Static Charge & Humidity: A Closer Look

[cepheid]

Why does static charge build up more easily in dry air? One site on the WWW mentioned that it has to do with the fact that under humid conditions, water molecules coat surfaces. I know that water molecules have net dipoles, but it's not really clear what effect that has.

 

[Doc Al]

The primary reason is that ordinary impure water is decent conductor of electricity. So the charge keeps leaking off and doesn't build up as well as on a dry day.

 

[cepheid]

Thanks! That makes sense. So, what about the dipoles? Do they factor in at all? Or would their be no conduction if (hypothetically) the water were pure?

 

[ArmoSkater87]

The primary reason is that ordinary impure water is decent conductor of electricity. So the charge keeps leaking off and doesn't build up as well as on a dry day.

But water vapor in the atmosphere isn't "impure" like liquid water is from the ions disolved in it. The water vapor is pure water since you can't evaporate ions that were once in the liquid water. So, if what I say is true, then the reason why static charge builds up better in dry air is because the water vapor in the moist air is a bad conductor.

 

[Doc Al]

But water vapor in the atmosphere isn't "impure" like liquid water is from the ions disolved in it. The water vapor is pure water since you can't evaporate ions that were once in the liquid water.

You make a good point, Armo. On further thought, a better answer is that the water that condenses on the surfaces of the insulators makes those surfaces more conductive. Thus it's harder to build up or transfer a charge.

So, if what I say is true, then the reason why static charge builds up better in dry air is because the water vapor in the moist air is a bad conductor.

That doesn't make sense. You want the air to be a bad conductor to build up a static charge.

 

[Gonzolo]

Can't the + side of a water dipole simply grab any loose electron, thereby eliminating any static? I think water just grabs the extra electrons as it would grab +ions.

With no dipole around, the electrons are free to build up and charge things, and shock us.

Basically, when its humid, the humidity gets electrically shocked, instead of us.