[alancalverd]

So what happens if you earth the bottom of the plate?

You can do the experiment and find out.

The problem with the given explanation is that if the metal plate is very thin, the potential gradient  between the upper and lower surfaces of the plate will be very large, so the charges won't separate. But the result is the same.

And why is the ground considered to be positively charged? You can do the same experiment with a different dielectric that retains a positive surface charge, so the earth suddenly decides to be negatively charged. How does it know? 

Beware of naive explanations of what is a rather more complicated phenomenon.

[Bored chemist]

Beware of naive explanations

The first thing I did was ask for a better explanation.
What are you waiting for?

How does this work then?

[Bored chemist]

so the charges won't separate.

Why not?
But, looking on the bright side, if you say that for a thin plate the charges won't separate, at least it looks like you have recognised the fact that they will separate for a thicker plate.
So the next question is how thick (or thin) does the plate need to be compared to say... an aircraft?

Is the length of a plane less than the thickness of an electroscope plate? (I think the ones I saw at school were made from coffee tin lids).

[paul cotter]

If there were active lightning in the area, even without a direct strike, I would expect enormous field gradients to exist and hence charge separation would occur. What started this controversy was petrochemical's suggestion that charge separation leading to a voltage differential between the front and rear of the aircraft would occur by reason of forward movement through the air: I don't see this happening, at all.

[Bored chemist]

If there were active lightning in the area, even without a direct strike, I would expect enormous field gradients to exist and hence charge separation would occur. What started this controversy was petrochemical's suggestion that charge separation leading to a voltage differential between the front and rear of the aircraft would occur by reason of forward movement through the air: I don't see this happening, at all.

PC talks a lot of twaddle; his signature says so.
But charge separation in a conductor is real

[Petrochemicals]

. What started this controversy was petrochemical's suggestion that charge separation leading to a voltage differential between the front and rear of the aircraft would occur by reason of forward movement through the air: I don't see this happening, at all.

Why not? Do you think that static electricity generation on a balloon is immediately equalised even though only one side of the balloon is rubbed?

[Bored chemist]

It seems that Alan doesn't understand the charge distribution an a conductor, but he's still ahead of PC who has yet to work out what a conductor is.

[alancalverd]

You need to distinguish between charge separation, a static phenomenon, and charge flow, which is dynamic.

[paul cotter]

Petrochemicals, a balloon is an insulator unless metalized and hence could have different charge distributions. If one removed the trailing static wicks from an aircraft I could imagine a large static charge building up but it would not lead to a differential between front and rear as you asserted.

[Petrochemicals]

Petrochemicals, a balloon is an insulator unless metalized and hence could have different charge distributions. If one removed the trailing static wicks from an aircraft I could imagine a large static charge building up but it would not lead to a differential between front and rear as you asserted.

Did I not mention paint on a plane yet? Plus someone I believe has mentioned magnetic fields. If it is in motion the electrons are being blown by the wind down the plane to the extremities in simplistic thinking.

Alan's conductor distribution whilst at rest is a different kettle of fish. Wherever ever electrons are moved or removed is an inbalance until the system is at rest and the charge equalises. 

[evan_au]

Did I not mention paint on a plane yet?

Yes you did.
But the interesting article from bored chemist about metallic mesh built into aircraft laminates already addressed this issue - the laminate itself is an insulator, as is the paint. So the combined thickness of both must be limited.

Here is the link again, for your convenience:
https://www.compositesworld.com/articles/lightning-strike-protection-strategies-for-composite-aircraft

If it is in motion the electrons are being blown by the wind down the plane

What is the wind resistance of an electron?

[Bored chemist]

But the interesting article from bored chemist about metallic mesh

Regrettably, I didn't say anything that interesting.
I just pointed out that Alan was talking hogwash when he said

You can't separate + and - static charges on a conductor.

because, you clearly can.
About 8 minutes in here.

https://www.khanacademy.org/science/physics/electric-charge-electric-force-and-voltage/charge-electric-force/v/conductors-and-insulators

[Edited to add the vid link].

[alancalverd]

Here's the problem

Dielectric materials are poor conductors of electricity because they do not have any loosely bound or free electrons that may drift through the material. Electrons are required to support the flow of an electric current. The current flows from the positive to the negative terminal and, in the opposite direction, as free electrons that flow from the negative to the positive terminal.

Dielectric materials support dielectric polarization, which enables them to act as dielectrics rather than conductors. This phenomenon occurs when a dielectric is placed in an electric field and positive charges are displaced in the direction of the electric field, while the negative charges are displaced in the opposite direction. Such polarization creates a strong internal field, which reduces the overall electric field within the material.

So we have two completely different models that give the same result! On the one hand, it is asserted that charging by induction works because conductors conduct electricity and thus sustain an internal field the cancels the applied field,  and on the other hand insulators sustain an opposing internal field because they don't conduct electricity.

It is left as an exercise to the reader to explain what is going on. [hint - think fields rather than currents]

But as far as the present question is concerned, there is no external electric field affecting a plane in horizontal flight so no polarisation or charge separation anyway!

[Bored chemist]

there is no external electric field affecting a plane

Unless... maybe, it's in a thunderstorm or something.

[Petrochemicals]

Did I not mention paint on a plane yet?

Yes you did.
But the interesting article from bored chemist about metallic mesh built into aircraft laminates already addressed this issue - the laminate itself is an insulator, as is the paint. So the combined thickness of both must be limited.

Here is the link again, for your convenience:
https://www.compositesworld.com/articles/lightning-strike-protection-strategies-for-composite-aircraft

If it is in motion the electrons are being blown by the wind down the plane

What is the wind resistance of an electron?

I think the laminates are the composite aircraft, very little of the airframe of a composite aircraft is metal. On a metal aircraft it has several layers of paint, this adds considerable weight to the aircraft. As paint is non conductive it effectively insulates the airframe. The breakdown voltage of the paint is probably not great. It doesn't answer the static conductor at rest in a neutral field question.

[alancalverd]

American Airlines abandoned all-over paint some years ago and saved a fair amount of fuel by just polishing the aluminum.The only large aircraft that carry substantial amounts of (nonreflective/anti-infrared/visual camouflage)  paint are military, but composites generally incorporate white pigment in the final gel coat to reflect solar heat. 

[Petrochemicals]

American Airlines abandoned all-over paint some years ago and saved a fair amount of fuel by just polishing the aluminum.

Yep and it probably had to be kept polished due to the friction from oxidised material. I imagine then you have the problem of damaging the integrity of the pressure Hull.

[SeanB]

No need to keep polishing, just have to do a weekly wash to keep salt accumulation from building up. They do however give the metal a clear coat protective finish, which bonds well to the thin surface oxide, keeping the surface shiny. You save weight, because you do not have multiple layers of heavy pigment, just a single coat to protect. Alternatively you use something akin to an automotive wash and wax liquid, that removes the dirt and salt build up, and leaves a thin film behind.

[alancalverd]

Salt on airplanes? We get bugs on the windshield and the leading edges at low level, and I guess the navy would have a problem if the left their assets on the flight deck, but American Airlines spend most of their time above 30,000 ft.....

The stuff you really want to stick is deicing fluid, and I've always wondered why it is sprayed on at such high velocity that most of it seems to bounce off. If you deice when airborne, it can be done with a "weeping wing" that just trickles the gunge into the airflow with minimal loss, but the ground deicers seem profligate, to say the least.

I recall a winter morning when all flights from Stansted were delayed for 2 hours because the the deicing trucks were parked outside overnight, and thanks to a thunderstorm they were, er, covered with ice. If you've time to spare, go by air. 

[Bored chemist]

Yep and it probably had to be kept polished due to the friction from oxidised material.

A lot of people know that many aluminium alloys essentially don't "rust"- particularly if they are anodised.
Could someone explain that to PC?