I had Electronics 100 in highschool and I’m a repair man and tinkerer, so I might have a hard time explaining myself since my math and theory are not the best, but here goes:
A circuit (in this example a DC one) is a flow of electrons from a source burgeoning with electrons, through a pathway to a ground which is a piece of metal which can take the excess electrons.
If I take the +5 rail from a power supply and attach a wire to that rail, I have just extended the rail of the +5 line. But if I touch that +5v line directly to a large piece of metal, won’t it short directly to ground? How do you determine how big a piece of metal is (or in the case of earth ground, the literal ground) for it to become a ground? On my arcade machines you can use the frame of the monitor as a ground, which is a relatively small stamped piece of steel, but I think those frames are usually looped back to earth ground as well for safety reasons.
edit: Another thing I realized is you can use another voltage line as your ground, if you use a 12v rail on one side and a 5v rail on the other, you will get 7v in the circuit as the difference between the two voltages. How does having a ground with a positive voltage happen?
There’s no such thing as “excess electrons.”
Circuits are, well, circular - all the electrons flow in a loop.
“Ground” is only a convenient point to take a reference from. Any part of the circuit can be “ground,” although typically the most negative point is used.
I am not sure your question makes sense, as such, but at a certain point large conducting surfaces isolated by a non-conducting material (including an air gap) will store a charge. They’re called capacitors.
Ground is whatever you define as having zero voltage. Typically, that is the Earth. The Earth is so large, it would take an enormous amount of charge to make the Earth be at even a single volt (relative to an Earth with zero total charge). So lets assume the Earth is at 0 volts, by definition, and take all other voltages relative to the Earth.
Now, if the frame of your arcade machine is connected to Earth, the frame will be at zero voltage as well. If you connect the 0 volt lead of your power supply to the frame, the frame will be at 0, and the +5 and +12 rails will of course be at +5 and +12.
If you instead connect the +5V rail to that frame, the Earth and frame will stay at 0, so that rail will also be at 0 volts, the +12 rail will be at +7 volts (relative to the Earth), and the 0 volt lead will be at -5 volts. Since all the parts are 5 volts lower, a few electrons will have flowed from the circuit into the frame and the Earth.
If the whole thing is isolated from the Earth, the total charge on it could be anything. The frame might be at -50V or +100V relative to the Earth. Then you could take the frame as your ground, and reference your circuit voltages to that instead of the the Earth. There’s not really a size limit for calling something ground in this case, but you’d probably want to have all you large pieces of metal connected and at a single voltage.
If you have one large piece connected to Earth and to the 0V lead, and connect another large piece to the +12V lead, it may take a lot of charge flowing to that piece to bring that second piece up to 12 volts, and you could have a large initial current flow when it is turned on. If you had a line in your circuit varying between 0 and 12 volts, and connected that to a large piece of metal, it could take a lot of charge to vary that piece’s voltage, and your transistor might have a hard time providing enough current.
ZenBeam gives a good overview of the relative voltages and ground.
It is worth adding - in many circuits there are multiple grounds - which sounds silly, but works like this. You have reference ground - which is simply what you define 0 volts to be. This curiously does not have to be any specific point, and there are circuits that define zero volts in quite odd places. You may have power ground. All current in the system must be conserved, so the various power supplies must get back exactly the same current as they supply. It is common that all the supplies are connected to a single reference point (which is usually conveniently defined as the reference ground). If this is done you can have a single heavy ground bus that returns all the current for all the power supplies. (Not everything actually returns all the current this way, a circuit with a split supply - having both +ve and -ve rails - may return little to no current via the ground). Understanding exactly how the return current flows can be critical to a successful design.
You have signal ground. In audio circuits and other systems where you are processing signals your signal connection need to provide a circuit. A single ended signal (which is what you have in the vast majority of home audio systems) is a signal plus ground wire. The signal ground is usually tied to the reference or power ground. For a host of technical reasons the exact manner in which this connection is done is critical, and is preferably done in one place only. After that the signal ground wires (usually the shield around the coax cable that the signal travels in) is not the same as reference or power ground, and you get problems if they are inadvertently connected.
Commonly the metal case of a device acts as a Faraday shield to reduce interference. Tying it to a ground helps in this. So often you see the signal grounds connected to the case. This is a mixed proposition, and can bring as many problems with it as it solves.
You have safety ground. If it is mains powered. This is usually tied to the frame or metal box the device is built in. It is connected via the power cable to the building earth and at some point to the neutral of the supply. The neutral is typically connected to earth stakes along the feed as it crosses the land. Safety ground is there to prevent the case being elevated to a potential that could cause enough power to flow between you and the earth to kill you. Usually a fault will trip a breaker or blow a fuse.
It is common to tie power ground and signal ground to safety/chassis ground. Again this is usually done at one point only, and the exact manner it is done can affect the circuit’s performance. Signal ground is often referenced via a resistor, or might only be coupled at AC (via a suitable capacitor), and is not capable of carrying any significant current between signal and safety ground.
Professional audio avoids most signal ground issues by using a balanced cable - with two wires, neither connected to any ground - to carry the signal current. Such balanced cables usually are wrapped in a shield that is connected to the chassis, but is not connected to the signal anywhere.
Power utilities can use current flowing in the earth to return supply current. Here in Oz, remote users can be supplied with a SWER - single wire earth return - feed. Because the earth is not that great a conductor (especially when it doesn’t rain for a few years) the voltage needs to be pretty high to reduce losses in the system. So there is a power transformer at the end of the run that drops the voltage down for domestic use.
There are certain similarities between a “large piece of metal” and ground.
The OP was mentioning the capacitance.
The ground is similar to the large capacitor in that the ground shall soak up all current and signal, never producing a voltage no matter how much current is delivered.
So too, the capacitor takes the current from an AC signal, and leaves no voltage signal to be seen. This is how it works as the ripple filter on the output of a power supply… The capacitor didn’t magically impose a DC voltage, it literally allows the AC current to flow, presenting a small resistance , and by voltage divider, there is no voltage in that AC signal.
Now if we look at DC, a really large piece of metal could lose even a DC signal … if it was a tiny DC signal , the signal could be lost as leakage into the air. But thats a bit obscure, its like using the metal roof of Boing’s largest hanger for an ADSL level signal … could happen, in theory, doesn’t happen in practice. )
The OP’s question implied that the only reason not to use large pieces of metal for the “+ve” , or “signal” was only if it was so large it was if ground. That is not so. There are many OTHER reasons not to use the large piece of metal for +ve (power) OR signal.
As noted, it’s very common that the “most negative” part of a circuit is considered ground. Because electrons have a negative charge, they flow opposite to the way you suggest, i.e. from ground to positive.
Mathematically, voltage measurements live in an R-torsor: They look like real numbers (hence the R) but there is no unique zero value; you can measure differences between two voltages, but it’s meaningless to say something is at a specific voltage until you’ve arbitrarily picked a point which you say has a voltage of zero. The planet Earth is convenient for this because it’s so utterly huge that nothing we do is likely to change its voltage appreciably.
If you remember calculus, and if you made it to integral calculus, you might remember that the indefinite integral of a function f(x) is F(x) + C, where F(x) is the antiderivative and C is any term or collection of terms which do not depend on x. Therefore, antiderivatives exist in a function-torsor, usually a real-valued-function-of-one-argument-torsor, which is a phrase so ugly you begin to understand why mathematicians come up with such insane names for things.
I think what rogerbox is looking for here is a link to a circuit diagram showing current flowing through a single-path circuit (with a power source) that connects two halves of a split planet/Earth. There’s no direct return path for the electrons, as the planet is split in two with the halves floating apart in space, but current flows as one half has electrons removed (making it more positively charged) and the other half has electrons added (more negative). My google-fu is weak today, so bonus points to anyone who can provide a link.
Everything is relative, so let’s assume our planet has a uniform charge before we split it in two, so both halves have the same charge and therefore no voltage difference can be measured between them. Our circuit is a 9 volt battery in series with a resistor, each free wire connected to a planet half. Current will flow until one half of the planet is charged up to 9 volts relative to the other half of the planet. Essentially the split planet is a giant capacitor, and due to its large size (and assuming the two halves aren’t too far apart) it will take a lot of electrons to charge up a hemisphere to 9 V relative to the other hemisphere, and there will be a large capacitance between the planet halves.
Bringing this thought experiment down to practical scales, and have the same circuit, but with two halves of a split ball-bearing instead of a planet. It doesn’t take many electrons to charge up half a ball-bearing to 9 volts, and while current will still flow between the two separated halves while the bits of bearing are charging up, they will charge up very, very quickly as the capacitance between is very small.
In summary, to finally answer the OP, for a free-floating conductive chunk of something to be an effective “ground” for a circuit there must be:
[ul]An assumption of equal charges on both the original circuit “ground” and the free floating conductor so that the initial voltage measured between them is zero
[/ul]
[ul]
[li]There must be sufficient capacitance between them so that it takes a very long time for a workable current to charge up the floating conductor to the applied voltage potential. This capacitance is a function of the intersecting areas of the original circuit “ground” and the floating conductor, and the distance between these two parts.[/li][/ul]
[ul]
[li]The floating conductor and/or original circuit are not charged by other means, e.g. atmospheric charges, ionic storms etc.[/li][/ul]
Meeting these conditions is not practical, so for convenience the planet Earth is taken as the ultimate reference for mains electrical ground (US) or Safety Earth (UK). I don’t like the term “ground” as it’s too ambiguous - for any circuit diagrams I create I prefer to label the reference conductor as “0V”. Where’s the “ground” of a circuit on a satellite or aeroplane?
Even using the planet beneath us as a reference is not ideal, as the non-zero resistivity of the planet and the telluric currents flowing all over the place mean that you’ll measure a voltage difference between any two stakes driven into the soil. This is why single point grounding schemes are preferred for Safety Earth installations, and if multi-point earthed systems are unavoidable then the conductors between them have to be of sufficiently low resistance that all the earthed points are at the same potential (give or take the few volts that local regulations allow).
It’s not necessary to have a direct return path for electron flow to enable current flow in a circuit - no actual electrons pass in the gap between the plates of an ideal capacitor, yet current flows in any circuit that charges or discharges the capacitor’s plates.
I just want to approve of the OP describing electricity as “a flow of electrons from a source burgeoning with electrons”
Once you get past Electronics 100, they start describing electricity as ‘holes’ flowing from positive to negative. That’s hogwash, there really are things called ‘electrons’ and there is no such thing as a ‘hole’ (Unless you want to consider a ‘hole’ as “someplace an electron isn’t”)
I believe this all has something to do with the way Benjamin Franklin described some of his electrical experiments.
Well, there’s conventional current flow and actual electron flow. We know that actual real-life electrons flow in the direction from negative to positive (or really, less negative) potential. But when drawing circuits we conventionally assume that current flows from positive to less positive (or source to ground.) In the end it doesn’t actually matter that much, most of the time.
When you get to semiconductor electronics, holes is a very useful and accurate concept. In a semiconductor, there’s a small gap in the energy levels between filled and unfilled electron energy bands. If the semiconductor were at absolute zero, all the electrons would be in the lower energy band, and none in the upper band. The electrons in the lower band aren’t conduction electrons, so the pure semiconductor won’t conduct current.
As the temperature increases, thermal energy kicks electrons from the lower band to the upper band, leaving holes. The electrons and the holes both transport current, so the semiconductor conducts. Calling it a hole is a very descriptive name. It’s the absence of a single electron state from an otherwise filled level. Just as a regular hole is the absence of dirt from what would otherwise be ground.
I have never heard the term “hole” used in regular circuits, only in semiconductors. And those were at least junior or senior level classes.
I think what you’re getting at, rogerbox, is the distinction between a “chassis ground” and an “earth ground”.
There is exactly one earth ground, and you make an imperfect connection to it by driving a metal rod into the dirt, or by connecting to anything else that is connected to it. Earth ground is indicated on a schematic diagram by a triangle pointed down and with a wire coming out of the middle of the top side.
You make a connection to chassis ground by connecting to the frame or body of an object, assuming this is conductive. Chassis ground is indicated on a schematic diagram by a stack of longer and longer horizontal lines, the longest at the top. It looks like you filled in the earth ground triangle with horizontal hatching, and then erased the outline.
The bigger a chassis is, the more like earth ground it acts. If you’re in the middle of a big airliner doing your electronics hobby, you’d think of them in the same way. Earth ground is the chassis ground on our home planet.
I suffered through many years of the EE meat grinder, and the only time we talked about electrons and holes was in the microelectronics (“solid state physics lite”) class.
Unless they are involved in the design of semiconductors or electrochemical batteries, most EEs don’t talk about electrons. We simply talk about current. Conventional current, to be more precise.
On a related note, I was talking with an old ham radio operator a few years ago, and he referred to voltage as “EMF” and used the letter E as a symbol for voltage. When I casually told him we no longer do this, he insisted I was wrong, and questioned my academic background. :rolleyes:
On the subject of ground, I think some people are overcomplicating the issue.
For starters, think about your cell phone (or any portable and battery operated device). The negative battery terminal is (probably) referred to as “ground” in the design, even though it is not physically connected to the earth. It is simply a reference point in the circuit.
(In actuality, there are probably multiple grounds in the cell phone, such as analog and digital grounds. But I won’t get into that since it muddies the discussion.)
For devices you plug into your 120 VAC outlet, it’s the same deal… the circuitry has a reference we call “ground.” This reference may – or may not – be connected to the earth via the power cord. If the device has a metal chassis, then the chassis is probably connected to the earth. There are no guarantees, though. I have a home theater amplifier with metal chassis, and the chassis is not connected to the earth by design.
Ummm, what? I’m not seeing anything in the OP talking about splitting the Earth in two.