# In order to charge the glass rod, why do we RUB it with a sillk cloth?

Why can't we get the same results if is wrapped with a silk cloth for some time? What actually happens when we rub it?

-
I'm voting to close this question as off-topic because it's about physics – endolith Feb 17 at 15:55

If you left a glass rod wrapped in a silk cloth for "some time" there would be a net transfer of charge.

The difference rubbing makes is accelerates the process due to charge transfer aided by friction

Look up the Triboelectric effect

Materials close together on the series may not experience any charge transfer if simply touched (add friction into it however...) while materials further apart will.

What this also means is two materials that were initially GROUNDED that are then placed in contact with each other & then ungrounded will experience a charge transfer over time.

The rule of thumb is the better the insulator, the great the affinity for charge (there are obviously exceptions - see the chart). Walking on a carpet will generate 35kV of static, sliding down a fibreglass slide will generate 50kV etc...

-
Thanks for the answer. I am clear with most of the things, except what exactly is this friction. Friction is generally something that opposes the flow of. Here I cannot relate to it. I searched on net, and everywhere I can find, rubbing is necessary for friction, friction is necessary for static electricity. – explorer Feb 15 at 17:16
Think of it like this... rubbing two materials together actually "rubs" electrons off one material onto another. This isn't just a discworld "lies to children", this is essentially what occurs. – JonRB Feb 15 at 17:27
@JonRB: +1 for the LtC reference. But "rubs" remains unsatisfying. Why doesn't blowing on things charge them as it cools them (come to that, how does cooling even work?) When the sticky-uppiest amber atoms are forced into brief, high-pressure and thermally-energetic contact with silk atoms, which adjective is important there? How deep does the charge go? If it's any deeper than the surface atoms, then how does the charge easily permeate a material, but not easily pass between two materials without rubbing? (answer for metals is that they're conductive; but what of insulators?)... Etc. – Dewi Morgan Feb 15 at 18:18
i'm a little confused by "human hands" being so far on the positive end. is this practically mitigated by conductive sweat? from the table, it certainly seems like the best high-school science could be done by rubbing a PVC pipe or a teflon-coated skillet with your hands. (it does support nylon+polyester in the clothes dryer making a crazy staticky mess :) – Rob Starling Feb 15 at 22:57
Thats human hair – JonRB Feb 16 at 7:45

The friction of rubbing the two is what dislodges electrons from the glass rod - the friction is physically stripping off electrons - which stick to the silk cloth.

This answer on Physics.StackExchange does a good job of expounding on the details of why charge goes to the cloth and not the rod. The Triboelectric Series is essentially the categorization of certain materials' reluctance/admittance to gaining/losing electrons through interactive friction.

The outer electron of some atoms are mobile, meaning they can be removed, or "elevated to a higher energy state." (See the definition of laser for more details on the latter.) Electricity flow is simply the movement of these electrons, loss and gain, repeatedly, across a material. If a material has a net deficiency of electrons, it is said to be "positively" charged. A surplus, "negatively" charged.

If there is a net deficiency at point A and a surplus at point B, and some type of a conductive path is placed between them, then an electric current will flow from B to A, neutralizing the net difference to zero. If there is a charge difference but no flow, then the charge is "static", meaning it's there, but isn't flowing anywhere. Hence, "static electricity."

Learning about static electricity is great for fundamental knowledge of electricity, but electronic circuits don't use this very often. Instead the focus is on that flow of electricity from point B to A, and not how that charge got there or by what means.

Some materials have tightly bound outer electrons. Since these refuse to budge, they do not work like glass and silk do together to form static charge when rubbed. In fact, they pretty much refuse to conduct electricity at all... so are called "insulators." Wax is a good insulator, and so are certain plastics, ceramics, glass, and resins. This is very different from metals, of which most do have loosely bound outer electrons, thus can conduct electricity. This is why metal wires are used as a conductor, with a plastic jacket to "shield" the conductor from accidentally touching some other conductor.

-