I was curious about why the pins on my Raspberry Pi didn't have a positive and negative pole, but positive and ground. I read about it on various web pages, but each article I read seemed to answer a different part of the question. I think I got it, but I'd like to have confirmation.
I will use a metaphor for electricity, as I'm not an expert. I will compare electricity to water flowing down pipes. The height it pours down will be electric potential, or voltage and the amount of water will be the current, or amperage. The product of these two will be power, or wattage. Not a perfect analogy (there's no resistance in this system) but it will have to do.
In reality, electrons flow from the negative pole to the positive pole, not unlike in my water system. So if I continue my analogy, I can call the ground floor (European ground, numbered zero) of a building "ground level," and each floor a volt. Let's start looking at what ground means.
Floor +3V will need to receive a flow of electrons or water. It will therefore need to be underground. It will be three floors beneath ground level. There will need to be a faucet at ground level and a drain (positive pole) three floors below.
And, contrariwise, floor -3V will need to send electrons or water. It will need to be above ground. It will need to be a faucet. That means that, for this floor, I will no longer need a faucet at ground level, but a drain.
So, at ground level, I will need either a faucet or a drain or both, depending on whether the water comes from above or is needed below, or both. In other words, the ground can act as a positive or a negative depending on what the other pole is.
Is that more or less correct?