How Does grounding work - the theory?

[christian0710]

Hi,
I've often wondered: If you connect a negatively charged piece of metal to the earth, the electrons flow to the ground so the metal becomes neutrolized, but what is in in the ground that the electrons flow to? Is it the dirt or the water under the ground? How could dirt particles absorb electrons and how could particles be evenly conducted around the ground of dirtparticles are not electrical conductors such as metal and water?

 

[Simon Bridge]

Hi,
I've often wondered: If you connect a negatively charged piece of metal to the earth, the electrons flow to the ground so the metal becomes neutrolized, but what is in in the ground that the electrons flow to?

A very large amount of matter in all kinds of states.

Is it the dirt or the water under the ground? How could dirt particles absorb electrons and how could particles be evenly conducted around the ground of dirtparticles are not electrical conductors such as metal and water?

There is no such thing as "dirt particles" ... there are lots of kinds of materials in the ground that may be ionized in different ways.

A simpler way of thinking about it is to imagine the ground is a very very large conductor ...

Take an initially charged, but much smaller, conductor and attach it to this "ground" ... what happens to the charge on the small conductor?

 

[abitslow]

The electrons flow to the nearest positive charge, based on the 'pull' of the electrical field. The positive charge may be an ion, or may be a partial charge (called polarization). The flow may be through the ground or across the surface, depending on the various resistances (it follows the path of least resistance to the volume of highest postive field strength.) Dirt, rock, and the ground conducts electricity. Atoms are composed of a nucleus (which is positively charged) and about the same number of electrons (which have negative charge (one each)). This means that matter, all matter, is electric in nature. The flow through something with fairly high resistence is more like dominoes falling than a bowling ball rolling (or a bullet flying).
The ionization energy of the elements vary between 4 and 25 eV. That means that it takes no more than four millionths of a billionth of a billionth of a kilowatt.hour of energy to ionize any atom. That means that there is no atom that isn't easy to ionize when dealing with measurable voltages. Even a vacuum fails to prevent the flow of electricity. Why? Because whatever is holding the charge will spit out an ionized atom when the field is high enough. Nature abhors charge separation.
In a world surrounded by electrical power, it is important to understand that thinking it takes metal or water to conduct electricity is a recipe for death or serious injury. Perhaps if I mention the word "lightning" (which most people know flows through the air and into and through the ground), it will help? There are some cool pictures of lightning caused by dust storms in desert areas on the web. It is NOT rain that allows the current to flow.

 

[christian0710]

The electrons flow to the nearest positive charge, based on...

Thank you so much for this very clear explanation - Very well explained and interestign way of putting it into a larger context. So even in vacuum where ions are not held together by water in a lattiice structure ions can float around? AMAZING! Is this because the Field is stronger than the energy holding the electron in place? So it breaks covalent bonds in pure metals such as steel?

Take an initially charged, but much smaller, conductor and attach it to this "ground" ... what happens to the charge on the small conductor?

Nice example: The charge evens out, so if it's negatively charged compared to the ground, electrons flow from the conductor down to the ground until the Voltage between the conductor and the Ground is zero.

 

[Simon Bridge]

Nice example: The charge evens out, so if it's negatively charged compared to the ground, electrons flow from the conductor down to the ground until the Voltage between the conductor and the Ground is zero.

Not bad - so if you are using ground as your reference voltage, then the voltage on the charged sphere ends up at zero.

But what I want you to notice, as well, is that the majority of the charge on the small sphere ends up on the big one ... in proportion to their sizes.

If the big sphere is the size of, say, the Earth, and the small sphere is the size of, say, a golf ball, you'd be hard pressed to tell the difference between the tiny residual charge on the small sphere and a completely neutral sphere.

This justifies the treatment of the large sphere as zero volts and the model that all the charge from the small sphere is gone. It's as near as makes no difference.

 

[dmummert]

Some chemistry

Thank you so much for this very clear explanation - Very well explained and interestign way of putting it into a larger context. So even in vacuum where ions are not held together by water in a lattiice structure ions can float around? AMAZING! Is this because the Field is stronger than the energy holding the electron in place? So it breaks covalent bonds in pure metals such as steel?

Nice example: The charge evens out, so if it's negatively charged compared to the ground, electrons flow from the conductor down to the ground until the Voltage between the conductor and the Ground is zero.

A couple of things, christian0710; ions are not confined to water as a rule. It's easy to get that idea because of high school chemistry, where all the reactions take place in water, but ions are just molecules or atoms with unsatisfied electron orbital pairs. They can exist freely in any medium that doesn't ultimately fill Pauli's Exclusion principle. A very good example is common table salt. It is an ionic compound without any covalent bonding. Sodium and chlorine react (violently, exothermically) to form a salt where outer shell electrons attempt to jump to the other atom to fill an empty orbital. The S and P orbitals don't like to release electrons - they are electrostatically bound to the nucleus very strongly. Instead of covalence, where the two atoms share an orbital to balance the books, sodium and chlorine are held together purely by electrostatic force (ionically). You mentioned also the idea that a lattice structure might be associated with a water solution - this is actually true in some cases, but mostly solutions are ruled by Brownian motion.

I'm not quite getting where you were headed with the last two sentences in the quote. Here's how some of it works. An electrical field exists between any two points that differ in potential, or voltage. Most solids are crystalline in nature, meaning they self-organize into categories of atomic packing (and here is your lattice structure). The crystal - for simplicity's sake, look again at table salt. The crystal structure is cubic, with a grid of sodium and chlorine atoms all lined up and alternating. The crystal plane just obtained is placed again on the next layer and the next, etc, being offset by 1 in each x,y,z direction. From this simple example it turns ugly quickly as you examine different substances with different crystallizations. HCP - Hexagonal Closed Packed; BCC - Body Centered Cubic; FCC - Face Centered Cubic, and 20 some other variations of a few simple classes. You get into pretty interesting physics by studying the properties of crystals.

Anyway, the method of loosening molecular bonding is phase change, not electrostatics. You can melt steel and then proceed with chemistry involving covalent bonds, but an electrical field is not going to materially affect a chunk of cold iron. Another example, perhaps. The electron beam in a CRT or an electron microscope is generated by a hot filament. The filament is set at some arbitrary voltage to prevent charge accumulation, and the electrons that are boiled off pass between two plates with a potential difference of 5 - 20 kV. The beam goes through some shaping after that, but has no bearing on Ground Potential. In this case, ground or zero volts is the plate nearest the filament. The electrons illuminate the specimen in such a way as to provide either an electrical signal that can be decoded into a picture or as a black-on-green image on a phosphor screen that can be photographed. When the electrons have impacted the target, they once again complete the circuit to ground. This current can even be measured (and in fact, you'd better pay attention to it, otherwise cooking the sample is a possibility).

Grounding in the laboratory is a big issue. You have to account for every signal source, every potential difference. All of the instruments are grounded at a common point to eliminate what are affectionately known as ground loops. Real-world isn't any different. All of your outlets run back to a common ground in the electrical box, where they are connected to grid ground. As much as is possible, this is kept at zero volts.

So, a parlance definition would be to equalize your voltage with your surroundings.

 

[mishima]

If the big sphere is the size of, say, the Earth, and the small sphere is the size of, say, a golf ball, you'd be hard pressed to tell the difference between the tiny residual charge on the small sphere and a completely neutral sphere.

How much charge would it take to allow a golf ball sized conductor attached to the Earth to actually hold a noticeable charge? How could I ballpark estimate this (ridiculously large) figure?

 

[Simon Bridge]

That's a good point!
The results would actually be smaller than one would think.

If the two spheres were far apart compared with their radii, say, put the golf ball on the Moon, then the charge would be distributed in proportion to the radii. You can work that out.

The Earth Earth is actually lots more complicated than the simple model.