# Does electrons not travel across GND?

Hi,

Another simple question from a simple man.

I see on nearly all example projects that it is made clear that the ground of every different components must be connected together, example, a power supply, Arduino, LED strip, PIR sensors, all ground will be connected.

But I was wondering, can this not be a route for electrons to pass and trickle on to another device? Say the power supply is different voltage, will that trickle not cause problems to the other devices?

Current goes round a circuit. Two circuits can share a single point (such as ground) and still
be independent as current can't flow along a single wire(*), only round a circuit.

There can be a problem if grounds themselves form a loop: "ground loops" can cause issues
with interference.

Bottom line, circuits are loops. At each point in a simple loop the current is the same, charge
has to move in lockstep.

(*) Well at radiofrequencies a single wire can be an antenna, but that's not what you are
Also minute amounts of charge can flow to equalize the potential when grounds are first
connected, but these really are tiny with low voltage circuitry.

The ground is the pathway that the electrons use to get
back to the power supply they came from.

Your Arduino should be connected to +5v.

Hi Yamiprem,

For the moment I'll talk about charge. It's exactly the same as electrons but it's got the opposite sign convention, i.e. it goes from positive to negative.

Yes, charge can travel across ground as you put it. The ground line is a conductor, same as any wire. The way you pose the question makes it sound like that's a problem - but it's not.

That's because (in most circuits) ground is the lowest voltage there is. Charge won't be able to trickle back up through other devices because that's going up a voltage.

I love an analogy so think of it like this. You have a mountain range with peaks of lots of different heights (different devices at different voltages). Each mountain has a river going from the top to the bottom (current). At the bottom you have a lake that all the rivers drain into (common ground). Each mountain has a pump which takes the water from the lake back to the top of the mountain (voltage sources).

The question you asked was equivalent, in this analogy, to asking if the water can flow down one river and then back up another. Not really! Random stochastic motion might mean that a molecule of water here or there makes it some way upstream, same as electrons exhibiting essentially random motion, but, on average, no.

Why do you need a common ground? Well there's a pinned post at the top of the forum explaining why. I mean, it's not absolutely essential in some projects and in some cases desirable not to but you need to make sure there's no electrical interdependency in those cases. An example might be when one circuit operates a relay which switches another, much higher-voltage, circuit. But there's a complete air-gap here so it's OK - it's the nature of relays.

You need a common ground in most circuits for two main reasons.

First, when you're switching with semiconductors, i.e. transistors and the like, you need to make sure that when you think you put the activation voltage onto the gate the transistor has the same point of reference as to what zero volts is. Voltage isn't absolute - it's just higher or lower than the voltage at another point in the circuit. If you tried switching a transistor without a common ground, then it may switch off or on at unexpected times. Two separate grounds are not necessarily at the same voltage.

Second, it plays havoc with the flow of charge. If you had that circuit where you're trying to switch a transistor, say you're trying to amplify a signal, and the only link between your power circuit and your controller circuit was the single line to the transistor's gate, then you're basically trying to send charge on a one-way trip from your controller circuit to your power circuit. And that's not sustainable. It'll just stop working really quickly.

Like I say, if you have no electrical connection between two circuits they don't need a common ground. An MCU controlling a relay doesn't need common ground with the circuit it's switching. A radio controller doesn't need a common ground with the model car it's controlling. My mobile phone doesn't need a common ground with my laptop.

But say I'm trying to transfer data from phone to laptop by a USB cable - there's a ground-to-ground line in there that is necessary. It's not just to allow you to charge the phone. It means that both the phone and the laptop have a common understanding of just how much energy per electron is a '1' and how much energy per electron is a '0'. In this case think of it like going bowling. Every time you bowl a 1 from your Tx to another device's Rx, you need to get a bit of charge back because electrons are flowing. The ground line is the ball return chute! OK maybe that's an analogy too far...

Hello yamiprem,
Welcome to the forum, your question might be simple, but it's not stupid.

Have you read this: https://forum.arduino.cc/index.php?topic=653831.0 ?

Also, a bloke called Kirchhoff had a lot to say on this subject, have a read of Kirchhoff's circuit laws. These laws are fundamental to electronics, once you understand them a lot of things will become clear.

Not directly related to your question but just as important is Ohm's Law

EDIT
++Karma; // To jbpeckham for your explanation!

Hi,

In addition to Kirchhoff's voltage laws (as Perry mentioned) there is another important "law" named Norton's current law.

Not trying to be picky but there is no "ground" in your circuits. Look at the power supply it has two terminals V+ and V- . Current leaves V+ goes through your circuit and returns to V- .

It is common to call the V- "ground" but I prefer "common" because it is common to your circuit.

So to answer your question, current flows "across ground". But better stated "current flows through the common of your circuit.

BTW at this point in your understanding you should ignore any reference to "electron" flow. While it is scientifically correct, the concept will needlessly confuse your understanding.

Good luck.

jbpeckham:
Like I say, if you have no electrical connection between two circuits they don't need a common ground.

So in the diagram I posted, assuming the Arduino is getting power via a USB cable (not shown), is the data line an "electrical connection" between the PIR circuit and Arduino circuit? Is this the reason why these two devices will be connected to a common ground?

Not sure it matters but we are assuming the power supply will be Earthed.

Thanks, everyone!

But I was wondering, can this not be a route for electrons to pass and trickle on to another device? Say the power supply is different voltage, will that trickle not cause problems to the other devices?E

Electrons don't 'trickle', they 'flow' , but in general we do not discuss electronics at the electron level. We discuss
current flow or direction or magnitude.
I am not exactly sure what it is you are trying to ask. Current flows based on the circuit configuration.
What is it you are asking ?