Since electrons flow from negative to positive, why are cars grounded to the negative terminal on the battery?


Seems like you’d want to be careful with the source of the electrons so the negative terminal would be covered/protected and the positive would be wired to the frame. But it is the opposite. I presume there is a good reason that I do not understand.

In: Engineering

It really doesn’t matter whether you use positive or negative ground. Some vehicles have had positive ground. The convention of saying that current flows from positive to negative was established by Ben Franklin before we knew about electrons. Conventional current flow is opposite electron flow. Current flows in a loop from source to load and back to source.

Interestingly, electron flow is very slow compared to the speed of electricity. It is usually less than 1mm/second while electricity flows at 50% to 90% of the speed of light. Electricity travels as an electromagnetic field. I like to think of it as one charge pushing the others along like a drinking straw full of peas. Add a pea to the end and one will immediately pop out the other end.

The direction electrons flow is, in terms of circuitry, irrelevant. Voltage is actually relative, and does not exist without two terminals. For instance, if you have the negative terminal grounded (at zero volts) and the positive terminal is “live”, then you can think of it as the positive terminal sucking electrons from the grounded terminal. This also means that grounded terminals are almost always safe to touch, no matter if they’re “positive” or “negative” (I use quotations because if it is grounded, it is at zero volts and therefore neither positive nor negative). In q high voltage system, if you’re shocked by a positive terminal, it is because the positive terminal sucks electrons out of you, which causes a shock. In this case, you are serving as the negative terminal.

It’s mostly for liability purposes.

Batteries generate hydrogen gas when the are charging. This can sometimes explode when connecting the battery cable as there are usually sparks. Ideally, connect the positive first, then connect the ground further away from the battery.

By jumping directly to the positive and negative terminals, you increase the risk of igniting potentially flammable gasses coming from the battery via the spark that often happens during the connection. These gasses can occur in certain scenarios, though is much more rare in this day and age.

If they tell you to connect both the positive and negative, and you cause an explosion, they could potentially be held liable.

If you find a CLEAN ground, you can successfully jump a car in the manner described, and it IS safer. But it is often hard to find a clean ground inside an engine chassis on a vehicle that needs a jump.

Ground is simply defining a given point in the circuit as a 0 reference. It doesn’t much (there are some secondary considerations) matter which way it is as long as you’re consistent about it. Lots of old vehicles, military vehicles, and old military vehicles do it the other way around and define the positive side of the battery as ground. It’s even fairly straightforward to convert a vehicle’s grounding.