How do sharp ended lightning rods reduce the probability of lightning strikes?

[ChessEnthusiast]

A few dozen years ago the endings of lightning rods were sharp.
I read somewhere that it served one purpose - it enabled the flow of charge between the ground and the cloud, thus reducing the voltage and lessening the probability of another lightning.

What I don't understand is why this sharp end enabled such flow of charge. Could you help me to explain it?

 

[mfb]

Sharp ends have stronger electric fields at their surface, which makes an impact more likely, but it also dissipates existing charges more easily.

Some discussion in an older thread
A review here

 

[anorlunda]

I use something called a static dissipator with lots of sharp pointy ends.

The name suggests that it helps to dissipate the buildup of static charge. That should make lightning less likely.

But less likely does not mean impossible, my dissipator got hit by lightning.

 

[rootone]

... I read somewhere that it served one purpose - it enabled the flow of charge between the ground and the cloud, thus reducing the voltage and lessening the probability of another lightning.

A lightning conductor also provides a path to Earth which is an easier path for the lightning to take when it does strike.
Thus the lightning is more likely to hit the expendable conductor than it is to hit people and other more important things in the area.

 

[lychette]

Charges collect at the sharp ends of conductors and increase the voltage at the point which breaks down the air.
To reduce the field strength and reduce the chance of breakdown large curvature surfaces are required, hence the large metal spheres on van de Graaf machines etc.

 

[FactChecker]

It might reduce the strength of a lightning strike a little, but it certainly doesn't do much to reduce the number of strikes. In fact, it attracts them and conducts the electricity to ground. The Empire State Building has a lightning rod at the top which gets struck on average 23 times a year.
(see )

PS. There is also danger on the ground near a lightning strike because the potential gradient through the ground is so great. If you are a camper sleeping on the ground with your head and feet touching ground, you can get hurt.

 

[davenn]

It might reduce the strength of a lightning strike a little,

there's no reason that that should be so

but it certainly doesn't do much to reduce the number of strikes.

they are not supposed to

In fact, it attracts them and conducts the electricity to ground. The Empire State Building has a lightning rod at the top which gets struck on average 23 times a year.

that's the whole point of lightning rods
it helps avoid the lightning striking more sensitive areas of the building by controlling where the strike should occurDave

 

[FactChecker]

It might reduce the strength of a lightning strike a little,

there's no reason that that should be so

If it dissipates the static charge a little, it may reduce the strength of a lightning strike. I assume it would be negligible, but I don't know how to estimate or prove it.

 

[anorlunda]

The OP's question was not about lightning rods, but rather why the sharp tips. @mfb gave the answer in #2.

 

[ZapperZ]

Charges collect at the sharp ends of conductors and increase the voltage at the point which breaks down the air.

There is a problem with cause-and-effect here. The "cause" in this case is that surfaces with a high aspect ratio (i.e. sharp, pointy objects) will have a high field-enhancement factor (the β in the Fowler-Nordheim model). The effect of this high field-enhancement is that charges will tend to congregate in such a region.

What you've stated is the other way around, i.e you're attributing the that charges will collect at sharp ends, and then this causes an increase in "voltage". Charges will not collect into a very small region spontaneously. Field enhancement are due to the geometry and the presence of external field. Only then will there be a high concentration of charges in these field-enhancement regions.

Zz.

 

[lychette]

There is a problem with cause-and-effect here. The "cause" in this case is that surfaces with a high aspect ratio (i.e. sharp, pointy objects) will have a high field-enhancement factor (the β in the Fowler-Nordheim model). The effect of this high field-enhancement is that charges will tend to congregate in such a region.

What you've stated is the other way around, i.e you're attributing the that charges will collect at sharp ends, and then this causes an increase in "voltage". Charges will not collect into a very small region spontaneously. Field enhancement are due to the geometry and the presence of external field. Only then will there be a high concentration of charges in these field-enhancement regions.

Zz.

perhaps I should have said electric field strength rather than voltage...the voltage (potential) over the surface of the conductor is, of course, constant

 

[ZapperZ]

perhaps I should have said electric field strength rather than voltage...the voltage (potential) over the surface of the conductor is, of course, constant

That isn't the problem, it is what comes first and what comes next as the cause of that first part. You put the charge accumulation as the primary cause, and then effect of that is the high field. This is not true.

Zz.

 

[davenn]

That isn't the problem, it is what comes first and what comes next as the cause of that first part. You put the charge accumulation as the primary cause, and then effect of that is the high field. This is not true.

could you expand on that please ... I would like a better understanding.
As I too would have thought that the charge accumulation or in the case of a thunderstorm a charge separation between the cloud and ground
is what is producing the electric field between those areas

I guess my Q then is how can you have a build up of the electric field if there isn't a build up of charge to cause that to happen ?
( if there is no charge, then there is no electric field ?)

curious minds want to learn

Dave

 

[ZapperZ]

could you expand on that please ... I would like a better understanding.
As I too would have thought that the charge accumulation or in the case of a thunderstorm a charge separation between the cloud and ground
is what is producing the electric field between those areas

I guess my Q then is how can you have a build up of the electric field if there isn't a build up of charge to cause that to happen ?
( if there is no charge, then there is no electric field ?)

curious minds want to learn

Dave

The concept of field-enhancement is that, in an external applied field, the sharp, pointy regions will "enhance" the magnitude of the field. So the magnitude of the field in such a region will be amplified many times.

Because of this enhanced field, charges on a metallic tips will accumulate a lot more than in flatter regions.

In the case of thunderstorms, the external field happens due to a potential difference between the Earth and the clouds. Lightning rods are there because these are tips that have significant field enhancement and thus, charge accumulation at the tips. If an electrical discharge were to happen in a vicinity of one, it will be more likely to occur at one of these tips.

Zz.