Charging and discharging objects....

[fog37]

Hello Forum,
There are three types of objects: conductors, insulators, and semiconductor. Let's leaves semiconductors aside for now.

--Conductors are said to have a very large number of free charges. Neutral conductors can be electrically charged (either positive or negative) either via contact with another charged conductor or via electrostatic induction. It is not possible to charge a conductor or conductors by rubbing them against each other.

--Insulator can only be charged by rubbing. The rubbing action takes two insulators and does not work if one object is an insulator and the other a conductor

That said, when air is dry, we can acquire some charge by rubbing our shoes over carpets. The human body is said to be a decent conductor and this accumulated charge spreads over the body to our hands so when we get close to a metal object, like the knob of a door, electrostatic discharge takes place (spark). But if we acquire that charge by rubbing it means that our shoes are insulators. The charge on an insulator does not spread on a conductor if contact takes place between the insulator and the conductor, correct? So how do we explain that the charging takes place on our shoes but the charge eventually reaches our hands?

If an insulator is charged, the charge is localized and does not spread on the surface of the insulator. How do we discharge a charge insulator if its charge is fixed and cannot be moved?

thanks,
fog37

 

[anorlunda]

Good for you. You got it partially right, partially wrong.

If your shoes conducted and you were standing on the ground, the charge would leak away so you would not charge up.

But conductors can hold a charge too. You probably have seen pictures like the ones below of Van De Graff generators.

Also, what you said about free electrons is right but it is not a yes/no question. There is a continum; good conductors to poor conductors to poor insulators to good insulators.

 

[fog37]

There is no perfect insulator and no perfect conductor. Even the best conductor has some finite resistivity even if very low. Also, the frequency of the current passing through the material matters: copper is a good conductor at DC but becomes an insulator at X-ray frequencies...

That said, my interpretation of what happens when our body gets charged is the following: we rub our shoes on a carpet and charge accumulates on the shoes. Because we are not perfect insulators, that accumulated charge will eventually spread to our areas of the body (like the fingers of our hand). At that point, once we get to close to the door know, electrostatic discharge takes place. Our body is somewhat a conductor and somewhat an insulator.

When we rub two insulators together, after some time, the charge is lost. Ideal insulator would have localized charge that does not disappear. But since insulators are also conductors, they lose charge to the water molecules in the atmosphere, i.e. they lose charge by contact. Ideal insulator cannot transfer charge by contact (technically it seems we could never discharge an ideal, perfect insulator since charges cannot be transfer to or from the insulator. But ideal insulators don't exist in practice...)

thanks,
fog37

 

[William White]

There is no perfect insulator and no perfect conductor. Even the best conductor has some finite resistivity even if very low.

https://en.wikipedia.org/wiki/Superconductivity