what is ground?

[RivaultUser]

in DC, is ground is the same as negative lead?

I read a lot of articles and each time people say ground it just reference to negative. Is this correct ?

And why sometimes a complete circuit still has a ground? ( imagine a circuit with resistor and LED has a ground)

[lewis]

Ground is just the name for the reference point of all voltages in a circuit. It's often called '0V' (zero-volts) or 'earth' in automotive electronics. If you have a 6V supply rail in your circuit somewhere, that 6V will be referenced to a ground - in other words to measure the 6V you need to put your multimeter positive lead on the 6V terminal, the negative lead on the ground terminal.

In simple circuits with only one power rail, ground is often the same as 'negative'. If you had a battery-bulb-switch circuit, the 'ground' would usually be the negative battery terminal (although you can have whatever terminal you like as 'ground', remember it is only the name for the reference point for all the voltages, and as a designer you can decide to put that reference point wherever you like. But nearly always it makes sense to have the ground the same as the negative of the power source in a single supply circuit).

But you can't always say that 'Ground equals negative'. In op-amp circuits, and many other circuits, there is a negative AND positive supply. That means you have, say, a positive 15V supply AND a negative 15V supply, referenced to... you guessed it... ground. So there's a power rail that is 15V ABOVE ground (+15V), and another rail 15V BELOW ground (-15V). To put it another way, the two rails have 30V between them with ground sitting in the middle.

So be careful with the terms 'ground' and 'negative', they don't necessarily mean the same thing. 

[RivaultUser]

Ground is just the name for the reference point of all voltages in a circuit. It's often called '0V' (zero-volts) or 'earth' in automotive electronics. If you have a 6V supply rail in your circuit somewhere, that 6V will be referenced to a ground - in other words to measure the 6V you need to put your multimeter positive lead on the 6V terminal, the negative lead on the ground terminal.

In simple circuits with only one power rail, ground is often the same as 'negative'. If you had a battery-bulb-switch circuit, the 'ground' would usually be the negative battery terminal (although you can have whatever terminal you like as 'ground', remember it is only the name for the reference point for all the voltages, and as a designer you can decide to put that reference point wherever you like. But nearly always it makes sense to have the ground the same as the negative of the power source in a single supply circuit).

But you can't always say that 'Ground equals negative'. In op-amp circuits, and many other circuits, there is a negative AND positive supply. That means you have, say, a positive 15V supply AND a negative 15V supply, referenced to... you guessed it... ground. So there's a power rail that is 15V ABOVE ground (+15V), and another rail 15V BELOW ground (-15V). To put it another way, the two rails have 30V between them with ground sitting in the middle.

So be careful with the terms 'ground' and 'negative', they don't necessarily mean the same thing. 

Thank you for your reply man!

Before I want to ask again, I want to tell you that I'm a newbie and I have a slow brain to catch thing, so please bear with me

Now I understand that Ground isn't always negative rail of circuits, but sometimes ground also refer to something else ( can you tell me??? )

What I want to ask is :
1. My friend once told me that buzzing sound that sometimes we here in the speaker is because the speaker doesn't have ground, and when he touched the metal thingy he said to me : " see, when I touched this now the speaker has the ground through my body because I touch the earth ". What does it mean by ground in his sentence ? ( Oh and if you don't mind, can you also explain me about the speaker is buzzing ? )

2. I just saw that there are also regulators that make negative volatge ( -5, -9, etc. ) Why would we need negative voltage ? I mean, we cannot power something with it right ?


Correct me if I'm wrong about this :
10 V means that I have : 10 V on the positive lead & 0 V on the negative lead
can 10 V also means : 5 V on the positive lead & -5 V on the negative lead

[grumpydoc]

What I want to ask is :
1. My friend once told me that buzzing sound that sometimes we here in the speaker is because the speaker doesn't have ground, and when he touched the metal thingy he said to me : " see, when I touched this now the speaker has the ground through my body because I touch the earth ". What does it mean by ground in his sentence ? ( Oh and if you don't mind, can you also explain me about the speaker is buzzing ? )

Loudspeakers tend to "buzz" because you are hearing a 50/60Hz signal induced in the audio path due to proximity of mains wiring. Also, quite a large induced mains frequency signal is present on the human body and if you touch the input pin of an amplifier you will get a loud buzz as you effectively inject a 50/60Hz signal into the electronics.

2. I just saw that there are also regulators that make negative volatge ( -5, -9, etc. ) Why would we need negative voltage ? I mean, we cannot power something with it right ?

Wrong, you can power things with it quite well. Look up old "positive earth" automobile electrics - any point you measured in such a vehicle would be negative with respect to the chassis but everything was powered just fine.

The thing to realise with voltage is that it is all relative - there is no "universal" reference point that is at 0V so we choose a convenient one according to need. When you measure voltage you are measuring the potential difference between two points in the circuit.

Think of it like height, or floor number in a building. Some buildings have a ground floor and go up only. Some have basements and, starting at the ground floor, you can go either up (+height or voltage) or down (-height/voltage).

[FenderBender]

Every circuit has a "ground". If it didn't...well then it's not a circuit. As grumpy said, a voltage is a difference in electrical potential between two points. My high school physics teacher never taught us the word "voltage". He always used "potential difference". You might always want to keep that in the back of your head.

Your friend is correct, but I think he means earth-ground. Think:  the 3rd prong on your outlet that gets connected to the planet Earth.

This is a different matter. In most audio applications, you do not tie signal or output ground to earth. That causes problems. The general practice is to tie the metal chassis of the amplifier to ground. This helps to eliminate EMI and can keep some of the "buzz" out of your system by rejecting external noise from other appliances like switching power supplies, or fluorescent lighting, and others.

In regards to your second question. Voltage is only a measure of a difference. Think of it as absolute value. You can make ground almost any point you want. I agree it is a little confusing but don't let it bother you too much!

One real example might be when you are working with op amps. If you have an AC input to the op-amp, in order to have an AC out, the op amp needs to be provided with a positive and negative supply voltage so that it can generate output voltages that are both positive and negative (AC).

[Shuggsy]

There's a song in there somewhere...

What is ground?
Baby don't short me,
don't short me,
oh no...

[HardBoot]

Ground is funny because G 10v 20v can also be 10v G -10v
Also ground is an evil thing of magic in HF.
Voltages are relativistic unless you're dealing with static charges and chemistry/physics.
There's a minimum voltage, but no maximum voltage, but you'll just shed power away like nuts so there's a practical limit... and where the min is, arguable.

[AlfBaz]

The word ground gets used for just about anything even when its not tied to "ground" or earth. Its nearly alway referring to the return path for the supply.

Conventional current flow means current flows from a high voltage to a low voltage. If you choose to place the black, negative lead of your multimeter to the lower voltage and probe the circuit with the positve lead you'll get +ve readings (mostly). With the black lead still on the lower voltage and you place the positve lead on the lower voltrage also, your meter will say, wait for it... 0V!

As the physical earth or ground is synonimous with zero potetial, that is zero volts, any supply point used as a referrence for other voltgage levels invariably gets called ground.

As for neagtive supplies, if you take a battery and place your black meter lead on the +ve and start probing the circuit with you red lead you will get neagative volteage values and you could say the +ve is ground. Case in point... Connect two batteries in series and place your black lead where the 2 batteries are joined and start probing with your red lead you will see both +ve and -ve voltage values. You can then call the series connection ground, current will flow from the positve to ground, fromn ground to the negative and fromn positive to neagtive... its all relative.

If the 2 batteries were 9V and you decided to call the +ve ground then you would have -9 and -18V. If you call the -ve ground then you have +9V and +18V if yo call the midlle ground then yo have -9V and +9V.
The circuit you are working with will dictate how you choose to use the available voltages and wether or not you wish to tie the zero voltage to ground.

Having said all that I am assuming you are dealing only with DC and only with batteries. If you are using power supplies you must be aware of how the power supply is hooked up and you may not be able to arbitrarilly tie any voltgae to ground because one of them may have already been tied to ground in the supply, so by tieing another to ground you will be createing a short circuit

Hope that helps

[JoannaK]

There's a song in there somewhere...

What is ground?
Baby don't short me,
don't short me,
oh no...

(like the one describing Opamp connections)

Put me up,
put me down.
Put my feedback to the ground,
Put me up,
and make me happy ...

(I think this one needs a Dave-Cad like draving.. )

[SamD]

"Potential difference" reminds me of the all too common error of adding "S" to "RPM": "RPM's". There is a "potential" between two points. The addition of "difference" translates into the two points being- potentially- somehow different from one another.

[rolycat]

"Potential difference" reminds me of the all too common error of adding "S" to "RPM": "RPM's". There is a "potential" between two points. The addition of "difference" translates into the two points being- potentially- somehow different from one another.

Not really - there is a useful distinction between potential and potential difference.

Potential difference is the change in electric potential between two given points, whereas potential is the potential difference between a given point and an assumed reference point.

[peter.mitchell]

ATM machine!

[rolycat]

ATM machine!

And of course you can't operate an ATM machine without a PIN number...

[lewis]

ATM machine!

And of course you can't operate an ATM machine without a PIN number...

Which of course has an LCD display and some light emitting LED diodes on it.

[SamD]

[quote

Not really - there is a useful distinction between potential and potential difference.

Potential difference is the change in electric potential between two given points, whereas potential is the potential difference between a given point and an assumed reference point.
[/quote]

Wait a minute- we're talking about a specific potential or voltage- not a [change] in potential. Whether one side is considered reference, positive, negative, ground, common- what have you- the net result (if present) is some specific potential (of electricity). I'm assuming, of course, that we are not using the sine wave of AC to represent change.

The intent isn't to be pedantic- despite my seeming persistence- but to be definitive and concise; a rare commodity on all too many forums.

[rolycat]

Not really - there is a useful distinction between potential and potential difference.

Potential difference is the change in electric potential between two given points, whereas potential is the potential difference between a given point and an assumed reference point.

Wait a minute- we're talking about a specific potential or voltage- not a [change] in potential. Whether one side is considered reference, positive, negative, ground, common- what have you- the net result (if present) is some specific potential (of electricity).

Real circuits don't usually have just two sides. If one point in a circuit has a potential of 3V relative to the reference point (let us say ground), and another has a potential of 5V relative to ground, then there is a change in potential between those points, and we say that the potential difference between the two points is 2V. I don't see why you consider this standard terminology to be erroneous.

I'm assuming, of course, that we are not using the sine wave of AC to represent change.

Naturally  not.

The intent isn't to be pedantic- despite my seeming persistence- but to be definitive and concise; a rare commodity on all too many forums.

If you intend to be definitive it is necessary to ensure that your definitions are clear. Concision is admirable, but omission is not.

[SamD]

I'm dispensing with quotations due to my ineptitude in with the iPad touch screen.

This is bordering on semantics; to regress, my original intent was to clarify terminology. When speaking of two (2) electrically charged points, a voltage or potential exists. There is no potential 'difference'; ignoring polarity for the moment (if DC), the voltmeter will read the same regardless of connection- there is no 'difference'.

Enter the "Third Man" (Great movie with Orson Wells, BTW). This third point (we'll call it ground) now gives validity to the prospect of differing potentials- or potential difference.

To generalize about "..real circuits…" not usually having just two (2) sides is expanding on and going further than my original comments- between two points, exists a potential. It may be a high potential or low potential- but not a potential difference.

[IanB]

Potential difference (sometimes abbreviated to P.D.) is the standard (and accurate) term used in physics to describe the difference in voltage potential between two points. This naturally corresponds to the two test leads on a voltmeter that  may be used to measure that difference. When a potential difference exists then current will tend to flow between the points considered.

[AlfBaz]

This is bordering on semantics; to regress, my original intent was to clarify terminology. When speaking of two (2) electrically charged points, a voltage or potential exists. There is no potential 'difference'; ignoring polarity for the moment (if DC), the voltmeter will read the same regardless of connection- there is no 'difference'.

Not only is there no potential 'difference', there's no potential to do work so there's no potential, because there is no electromotive force. There has to be a difference in potential energy for a voltage to exist.
 To then turn around and introduce a third point that creates a potential difference to prove there was a potential in the first two points is somewhat disingenuous and quite frankly ludicrous   

[Teneyes]

A grounding Story,
It North American grounding is what other places call Earth and most Mains Power  is wiring so that if any HOT 110 , 220 Vac connects to the case of Equipment the  electric power flow will flow into the Grounding wire and cause Circuit breakers to trip open. 
But here is a good story.
     My neighbour across the street had a roof of 1 meter of snow Side off Her roof and break her AC Power conduit off her House,  bad place for the 6cm pipe.
Now the AC power shorted to the Pipe, and because this was before any of her curcit breakers, it did Not disconnected and the Electricity flowed into her grounding, and into the ground/earth and on to the Water pipes , then the flow went along the water pipe and into the next neighbours hot water tank pipe and along the grounding wire for the Hot water tank, This 14 AWG grounding wire connects to the AC neutral/ return line at the Second neighbours circuit breaker box. from there the power flowed back to the Main electric company's Transformer outside on the power pole. So this circuit with no circuit breakers, had low resistance and a 200Kw supply , Needless to say the ground wire got red hot , and was smoking in the walls, the neighbour shut off the all breakers , but that could not interrupt the electrical flow. Eventually the wire burnt to an Open Circuit.  and there was $20,000 fire damage.
Another neighbour had this electricity go to his Gas Furnace from the gas Pipes and damaged the electrical controls on his Furnace!!
Be careful grounding is used correctly.

A good design is to have Separate grounding system for Power and Electronics/instrumentation.

[TerraHertz]

And of course it all depends on what voltage is considered significant, ie 'not ground.'

http://science.nasa.gov/science-news/science-at-nasa/lis/lis_4/
"The voltage difference between ground and ionosphere is 200,000 to 500,000 volts (200 to 500 kV)."

Note that the ionosphere is a conductive medium, so is the 'ground', and the atmosphere between isn't a very reliable insulator, especially when wet. Hence lightning.

Also there are flavors of 'ground.' Chassis ground, circuit ground (which may or may not be connected in some way to chassis ground), shield ground (ditto), summing ground points, etc. Ground isn't necessarily fixed either, hence 'ground bounce' as referred to in logic design. (It's undesirable btw.) While in RF there's no such thing as 'ground' since everything is a circuit element with complex impedance - even the 'ground' plane.  While in waveguide systems, where is your ground now? Or there are 'grounds' that are so localized in a circuit that people use a different word - 'common', eg "common emitter transistor circuit" where the reference is just one circuit node, but it's still a ground of sorts.

Then there's high voltage discharge devices, like can crushers, traveling arc lasers, and so on. In which the entire circuit is 'ground' more or less, but with extreme transient effects.

'Ground' is just a word, that can mean many different things depending on the context. Ideally it's a circuit-wide reference, but wherever there's current flowing and non-perfect conductors, there's a voltage difference.
 
I sometimes think that understanding 'ground' is about 50% of understanding all of electronics.

[nardev]

You can read more interesting things about grounding here: http://www.esgroundingsolutions.com/about-electrical-grounding/what-is-step-and-touch-potential-and-resistance-to-ground.php

As i see, some of people have pretty interesting stories about grounding.

At my faculty, there was professor explaining what is step and touch potential. And she would always mention case of mysterious dying of cows and horses in villages near Jablanica and Konjic (Small places in Bosnia with few large hydro power plants) just after II world war.

Engineers were inexperienced and all those high powered transmission lines were poorly protected and grounded so that animals, who tend to have larger step, started dying and man didn't feel such spike since man has smaller step and could not be endangered.

[Zero999]

Ground can mean two different things, when one circuit is driving another. In the attached circuits an astable is flashing two LEDs alternately and activating a piezo buzzer. As far as the astable is concerned, 0V really is 0V but the piezo buzzer's 0V is oscillating between +V and 0V at 2Hz. If you put the meter across the piezo buzzer, the reading would change between 0V and 5V.