[PmbPhy]

Since the king of insulting has posted twice since I posted I assume he was insulting again so I choose not to see his posts. However I can guess where his confusion lies this time and its about the "direct physical connection" comments. Unfortunately online definitions are never perfect. But textbooks are.

From Physics for Scientists and Engineers with Modern Physics -- A Strategic Approach by Randall D. Knight, (2004), pages 793-94

Any object which is physically connected to the earth through a conductor is said to be grounded. The effect of being grounded is that the object shares any excess charge it has with the earth. But the earth is so enormous that any conductor attached to the earth will be completely discharged.
   The purpose of grounding objects such as circuits and appliances. is to prevent the build up of any charge on objects.

Whomever is saying otherwise is quite wrong. Ignore them!

[Bored chemist]

Whomever is saying otherwise is quite wrong. Ignore them!

OK
You just told everyone to ignore you.

"Any object which is physically connected to the earth through a conductor is said to be grounded."
So, for example, a door handle, which is connected to earth via the timber door (timber is a poor conductor, but good enough if there's a high voltage, and a large contact area- like a doorknob) is grounded.
Now, lets see who
.

is saying otherwise

The handle in fact is connected to the ground through the door .

That is incorrect. Please look up the definition of grounded.

[Bored chemist]

Quote from: PmbPhy on Today at 15:59:00
Do you want the definition from a physics text?

No, but it looks like you should check it out.[/quote]


Looks like I was right.
You checked it out and found you were wrong.

[Bored chemist]

I choose not to see his posts.

Great way to stay uninformed there.
Hows it working out for you?

[evan_au]

But, like wood, earth isn't a great conductor.
So, you really can't need a good conductivity to earth something.

It's true that some rock types like granite have low conductivity, and safety earth connections in granite country need alternative methods (eg relying more on the Neutral wire). But most rock types give reasonable protection.

I agree that rock would not be good to make long, thin wires. But in its protective/electrical safety role, it is used to make short, really fat wires, and it's not too bad at this - 1 Ohm resistance is often achievable, especially in areas using "Multiple Earthed Neutral", where every house has its own earth stake.

The other important role of "Earth" is as a capacitor.
- This is why the old-fashioned "wireless" had a terminal for an antenna wire and an earth wire - the Earth is able to absorb the radio-frequency currents picked up by the antenna wire.
- Most of the capacitors we put in electronics have two plates, spaced very close together. This allows high capacitance in a small volume.
- But there is another form of capacitor which has just a single conductive element. It doesn't pack nearly as much capacitance into the same volume as a two-plate capacitor.
- A doorknob insulated by the door can act as a small capacitor, and can cause a spark when you touch it
- A car, somewhat insulated by its tyres can act as a larger capacitor, and can cause a spark when you touch it
- An earthed object is connected to the Earth as a much larger capacitor, and can cause a significant spark when you touch it
- I remember at university calculating the capacitance of the Earth - I don't remember the technique, but I do remember that I wasn't impressed with the comparison with what I could get at my local electronics store...

Of course, the Earth, insulated by the vacuum of space, has a higher breakdown voltage compared to a capacitor I could hold in my hand!

Even for an "Earthed" object, the inductance of the Earth wire and skin effect in the wire significantly reduces the current in an impulse with a fast rise and fall time, like an electrostatic discharge.

PS: These MIT lecture notes calculate that the Earth has a capacitance of around 0.0007 Farads, or 700 microFarads.
http://web.mit.edu/sahughes/www/8.022/lec06.pdf

[Petrochemicals]

Metals are electron hungry sooo.. 

When I wear shoes and spent time on the bed, I get an electric shock when touching the metal door-handle. I must be positively charged. So how is this allowed if the door handle is not connected to ground.

Try touching a wired earth object when you get out of bed , my understanding of electrical regulations is any wired 240v appliance with a metal case.

[PmbPhy]

It's true that some rock types like granite have low conductivity, and safety earth connections in granite country need alternative methods (eg relying more on the Neutral wire). But most rock types give reasonable protection.

That's beside the point. Whenever an electric circuit is grounded or a conductor is grounded it can't build up a charge. Try. It won't work. The earth is a charge sink. I used to be an electronics technician. That was when I was first acquainted with the subject. Every time I worked on an electronic device I always grounded the chassis. Not once in those years did I ever get zapped. You won't get sapped from a conductor which is conducted to ground with a conductor. - Just as the text said.

[Bored chemist]

These MIT lecture notes calculate that the Earth has a capacitance of around 0.0007 Farads, or 700 microFarads.

Which is pretty small if you are considering impedance at 50 or 50 Hz,

But it's huge compared to a person- have a look at fig 2
https://incompliancemag.com/article/human-body-model-and-electrostatic-discharge-esd-tests/.

Now, let's get some order of magnitude for the capacitance of the doorknob (on its own)
The capacitance of an isolated sphere is proportional to the diameter.
The Earth is about 13 million metres across a doorknob is about 0.065 metres So it's about 200 million times smaller.
So the capacitance is about 3.5 pF.
I don't think that's big enough to transfer enough charge to give much of a shock.
So I still think it's connected to Earth. How good does that connection need to be?
Well, the body is something like 100pF of capacitance. And the spark duration is something less than a tenth of a second- it's hard to say how much less.
That implies that the resistance of the circuit must be small enough that RC is less than about  0.1.
C is about 10^-10
So R must be a gigohm or less.

There are some values here
https://link.springer.com/chapter/10.1007%2F978-3-642-73683-4_6
for the resistance measured between two needles in a piece of wood. They vary from about 0.6 Meg to about 5 Gig.
So, it's in the right ballpark, especially considering that doorknobs are not just held in with needles.

[Bored chemist]

You won't get sapped from a conductor which is conducted to ground with a conductor. - Just as the text said.

You will if you are charged to a high voltage- which is the relevant case for the OP's circumstances.

[alancalverd]

The doorknob spends 24/7 connected to ground through the bulk and surface resistivity of the door, so it is reasonable to assume that it is at earth potential first thing in the morning. You could measure its resistance to true earth and it is unlikely to exceed a gigohm.

Walking across a carpet in insulating shoes generally charges the body to around 15,000 V - the value assumed for most ESD protection systems, though 50 kV is possible with really dry shoes.

This is enough to give you a very nasty surprise when discharged. Remember the door knob is not isolated but connected through a few megohms to a very large surface (the door) with roughly the same capacitance as your body, so you will lose half of your accumulated charge on contact. The duration of the discharge depends pretty much on how well the doorknob is connected to the surface of the door.

One of my more entertaining clashes between engineering and accountancy was in specifying the carpet to be fitted in a mammographic x-ray room. We had designed a special unit accessible to wheelchair users, and the radiographers worked on their knees. The superintendent asked for a pure wool carpet treated with industrial antistatic solution, but having paid over the odds for a special x-ray machine the finance department insisted on tendering for the carpet (the bit they could understand) and supplied a domestic wool-nylon mix without telling us. The first radiographer knelt down, shuffled across the carpet, and touched the first patient's nipple. The noise was unbelievable and the carpet was replaced the next day.

[Bored chemist]

I hope someone in finance ended up paying for the useless carpet.

[PmbPhy]

The doorknob spends 24/7 connected to ground through the bulk and surface resistivity of the door, so it is reasonable to assume that it is at earth potential first thing in the morning. You could measure its resistance to true earth and it is unlikely to exceed a gigohm

The door is not a conductor so the doorknob is not grounded and therefore the doorknob can have static charge built up upon it.

[Bored chemist]

Will someone explain to pmbphy that his decision not to look at what I type makes him look dim.
In particular, could someone quote the reference I gave showing that the resistance of a piece of wood can be less than a megohm.
https://link.springer.com/chapter/10.1007%2F978-3-642-73683-4_6
That might stop him repeatedly posting this sort of tosh.

The door is not a conductor

[Bored chemist]

I just measured the resistance between the front and back door handles of my house,
It's about 400 Mohm.
So, about 200M per door handle.

That's only a fact; I don't see it changing Pmbphy's mind.
(Again, I'd be grateful if someone would quote this, otherwise it won't reach the eyes of the "man who will not see".)

[evan_au]

My past lessons on handling ESD-sensitive electronic components are starting to coming back to me...
            ESD=ElectroStatic Discharge

Try touching a wired earth object when you get out of bed

The recommended way to discharge yourself to Earth is through a resistor of several hundred kilohms to a Megohm.

This limits the discharge current to something that humans can't feel - and also won't damage sensitive electronic components.

Don't use a single 1MΩ resistor from the local electronics store - they arc over at high voltages. Put 10x 100KΩ in series for better protection against high voltages.

I just measured the resistance between the front and back door handles of my house,
It's about 400 Mohm.
So, about 200M per door handle.

How did you measure that? My multimeter only goes up to 10 MΩ - and the leads aren't nearly long enough!

[Bored chemist]

The recommended way to discharge yourself to Earth is through a resistor of several hundred kilohms to a Megohm.

A megohm at 20KV "limits" the current to  20mA. That's enough to give you a shock.

[PmbPhy]

The recommended way to discharge yourself to Earth is through a resistor of several hundred kilohms to a Megohm.

That's so you don't electrocute yourself with a high current. You are neither an electric appliance or an electric circuit. High current destroys circuits. There's a good reason to ground circuits as you may know.

I'm tired of this thread. Think what you want but don't come asking questions if you won't accept the answer - From a legitimate source like a  physics text.
This limits the discharge current to something that humans can't feel - and also won't damage sensitive electronic components.

Don't use a single 1MΩ resistor from the local electronics store - they arc over at high voltages. Put 10x 100KΩ in series for better protection against high voltages.

I just measured the resistance between the front and back door handles of my house,
It's about 400 Mohm.
So, about 200M per door handle.

How did you measure that? My multimeter only goes up to 10 MΩ - and the leads aren't nearly long enough!
[/quote]

[alancalverd]

Not many people know that BC is a gnome, with a very small house and a tiny but very good insulation tester. Or possibly someone with a brain and a long piece of wire. 

Back in the day I used to design electrometers with >10¹⁵ ohm input impedance. No big deal measuring 400MΩ if you know what you are doing (a standard "Megger" insulation tester runs to 200 GΩ). We occasionally used reformed amber as an insulator (non-triboelectric, non-piezoelectric, and more dimensionally stable than polyethylene) but never wood.

[alancalverd]

I hope someone in finance ended up paying for the useless carpet.

Knowing the way of the world, I suspect he was promoted for challenging the "experts" and saving money, long before the carpet was delivered. 'Twas ever thus.

[Bored chemist]

That's so you don't electrocute yourself with a high current. You are neither an electric appliance or an electric circuit. High current destroys circuits. There's a good reason to ground circuits as you may know.

Would it  be better if you stopped talking bollocks?
The OP is talking about static from moving round the room.
That's not gong to electrocute anyone, is it?

(BTW, the word electrocute might not mean what you think it does. It's derived from "execute").

How did you measure that? My multimeter only goes up to 10 MΩ - and the leads aren't nearly long enough!

A long bit of wire (of sufficiently small resistance as not to matter), and  a better meter than yours.
A maplin wg020 with a 2 gig range.
I did check it against a couple of 1 gig resistors. If anything, it seems to over-read.