Electrostatics and electric shocks

[Jimmy87]

Hi, could someone tell me why you get a shock from contact with a conductor when you are charged and not from an insulator. As I understand it, if you become charged by friction then you have an excess of charge. Let's say I have gained electrons from another material I have rubbed against. If I go to touch a metal door handle, the charge build up is so high that electrons ionize the air and jump onto the door handle. This I understand. But why don't you get a shock from, say, a wooden table. If the charge build up is high enough to ionize air between me and the table they why can't they flow onto the table? Especially if charges can jump off of insulators, why can't they also jump onto insulators? Surely a wooden table would be at a lower potential difference so why can't the electrons jump onto the table (if the air is ionized)?

 

[diegzumillo]

If I go to touch a metal door handle, the charge build up is so high that electrons ionize the air and jump onto the door handle. This I understand.

A conductor allows for charges to move freely, so when your charged hand (say, negatively) approaches a conductor, the negative charges of the material will move away, leaving its closest point to your hand positively charged. So this small distance between your hand and the conductor has a very high potential difference that could be high enough to overcome air's electrical resistivity and allow electrons to move from your hand to the conductor. In an insulator the emphasized text would not happen.

 

[Jimmy87]

A conductor allows for charges to move freely, so when your charged hand (say, negatively) approaches a conductor, the negative charges of the material will move away, leaving its closest point to your hand positively charged. So this small distance between your hand and the conductor has a very high potential difference that could be high enough to overcome air's electrical resistivity and allow electrons to move from your hand to the conductor. In an insulator the emphasized text would not happen.

That makes sense, thank you. So, if you bring a negatively charged object to an insulator, because the free electrons cannot move away (to leave a net positive charge), there will be no potential difference created? Although the net charge is zero (because electrons cannot move in the insulator), wouldn't a net charge of zero still create a potential difference since you have an accumulation of charge ( on the object) versus no accumulation of charge? So essentially it should act like a ground?

 

[diegzumillo]

Yep. A simpler case is to forget the air and just assume there is contact. If we are talking about insulators there can't be any current between the two materials.

 

[Jimmy87]

Yep. A simpler case is to forget the air and just assume there is contact. If we are talking about insulators there can't be any current between the two materials.

So are you saying that charge cannot jump to an insulator anyway because this requires a flow of charge which cannot happen for insulators? Why do excess charges flow to ground? I have never really understood this as the ground isn't a conductor so how can everything flow to ground?

 

[diegzumillo]

Ah well, 'ground' is kind of an idealized concept. It's meant to be a conductor so big that its potential doesn't ever change and any conductor in contact with it will become neutral by either sending or receiving charge from it. In practice, the actual soil has varying degrees of conductivity, but even not being a great conductor like copper, it's still big enough to fit the idealized concept of 'ground'.