Explaining the static electricity

[Passiday]

Hello,

I am preparing a material about static electricity, and it's basically finished. However, I have a question what puzzles me. I hope people in this forum can fill that in.

So, the core process is that electrons hate to float in the insulators, so they hang on the surface of those materials. When they have a chance to jump to less effective insulator, they happily do it, they opt for bad party over really bad party. If there is chance to jump over to a good conductor, ie some metal with low potential, than that they do it with great passion, thus the relatively high current ("ouch!" when getting out of the car and getting discharge between your hand and car door).

Well, and the puzzling question is, how the electrons are replenished on that first insulator. I mean, if it's so bad that all electrons desperately want to get away from it, and when the material is stripped of all free electrons (ie, piece of amber, used many times to generate the static electricity), from where then the new supply of electrons comes? Perhaps it's an photo-electronic phenomena? Or, the air is even worse conductor than piece of amber, so those few electrons that hang with molecules in the air, they are happy to jump to the surface of amber?

 

[DrZoidberg]

It's not true that electrons always try to jump to the best conductor they can find.
In fact most plastics tend to charge negatively. If you rub your hand over a plastic foil electrons will jump from your hand to the foil even though the plastic is a much worse conductor then the hand.
When two different materials touch they often exchange electrons. That's a quantum mechanical effect. It happens also when 2 different metals touch.
Usually only a tiny fraction of the surfaces really touch. Even if it looks like 2 surfaces touch each other, under a microscope you could see that at most places there is actually a tiny gap.
Rubbing the materials helps to increase the contact area. But it's really the contact and not the rubbing that causes electrons to jump from one surface to the other.
Missing electrons in a positively charged object are usually replenished either by a current flowing through the object or on the surface of the object or through the air since air is not a perfect insulator. If the electrons are not being replenished then the object simply stays charged.

 

[Passiday]

It's not true that electrons always try to jump to the best conductor they can find.
In fact most plastics tend to charge negatively. If you rub your hand over a plastic foil electrons will jump from your hand to the foil even though the plastic is a much worse conductor then the hand.

Oh, that's new for me that the insulator factor is not the decisive one. So, could we then establish a scale of materials, where the material to the left always will pass more electrons than receive from the material to the right (when put in contact, that is)?

If such scale exists, there is some leftmost material that receives no electrons from any other material, and gives away electrons to any other material. And so on to the right, and soon we find that all the electrons have moved to the rightmost material. What's wrong in this picture?

 

[DrZoidberg]

That scale is called the "triboelectric series"
http://en.wikipedia.org/wiki/Triboelectric_effect

In that series air is at the positive end and plastic is negative. So theoretically a negatively charged piece of plastic should never give off electrons to the air. But in practice air contains positive and negative ions. The positive ions are attracted to the plastic and can neutralize it's charge. I don't know the exact details of why a positively charged air molecule can neutralize a negatively charged piece of plastic against the triboelectric effect. It's difficult to find any detailed information about this. But it works somehow.

 

[Passiday]

Thanks, this was valuable.

This makes me wonder if there are some typical everyday situations where a static charge is accumulated and could be afterwards used as a form of alternative energy.

 

[sophiecentaur]

All this cuddly chat about electrons having minds of their own and "wanting`" to jump etc. is not really treating the Physics of the subject, is it?
Why don't you read what Wikkers etc. have to say about Static Electricity and take a bit of time finding out some solid Science? There's lots of stuff available on the Web.You'll emerge from the exercise much better off than using the fairy-tale approach.

 

[Passiday]

:)
Well, I have a reason to be cuddly, anthropomorphizing the electrons. The material is for elementary school kids, with some cartoons, etc. Of course, the material has a disclaimer that electrons do not have mind of their own, but I think that explaining kids the electricity by using metaphor of electrons like small creatures is both fun and also helps to remember better.

Pardon my ignorance but what is meant by "Wikkers"? Is it Wikipedia? If so, this is first time I hear such jargon.

 

[sophiecentaur]

Hi
I had a feeling that the 'cuddliness' was related to education. Having taught GCSE and A Level Physics for some time, I have a serious problem with the notion of trying to teach very young minds stuff that really requires some very sophisticated tools (like Maths, for a start) to deal with. Try anthropomorphising Differential Calculus and everyone falls over.

I sympathise that you are required to deliver this stuff and I really think it is an imposition on you if you are not completely happy with it, yourself. Your posts seem to imply this.
Statements like "electrons want to", "volts are a force, pushing electrons round", "resistance is how hard it is to push electrons round", "electricity and water don't mix", "current takes the easiest path" (not all yours and definitely not Your fault , of course) are all nails in the coffin of understanding. When you connect up electric meters in a circuit, you get nice concrete numbers on them which represent good old Volts and Current. These can really mean something to kids because they can see them. I wish they had stuck with that. Someone, a while ago, decided that Particles were the only way forward in the teaching of all Science and that, instantly, made life a nightmare for nearly all teachers. It has also left many kids with dreadful misconceptions that they have to unlearn again if ever they want a better understanding.

Here is my 'noddy' exposeé of Static Electricity. All the atoms in a substance have 'just the right number' of electrons each. That applies to any 'uncharged' object. If you attach or detach electrons (millions and millions, for any noticeable effect) , then the object becomes charged. Rubbing two materials together can transfer some electrons between the two surfaces and the two objects (plastic and cloth) become charged (oppositely). Charges have been separated. There is now a potential difference and an electric force between them. A good insulator will not allow electrons to flow through it so the end of a plastic rod remains charged because electrons won't flow along it, onto or off the hand that is holding it. The charges remain on the surface. Dampness in the air or brushing the surface with your hand can allow charges to flow back. This all applies to the cloth as well, of course.
A conductor will allow charges to flow very easily and, to charge it, you have to insulate it ( a car on rubber tyres, for instance, will charge by friction through the air). As the charge builds up on an object, the Potential increases and can cause a current (spark) through which the charges flow and rebalance the situation.

Essentially, charging Separates Charges. The Net Charge will always be zero. Any old electrons will suffice, to neutralise a charge and the electrons on a large object (you or the Earth etc. ) need very little potential difference to allow some to flow and discharge a small charged object. In fact, you and the rod will actually Share the imbalanced charge.
There is, of course, loads more . . . .