Just to beat this horse a little more, if I understand in the case of the battery circuit the negative terminal serves as the common reference. What I don't get is what tying it to ground would do electrically. I think I understand that it would serve as a common reference, but the negative terminal is already doing that, right? The thing that got me all confused was the circuit on the right in the attachment. How would the right-hand circuit differ electrically from that on the left, if at all (e.g. if I measured the voltage drop across the resistor, would there be a difference between the two circuits)?
For some reason "ground" has been a hard concept to grasp, only because there seem to be so many. I understand that to get current flowing, you need a potential difference, and that potential difference has to be referenced to some level so that you can determine how much of a difference you're dealing with. But when looking at circuits and in discussions, I see ground being used in many different ways.
No,both circuits are the same.You can call some point in your circuit "Fred" if you wish,but the circuit is still the same!
As previous posters have said,it is necessary to have a reference point for multiple power rails,as descriptions of the supplies need in practice to mean the same thing.
A -5volt supply & a +5 volt supply are both referred to the same reference point,although a meter connected between them will read 10 volts,so you could call the two in series a "10volt"supply.(But it would confuse people!)
The terms "ground" & "earth"have a historical background,from when telegraph systems were single conductor,"earth return".
One leg of the supply (battery) was connected to an earth stake,the other to the single wire,via a Morse key.
At the other end,a very sensitive "sounder" was connected between the single conductor & an earth stake.
In theory,if we can make a really good connection to the earth,even though dirt & rocks are not good conductors,the cross-sectional area of this grotty conductor is huge,so it will be a low resistance path between any two points.
The problem is in getting this good connection!
It made sense for old time Techs & Engineers to read voltages with respect to the "earth connection",& eventually the terms "ground" & "earth" became synonymous with the return conductor, even if there was no
real earth connection.
When 2-wire balanced systems came into use,the connection of "return conductor" & "ground" on the telegraph lines became lost,but it remained in use on the terminal equipment.
"Ground","common","reference","earth",are all terms that have been used to mean the same thing,although "earth" is more recently,usually reserved for actual physical connections to the Earth.
You can declare any point in your circuit to be "ground",but it most commonly refers to the negative or positive supply rail in electronic circuits.
The standard in many older British circuit diagrams was to have a "Supply rail" drawn along the top of the drawing,& a
"ground" or "common" rail at the bottom,with the circuit drawn between them.
American,Australian,& Japanese practice was to place "ground" symbols where necessary to indicate a return connection to the "common"rail.
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