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Static electric shock appears blue, but why?


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It’s winter in the Midwest US. The air is extremely dry. This leads us to more easily build up static... picking up electrons. Our hair goes in funny directions. We shock each other when touching. It’s a fun part of winter.

We also get shocked when touching the light switch on the wall since it’s connected to the ground of the house. 

The shock becomes visible at night, which adds a layer to the experience. 

Touching the light switch, feeling the jolt as the electrons flow from the finger to the lower resistance copper wire of the toggle switch into the ground, I can see the little micro lightening bolt jumping away from me toward the house ground. 

But why does it look blue, and not orange or white or something else? Is it just the temperature... the wavelength of the current being transferred?

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Comes from the Nitrogen mainly.

 

Quote

Deexcitation of nitrogen

The excited nitrogen deexcites primarily by emission of a photon, with emission lines in ultraviolet, visible, and infrared band:

N2* → N2 +

The blue light observed is produced primarily by this process.[1] The spectrum is dominated by lines of single-ionized nitrogen, with presence of neutral nitrogen lines.

https://en.m.wikipedia.org/wiki/Ionized-air_glow

 

You have an... interesting.. idea of 'fun' lol. Kills me visiting relatives up North during the winter.

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Scuffing your feet on the carpet while holding a fluorescent bulb is another way to pass the time leveraging static electricity (My childhood was pre-Star Wars, so we didn't know to pretend it was a light saber)

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  • 1 month later...
On 2/22/2021 at 7:11 AM, iNow said:

Touching the light switch, feeling the jolt as the electrons flow from the finger to the lower resistance copper wire of the toggle switch into the ground, I can see the little micro lightening bolt jumping away from me toward the house ground. 

light switch usualy plastic, not conductor

I think by the color of the discharge, everything is trite. It depends on the spectrum of light emitted during the discharge. That is, this or that discharge generates a certain wavelength. Apparently a blue tint indicates a large proportion of violet radiation.

By the way, the arc from electric welding generates light close to the sun, and includes the shortest wave of ultraviolet radiation, which is absorbed by ozone. Probably using electric welding without special protection is dangerous

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19 minutes ago, altaylar2000 said:

light switch usualy plastic, not conductor

True but:
It's next to the two earthed metal screws that hold the cover in place, and you can make sparks by putting your finger near them.

|It's also relatively thin plastic and may well conduct at the high voltages involved.

And, of course, it may be a metal switch  or a metal cover.

 

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8 minutes ago, John Cuthber said:

True but:
It's next to the two earthed metal screws that hold the cover in place, and you can make sparks by putting your finger near them.

|It's also relatively thin plastic and may well conduct at the high voltages involved.

And, of course, it may be a metal switch  or a metal cover.

I think it's just a static effect, like the charge on a comb or dress. Such a small charge will not pass through plastic, there is need about 5-10 killovolts to do that

If switcher is metallic that is possible

Edited by altaylar2000
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3 minutes ago, altaylar2000 said:

there is need about 5-10 killovolts to do that

And that is well within the range of typical static electricity.
https://en.wikipedia.org/wiki/Static_electricity#Static_discharge

You don't know much about this, do you?
 

5 minutes ago, altaylar2000 said:

Such a small charge will not pass through plastic

It isn't the charge that makes a difference, it is the voltage (though the two are related).

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29 minutes ago, John Cuthber said:

You don't know much about this, do you?

Okay. To begin with, why do you think that the charge can only be in relation to the earth and is necessarily discharged into the earth?

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2 hours ago, altaylar2000 said:

Okay. To begin with, why do you think that the charge can only be in relation to the earth and is necessarily discharged into the earth?

It won't necessarily be discharged to earth - you can shock yourself on a doorknob (which I regularly do in winter when stripping off a layer of the sticky mats in my lab; lots if static electricity generated)

But if there is a path to ground such as what JC mentioned, that's the usual path the discharge takes, because it is intentionally made to be a low resistance path when wiring up electrical power systems

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11 minutes ago, swansont said:

t won't necessarily be discharged to earth - you can shock yourself on a doorknob (which I regularly do in winter when stripping off a layer of the sticky mats in my lab; lots if static electricity generated)

But if there is a path to ground such as what JC mentioned, that's the usual path the discharge takes, because it is intentionally made to be a low resistance path when wiring up electrical power systems

The earth has different potentials in different places. Conductors drawn from different places of the earth can give a discharge, for this reason realized potential equalization systems.

It does not matter at all where the body is discharged, to the ground or not, so that the discharge needs a charge, the potential difference at the poles of the "capacitor"

And in fact, in most electrical systems, the main protective conductor is not the connection to earth, but the connection to the neutral of the transformer or generator. Connection to earth sometimes not used at all

And ground is not a low resistance path. As a rule, this path has more resistance than neutral, and in addition, this resistance is not stable (depending on the weather)

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On 2/22/2021 at 4:11 AM, iNow said:

It’s winter in the Midwest US. The air is extremely dry. This leads us to more easily build up static... picking up electrons. Our hair goes in funny directions. We shock each other when touching. It’s a fun part of winter.

We also get shocked when touching the light switch on the wall since it’s connected to the ground of the house. 

The shock becomes visible at night, which adds a layer to the experience. 

Touching the light switch, feeling the jolt as the electrons flow from the finger to the lower resistance copper wire of the toggle switch into the ground, I can see the little micro lightening bolt jumping away from me toward the house ground. 

But why does it look blue, and not orange or white or something else? Is it just the temperature... the wavelength of the current being transferred?

 

Endy has already answered this beautifully. +1 from me as well.

 

I would just like to emphasis that the colour of the discharge is not to do with the discharge electricity.

It is to do with the gases present.

This is why a 'spark' inside a discharge tube is not blue but the more usual shades of yellow/white.
There is no nitrogen in the flourescent tube.

Note the light from the tube is conditioned by the internal phosphor coating, not the spark or the gases present.

Quote
A fluorescent lamp tube is filled with a mix of argon, xenon, neon, or krypton, and mercury vapor. The pressure inside the lamp is around 0.3% of atmospheric pressure.

 

Edited by studiot
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8 hours ago, altaylar2000 said:

Okay. To begin with, why do you think that the charge can only be in relation to the earth and is necessarily discharged into the earth?

OK, imagine I get some sort of electrical generator- perhaps a Wilmshurst machine, and I put it on an insulating slab like a thick sheet of polythene or something.

And I set it running (let's say it has a battery drive motor, and a remote control- so I can do that without anything being connected to Earth). such that it charges the two spheres - one positive and the other negative with respect to eachother.

 

And then imagine that I bring an earthed wire near to one of the spheres.

 

What do you think will happen?

 

8 hours ago, John Cuthber said:

You don't know much about this, do you?


 

5 hours ago, altaylar2000 said:

The earth has different potentials in different places.

Not very, and not for long. It is generally quite conductive.

5 hours ago, altaylar2000 said:

And ground is not a low resistance path.

It doesn't need to be.

Static sparks are typically thousands of volts.

The capacitances involved are of the order of pico farads.
So even a bad earth connection with a resistance of 10 ohms would allow a current of hundreds of amps to flow. And that would discharge teh capacitor in a period of the order of a nanosecond or less.

 

But for that to happen, the current would need to start and stop within a nanosecond.

And that, in turn would mean that the rate of change of current was very large-hundreds  of gigamps per second.

And that could only happen if the inductance of the circuit was small enough.

But to drive a rate of change of current of 100 GA/S with a few KV you need an inductance of about 10 pico Henries or less.

And even a few inches of wire has much more inductance than that.

So the thing that limits the discharge in electrostatics is generally the inductance, rather than the resistance.

So... why did you point out the the resistance isn't all that low?

 

 

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On 4/1/2021 at 7:49 PM, John Cuthber said:

OK, imagine I get some sort of electrical generator- perhaps a Wilmshurst machine, and I put it on an insulating slab like a thick sheet of polythene or something.

And I set it running (let's say it has a battery drive motor, and a remote control- so I can do that without anything being connected to Earth). such that it charges the two spheres - one positive and the other negative with respect to eachother.

 

And then imagine that I bring an earthed wire near to one of the spheres.

 

What do you think will happen?

Typically a comb and hair is used for this example.
If the discharge is with the ground, does this mean that it will not be with other objects?

On 4/1/2021 at 7:49 PM, John Cuthber said:

Not very, and not for long. It is generally quite conductive.

a lot and a little is relative

always

On 4/1/2021 at 7:49 PM, John Cuthber said:

It doesn't need to be.

Usually it is in real systems

Edited by altaylar2000
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