Do static charge in a strong insulator flow to a weaker insulator when both stay in contact with each other? For example, when an insulator weaker than air placed in a medium of air, would the static charges on the insulator be absorbed to the air slowly and finally the insulator becomes neutral? If so then what is the rate of flow from the insulator to air?
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The Triboelectric effect is the process through which materials can become electrically charged through friction when they come in contact with other different materials.
These materials do not have to be insulators for this effect to take place however if they are good conductors the charge will usually flow away.
There is a series of materials ranging from those that become positively charged to those like to remain neutral to those that become negatively charged. This series is independent of a materials conductivity. With aluminium, steel and silver being good conductors and wool and polystyrene being good insulators in between them on the list.
user288447
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I got confused at something. I see that polyurethane is a good insulator. Let's say we have a charged polyurethane. If it comes in contact with human skin would the charges in it flow through the skin since charge affinities of both skin and polyurethane are not close? The point is as far as I know good insulators prevents charges leaking away so what would happen if charged polyurethane and skin comes in contact? – Starior Aug 25 '14 at 17:57
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When you put a charged insulator in air, the reason you lose charge is mostly due to humidity in the air. I gave details of this mechanism in a recent answer to a related question: https://physics.stackexchange.com/a/130988/26969
The curves in the referenced paper (some of which I reproduce in that answer) show you how the leakage current is a function of the relative humidity - conversely if you know the amount of charge, you can compute the rate of charge loss using these curves.
For example, if you had a sphere of radius 10 cm charged to 100 kV, you would have a charge of
$$\begin{align}\\ Q &= 4\pi\epsilon_0 r V\\ &= 4 \pi \cdot 8.85\cdot 10^{-12} \cdot 0.1 \cdot 10^5\\ &= 1.11 \mu C\end{align}$$
So the surface charge density (charge / area) would be
$$\begin{align}\\ \sigma &= \frac{Q}{4\pi r^2}\\ &=\frac{\epsilon_0 V}{r}\\ &= 8.85 \mu C / m^2 \end{align}$$
Now the leakage data we will use came from a setup with 40 parallel plates with alternating voltages, so 39 pairs of gaps of 0.66 mm with up to 1.5 kV across them, and a surface area of 25.4 $cm^2$. That makes the highest electrical field $E = 1.5/0.66 = 2.3 MV/m$ and the area 990 $cm^2$. For a relative humidity of 50%, this setup produced about 10 nA of leakage current (reading from the plot reproduced from the paper referenced in the above answer):
Now the sphere above actually has an electrical field at the surface of
$$E=\frac{Q}{4\pi\epsilon_0 r^2}=\frac{V}{r}=1MV/m$$
translating back to the given setup, that corresponds to an applied voltage (across the 0.66 mm gap) of 660 V, and a leakage current (hard to read a log scale without subdivisions) of maybe $\frac{3 nA}{990 cm^2} = 30 nA/m^2$. For our sphere with a surface area of $0.04\pi m^2$ this gives a leakage current of about 3 nA - or about 0.3% of the charge on the sphere per second.
I gave the above calculation as an example of how to interpret / use the tables given - obviously the answer for your geometry will be somewhat different. Note that regions of higher curvature will have higher fields, and initially greater leakage - but this will not last since charge will not flow on the insulator; so soon, the electric field will even out.
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Thanks. That was a good answer. I just want to ask something. If we have two charged and connected insulators, would there be a flow of electricity from stronger insulator to a weaker insulator? – Starior Aug 25 '14 at 18:38
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By definition charge does not "flow" in an insulator. You have to define "strong" but in general the answer is "no". Touching two insulators (eg rubbing them) will lead to charge transfer (see other answer) but I don't think that is what you are asking... – Floris Aug 25 '14 at 18:41
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I meant if charge transfers from one insulator to another. I also meant an insulator which has the higher resistance by "strong insulator" – Starior Aug 25 '14 at 18:48
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To clarify: If the potential at the surface of the two insulators is different, there will be a flow of electrons to equalize the potential. The direction of the flow has nothing to do with "which is the better insulator" - only with the potential. My example of a sphere in air has a high potential on the sphere, but none on the air. So the air will "charge up" with the charge from the sphere (and eventually both will appear to have no charge since the potential of the air doesn't noticeably change). – Floris Aug 25 '14 at 18:50
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Did you mean electric potential by "potential"? Also does potential related fith electron affinity as user288447's link suggests that the difference between electric affinity and the rate of charge transfer is directly proportional. – Starior Aug 25 '14 at 18:57
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Yes I mean electric potential; and no that does not really relate to electron affinity. The transfer mechanism described in user288447's answer is triboelectric: the act of touching two materials and removing them again causes a few electrons to "stick" to the material with greater electron affinity but that has nothing to do with how "good" an insulator is. – Floris Aug 25 '14 at 18:59
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Is there a way to prevent the charge transfer between the insulators which have different potential? – Starior Aug 25 '14 at 19:05
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If they are not touching, then you just need to make sure you have really dry air (or vacuum). If they are touching, there is not much you can do... – Floris Aug 25 '14 at 19:06
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One last thing...If we have a charged conductor and neutral insulator, would there be a charge transfer between them? – Starior Aug 25 '14 at 19:12
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If they are touching, then probably yes. A little bit of charge would end up on the insulator. Exactly how much depends on the exact geometry - because the charge cannot flow away (it's an insulator), it will quickly even out the potential difference (in other words, the electric field between them will go to zero and then no further transfer happens). – Floris Aug 25 '14 at 19:24
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