Static Electricity Shock on Door Knob

Thread Starter

rahulpsharma

Joined Sep 5, 2010
60
Many times, I get a momentary static electricty shock when I try to touch the door-knob in my room. The door knob is embedded in a non-conductive wooden door with no ground paths. So isn't a conducting path required for this static discharge phenomenon?

Thanks in adv and regards
 

MrChips

Joined Oct 2, 2009
30,927
Even though you may think that the wooden door is non-conducting, the door can still be a conductor.
The charge build-up is on your body when you move around, comb your hair, or change your clothes. You are discharging the static charge from your body when you touch the doorknob.

Static charge accumulation is a problem in cold temperatures, low humidity, high altitudes, and temperate climates. It is not usually experienced in warmer, humid, tropical climates.
 

GetDeviceInfo

Joined Jun 7, 2009
2,196
Think capacitance. The knobs charge will leak down through the door’s surface enhanced by moisture levels, surface finish, dust, etc, making it ready to receive a charge from your body. Given time to leak down again, ready for another recharge cycle.
 

Thread Starter

rahulpsharma

Joined Sep 5, 2010
60
Even if the door was a perfect insulator, there would still be charge transfer - until the charge equalizes.
So why doesn't the charge 'want to' equalize when I touch the wooden surface. Why only when I touch the metallic surface of knob. This is what made me query if it has to do something with the conductivity of surface.

Regards,
Rahul
 

WBahn

Joined Mar 31, 2012
30,236
Many times, I get a momentary static electricty shock when I try to touch the door-knob in my room. The door knob is embedded in a non-conductive wooden door with no ground paths. So isn't a conducting path required for this static discharge phenomenon?

Thanks in adv and regards
Given enough voltage, anything is a conductor.

Depending on the circumstances, you can build up a voltage potential between you and the surrounding objects of between about 1,000 V and 10,000 V or more.

But conduction really doesn't have much to do with it. If you have two isolated objects, conducting or otherwise, that come into contact, then any significant charge build up on either or both is going to tend to equalize (in the sense of distributing themselves so that the resulting electric fields balance out the other effects wanting to move (or resist the motion of) the charges). That redistribution takes the form of charges flowing, which is, by definition, an electrical current. It tends to be short and intense.

Now, why do you get zapped more if you touch a metal doorknob that, say, touching the door itself? Here is where conduction plays a role. When you touch the wooden door, some charge exchange happens, but it stays localized on the door where you touched it for a while since, because the door is a poor conductor, it takes a while to bleed away in response to that spots elevated voltage relative to the rest of the door. So while the same total charge (or something close to it) still gets transferred, it happens over a significantly longer period of time, perhaps milliseconds instead of microseconds. But when you touch the metal doorknob, the initial charge that is transferred is very quickly distributed away from the point of contact making able to continue accepting new charge. The result is a current that is perhaps thousands of times more intense and what we perceive is due to the current, not the voltage.

Once the charge is transferred, it will eventually (as in fractions of a second in most cases) get redistributed back to where it needs to be to balance everything out again, even if it is via very poor conducting paths. Of course, if the path is extremely poor, the charge can, indeed, stay there for quite some time.
 

WBahn

Joined Mar 31, 2012
30,236
So why doesn't the charge 'want to' equalize when I touch the wooden surface. Why only when I touch the metallic surface of knob. This is what made me query if it has to do something with the conductivity of surface.

Regards,
Rahul

It does. Get yourself all charged up and then walk up to the door and put your hand on the wooden surface for a few seconds. Now reach down and touch the door knob. At the very least, the shock will be considerably less and, very likely, won't happen at all. You are mistaking the absence of feeling a shock with the absence of the existence of a shock.
 

Thread Starter

rahulpsharma

Joined Sep 5, 2010
60
Even though you may think that the wooden door is non-conducting, the door can still be a conductor.
The charge build-up is on your body when you move around, comb your hair, or change your clothes. You are discharging the static charge from your body when you touch the doorknob.

Static charge accumulation is a problem in cold temperatures, low humidity, high altitudes, and temperate climates. It is not usually experienced in warmer, humid, tropical climates.
During the static charge discharge, is it the charge that is jumping over the gap OR is it the air gap that is breaking down like in case of a spark plug OR both these phenomena are two explanations of the same thing.

Regards,
Rahul

Trivia: I experience static electric shocks almost in all weather conditions. I always wonder if it has something to do with biology too... :)... I often hate to touch metallic surfaces cos the fear constantly plays over my mind to stay prepared for a possible shock...
 

Thread Starter

rahulpsharma

Joined Sep 5, 2010
60
Given enough voltage, anything is a conductor.

Depending on the circumstances, you can build up a voltage potential between you and the surrounding objects of between about 1,000 V and 10,000 V or more.

But conduction really doesn't have much to do with it. If you have two isolated objects, conducting or otherwise, that come into contact, then any significant charge build up on either or both is going to tend to equalize (in the sense of distributing themselves so that the resulting electric fields balance out the other effects wanting to move (or resist the motion of) the charges). That redistribution takes the form of charges flowing, which is, by definition, an electrical current. It tends to be short and intense.

Now, why do you get zapped more if you touch a metal doorknob that, say, touching the door itself? Here is where conduction plays a role. When you touch the wooden door, some charge exchange happens, but it stays localized on the door where you touched it for a while since, because the door is a poor conductor, it takes a while to bleed away in response to that spots elevated voltage relative to the rest of the door. So while the same total charge (or something close to it) still gets transferred, it happens over a significantly longer period of time, perhaps milliseconds instead of microseconds. But when you touch the metal doorknob, the initial charge that is transferred is very quickly distributed away from the point of contact making able to continue accepting new charge. The result is a current that is perhaps thousands of times more intense and what we perceive is due to the current, not the voltage.

Once the charge is transferred, it will eventually (as in fractions of a second in most cases) get redistributed back to where it needs to be to balance everything out again, even if it is via very poor conducting paths. Of course, if the path is extremely poor, the charge can, indeed, stay there for quite some time.
Thanks for your elaborate post... It pre-empted my subsequent questions too... :)... Thanks...

Regards,
Rahul
 

crutschow

Joined Mar 14, 2008
34,679
I was in a motel for a couple days one summer in Albuquerque where the weather was hot with very low humidity.
I walked down a long carpeted hallway to my room, and the large, visible arc when I touched the door knob was enough to remind me not to do that again.
So after that I used a metal key to discharge the charge, but I could still feel it some in in my fingers.

I think the charge was mostly going into the capacitance of the knob and the associated metal lock fixture.
 

cmartinez

Joined Jan 17, 2007
8,274
One more thing: it bothers me to no end every time I get shocked by a freaking doorknob ... if you wish to avoid feeling the painful jolt, make a fist and touch the knob first with one of your knuckles, instead of using your fingertips. The spark will still be generated, but the discharge travels through a path within your arm that is not connected to your body's most delicate nerves, avoiding practically all of the painful surprise.
 

WBahn

Joined Mar 31, 2012
30,236
During the static charge discharge, is it the charge that is jumping over the gap OR is it the air gap that is breaking down like in case of a spark plug OR both these phenomena are two explanations of the same thing.

Regards,
Rahul

Trivia: I experience static electric shocks almost in all weather conditions. I always wonder if it has something to do with biology too... :)... I often hate to touch metallic surfaces cos the fear constantly plays over my mind to stay prepared for a possible shock...
Air typically breaks down and ionizes at about something like one million volts per meter (there are a lot of variables involved). That works out to about 10 kV/cm, which is a pretty common ballpark figure for how much voltage you develop and the distance that your finger is from the doorknob when the spark occurs.

Certainly some people are much more sensitive to electric shocks than others, so it is possible that you are no more likely than anyone else to get a shock, just more likely to feel it.
 

Reloadron

Joined Jan 15, 2015
7,544
The door knob is embedded in a non-conductive wooden door with no ground paths. So isn't a conducting path required for this static discharge phenomenon?
The only thing needed is a high enough potential between your hand and the door knob. A ground path is not required. I build up a static charge on myself and reach for one of my dogs noses. I pull an arc and I get a mild shock as does the dog. I am not grounded and the dog is not grounded. I slide across a car seat and reach to put the key in. Zap a shock the car is not grounded. It's about potential difference. :)

Ron
 

Ya’akov

Joined Jan 27, 2019
9,230
The only thing needed is a high enough potential between your hand and the door knob. A ground path is not required.:)

Ron
This is correct, but thanks to the oft-mentioned ambiguity of the word "ground", it is also misleading.

Ground can mean the same thing as earth, but it can also mean—among other things—the 0V potential side of a circuit. This can also be at a higher potential that other things. That is, if we measure between two points and find, say, +10V, we know the red lead of the meter is connected to a point where there is 10V of potential relative to the black lead's point of connection—so we call the negative lead's side 0V, or incorrectly from the most rigorous point of view "ground".

This doesn't stop us from doing it, though, So, if you have developed a high voltage positive charge by stripping off electrons through friction (rubbing your shoes on the rug) it is actually possible to say, with a straight face, the door knob with it's free electrons represents ground!

This means there is a ground involved, it's just not earth. If we measured the potential between your finger and the doorknob, it would be very high. Let's say it is 35kV (carpet-based charging can produce that). So, we can say that your finger is at 35,000V and the knob is at 0V—ground you might say. This is because electrons always flow "downhill*", in terms of charge.

*At this point is quite possible to get into a conversation about the errors of Ben Franklin, conventional vs. electron flow; and even E and B fields, Poynting vectors, and many other refinements—but somehow that doesn't seem like it would help the TS at this time.
 

nsaspook

Joined Aug 27, 2009
13,418
*At this point is quite possible to get into a conversation about the errors of Ben Franklin, conventional vs. electron flow; and even E and B fields, Poynting vectors, and many other refinements—but somehow that doesn't seem like it would help the TS at this time.
The discussion is about static electricity instead of current electricity so no need to go there.
 

Ya’akov

Joined Jan 27, 2019
9,230
The discussion is about static electricity instead of current electricity so no need to go there.
Well, yes—until you get close enough to the door knob to exchange electrons with it. For the short time there is electron flow, there is current to deal with.

But I said I wasn't going to devolve into a discussion better suited to a different thread. Is it just impossible to avoid?! Is this an undiscovered law of... reality: "all (nerdtastic) discussions will end up in such tall weeds the interlocutors will no longer be able to see each other on account of them".
 

nsaspook

Joined Aug 27, 2009
13,418
Well, yes—until you get close enough to the door knob to exchange electrons with it. For the short time there is electron flow, there is current to deal with.

But I said I wasn't going to devolve into a discussion better suited to a different thread. Is it just impossible to avoid?! Is this an undiscovered law of... reality: "all (nerdtastic) discussions will end up in such tall weeds the interlocutors will no longer be able to see each other on account of them".
It is sort of a curious thing (most likely because the study of static electricity predating current electricity) that the models and analysis static discharge currents (release of static electricity) are normally under the header of ESD (as a cause and effect) as separate category from typical circuit theory.
https://www.esda.org/esd-overview/esd-fundamentals/part-1-an-introduction-to-esd/
Greek scientist, Thales of Miletus mentioned the earliest report of electricity. He found that after amber was rubbed, dust and leaves were attracted to it. The word "triboelectric", covered later, comes from the Greek words, tribo – meaning "to rub" and elektros – meaning "amber" (fossilized resin from prehistoric trees). When flowing electricity properties were discovered in the 1700s, static electricity became the term for the old form of electricity, which distinguished it from the new forms of electricity.
 
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