).
If the negative terminal is the source of electrons, why do we consider it a “sink” and connect it to the chassis ground? Wouldn’t it make more sense to ground the positive side, which is where the electrons are flowing to?

Additionally, I’m interested in learning about the thought process and design principles that electrical engineers follow when making this decision. What are the safety, performance, and practical implications of grounding one terminal over the other?

In: Physics

It actually doesn’t directly matter wich side you ground. Voltages are potential**differences** and grounding defines where your zero potential is. So if you ground the positive side you simply get a negative voltage at the negative terminal instead of a positive one on the other side.

The flow direction of electrons is irrelevant for this. They always flow from high density to low density area basically.

What is important is that you’re consistent in that decision. If you ground plus in one device and minus in the other then touching both at the same time will electrocute you. So we just decided that negative side always gets grounded to avoid accidants

Consider it like air for a moment. We have three spots:
– The wide open world, absolutely filled with air molecules.
– One end of a tube with a relatively many molecules/high pressure.
– The other end of that tube, with relatively few air molecules/low pressure.

A pump ensures that the *relative* pressures between the two are constant. I have a valve that I can open to allow air molecules to flow from the high pressure end of the tube to the low pressure one and the speed of *this* flow is what matters. I cap one end of the tube, and keep the other one open so that the pressure is equal to the air around it. In this case, it actually *doesn’t matter* which side I cap – because the pump moves a certain amount of molecules and creates a certain pressure differential no matter what. I don’t truthfully care about the actual pressure, I care about the differential.

That’s how it goes with electricity. The wide open world is ground – a massive source of air/electrons. The end with all the molecules is the negative end, and the one with less is the positive one. The pump is the battery, creating a relatively constant difference in potential between an anode and a cathode. It doesn’t *actually* matter if the anode has a different potential to the ground or not, what matters is the *difference* between the sides.

In electrical theory it doesn’t matter which convention you choose.

In the real world where we live in a wet, oxygen-rich environment though, positive ground encourages corrosion of the surrounding bodywork whearas with negative ground the electrical contacts tend corrode, and these are in theory easier to replace and/or protect.