What's the difference between Negative and Ground? Like a negative terminal and a copper ground? I was reading that they are both the same; one person stated that negative is just more negative than ground. What's the difference? Do I even need ground if I have a negative terminal?
Ground means whatever is attached to this symbol in the schematic:
Everything that touches this symbol in the schematic is actually connected to everything else that touches the symbol. Since so many things connect to it, this makes the schematic easier to read.
Usually the negative side of a battery is attached to that. But, there are many circuits that work differently. Some circuits need a negative voltage, so the positive side of a battery would be "ground". Some circuits need positive and negative voltages, in which case there could be two batteries, one with the negative side attached to ground, and the other with the positive side attached to ground.
This works because voltages are relative. Put three \$10k\Omega\$ resistors in series, and attach them to a battery. The difference in voltage from one side of the battery is 3V (because it's a 3V battery). The difference in voltage from one side of a resistor (any of the three) to the other side of the same resistor is 1V, because the battery's 3V is divided among 3 resistors of equal value.
Since voltages are relative, ground exists as a sort of assumed reference voltage. If we say an input is "5 volts", we mean "the difference between the input and ground is five volts".
In the context of AC, things aren't really different, except that tradition has done a good job of making the same term "ground" mean many things. It still could mean whatever is attached to that symbol, or it could mean that 3rd connector on the wall. More on that later.
As far as the circuit is concerned, live and neutral are no different. Pick either one, and the other oscillates between a higher and lower voltage, relatively. If all you have are those two wires for reference, they are indistinguishable.
The difference is more important when you consider safety. The things around you are at some particular electromotive potential (voltage). Current flows when there is a difference in potential. The neutral AC line should be about the same potential as most of the things around you, so in theory, if you touch it, and also Earth, you don't get shocked, because there is no difference in voltage. If you touch the live wire, you do get shocked, because there's a difference in potential.
However, I said neutral should be about the same potential as Earth, and since you are probably touching Earth, you. But, I wouldn't trust your life on it. There could be a faulty transformer on the pole near your house. There could be a lightning strike nearby. The house would be wired backwards. Or, as I mentioned the circuit will function even if the wires are reversed, it could be plugged in backwards. In the US, one of the prongs is a bit fatter to prevent this, but you never know. This is why there's the third connector, called ground or earth. This should go to a big copper rod near your house stuck in Earth, like this:
It doesn't otherwise connect to anything else. There are some times this is important for safety, and other times it's important for other reasons. Point is, it has nothing to do with the electrical power supplied to your home.
How can I tell if I need to ground something to earth vs. "ground" to the negative terminal? When do I ground to the chassis of my device?
If we are talking about a device that plugs into the wall, leave these questions to someone else. Each country has safety regulations, and these regulations exist for good reason. Buy a DC power supply that takes care of all that for you, and connect to its output, and nothing else. Don't connect to Earth through the 3rd pin on the wall or you may circumvent the safety features of your power supply.
If you are wondering if the "ground" symbol on your schematic should also be connected to box your project is in, well, it depends. Maybe you want to do that for RF shielding. Or maybe you don't, because you don't want some other device with a different idea of "ground" to touch it, which could result in noise in your circuit or melting something. In many circuits, it doesn't matter at all.
You mentioned two possible issues: type of ground in use and direction of of current flow (source of the electrons).
There are four common types of ground:
- Earth - Literally, a copper rod shoved deep into the ground.
- Chassis - The common connection using the frame of something.
- Digital - The common connection for digital circuits.
- Analog - The common connection for analog circuits.
Realistically, all of these grounds are the same, but they are usually kept separate for the most part, only being connected together in a single place to avoid loops, and often using an inductor. It is called "ground" because of the original reference point being the actual ground, as in the a rod shoved into the earth. However, anything not connected to the earth (such as a battery powered device, or a vehicle) still employs a common node commonly called a ground.
When you use the negative terminal of the battery as a "ground" it is really just the common node (return) of the circuit. A circuit has to have a closed loop for current to flow, and using the negative terminal ensures that all of the components have an identical reference point.
Occasionally, separate portions of the circuit will be completely isolated, each with its own "ground," but again, this really just means the common node of that portion of the circuit.
As far as the "source of the electrons," that is a tricky matter to discuss. Typically, in the US at least, current is shown flowing from positive to negative. While this is not entirely accurate, that is the convention used for modern circuit design. Take a look at the answers to this question: Does electricity go from negative to positive or vice versa?
Your voltage regulator is placed at the positive lead because that is the convention used to design the components - nothing too mystical about it. It is the positive voltage potential that is being regulated. The negative terminal is acting as the reference point (common node) in the circuit.
In the example of powering an LED from the 9V battery, it doesn't matter if the resistor was placed between the positive terminal and the LED, or the LED and the negative terminal. It is not "regulating the voltage" so much as restricting the current. What does matter is the orientation of the LED. The anode has to have a higher potential (voltage) than the cathode for current to flow, so the LED anode is connected towards the positive terminal while the cathode is connected towards the negative terminal.
The position of the resistor is insignificant, it just needs to be there because the current flowing through the LED at 9V would immediately burn it up. The LED more likely drops 2 or 3V. The remaining voltage is dropped across the resistor, and it is this value that determines the current through the LED according to Ohm's law: V = I*R.
I_LED = ( V_battery - V_LED ) / R, where V_battery - V_LED = V_resistor.
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