This question comes from reading Walter Isaacson’s biography of Benjamin Franklin.
In addition to answering the thread title, what about these related questions:
– Do all planets have low electrical potential?
– If not, and one those other planets also had lightning, where might the lightning go?
Jupiter has lightning storms (it appears that it jumps between clouds of different chemical composition).
The conductivity of the upper crust is due to the materials present as well as the presence of water which has a significant effect (desert areas can be notorious for making effective electrical grounding difficult). Once you get into the lower crust and into the mantle the high conductivity of the earth is due mostly to carbonates. I’m a bit fuzzy on the geological processes that cause the carbonates to form but it is my understanding that it is largely related to the earth’s natural volcanic processes. The earth’s core is mostly iron (with a bit of nickel thrown in) and is highly conductive. The outer core is molten and the inner core is solid.
I don’t what you mean by the earth having a low electrical potential. Are you talking about voltage (which is a measure of electrical potential)? Voltages exist between two points - when someone talks about the voltage at a point, it’s always in reference to some other point (often referred to as ground). So if the earth has a low electrical potential, it would mean the voltage between the earth and some other point is low. So what is this other point you’re measuring the earth’s potential against?
Whatever it is that makes the lightning establish a channel down to the earth.
I somehow got it into my mind that it was the low electrical potential of the earth that results in many lighting bolts going from cloud to ground.
I don’t know. Is it between the cloud and the earth? Or, since the cloud can’t know the earth is down there, it is between every point on the path down to the earth and the point right next to it?
Lightning is due to charge separation. In the case of cloud to ground lightning, the charge gets separated due to the transport of electrons due to rain. It has nothing to do with the inherent electrical potential of the earth.
Think gravity and falling. If you fall, it’s not the absolute height above the earth’s surface that matters but to the nearest ‘ground’.
Example: my bed is on the 6th floor. I fall out of bed. I only fall to the 6th floor, not all the way down. And that’s really the only gravitational potential that matters.
Ditto lightning. That’s why standing under the only tree on an otherwise treeless plain is a bad idea. Lightning will only ‘fall’ as far as necessary. In this case, “necessary” meaning the least energetic path.
The operative measure here is the potential between the lower atmosphere (which tends to be electrically charged) and the surface of the earth.
I think you still have some kind of misconception about what electrical potential is. Lightning can go both ways - cloud to earth or earth to cloud. It just depends on whether the atmosphere is locally positively or negative charged, which means the ‘cloud to earth’ electrical potential can also be positive or negative. The overall electrical potential of the earth (relative to any reference you want) is irrelevant to this local potential.
The gravitational comparison would be falling out of bed or, if suddenly a massive body appeared in the sky then “falling” up towards it. Lightning can do both.
The atmosphere itself is not a uniform thing. There are several distinct layers and each layer has different electrical properties. Electrical charge varies greatly at different elevations and geographically over time. Generally, the lower atmosphere (the part closest to earth) has a small net positive charge on a nice day. But it can go largely negative in local areas during storms. It’s that large local difference that can create cloud to earth lightning strikes. But note that the local atmospheric charge can be sufficiently positive to create earth to cloud lightning.
So if it’s differences in electrical charge (hence electrical potential) that cause lightning strikes, why isn’t electrical neutrality the norm? It takes some effort to move electrical charge. Just as I can’t fall through my bed to get to a lower gravitational potential state, electrical charge can’t move freely to reduce potential.
It’s a strong potential gradient that causes lightning - i.e. a relatively localized large difference in the electric potential. The actual value of the electric potential itself is subject to ‘gauge arbitrariness’ and so has no direct physical significance.
Mathematically, we call that a torsor; in this specific case, voltages lie in an R-torsor.
Even a single cloud can have such a charge differential between its own layers that lighting will jump from one part of the cloud to the other without involving the ground at all.
Yes. What happens is that the raindrops from thunderstorms carry charge with them. Something causes electrical charges in the clouds to separate, so that part of each cloud has more negative charge, and part of it has more positive charge. When raindrops fall from one of these clouds it carries away the charge local to the part of the cloud that it came from. So, for example, if a raindrop comes from a negatively charged part of the cloud, it carries away electrons when it falls. The raindrops carry their charge to the ground, which results in a difference in electrical potential between the clouds and the ground. Lightning happens when the potential difference gets high enough to overcome the resistance of the air gap between the clouds and the ground.
The cause of the initial charge separation in the clouds is not well-understood. It may have something to do with friction within the clouds, or with cosmic rays.
True story:
Scene: My junior year level EE class “Fields and Waves” Many weeks into the course. (the question below would have been covered the first week, Perhaps not the first day)
Cast:
Herb R., an excellent, if irascible instructor.
Glen C., a student prone to asking lots of inane, disruptive questions.
HR:…so lets assign a potential of zero to earth…
GC:< interrupting >How can you just say that?!
HR: <Wheeling around from the chalk board> IT’S JUST A REFERENCE! WE COULD CALL IT 17.9368572143 VOLTS IF WE WANTED TO, BUT IT WOULDN’T BE VERY DAMNED CONVENIENT!
Class (excepting GC): <Barely stifled snickering>
GC: Well, I guess I’ll just let that stew in my mind for a while.
HR: Very Well, if you think the fire is hot enough, Mr. C.
Class: <Applause>
Quotes are doubtless not exact, but this is how I remember it. The “stewing” and “fire” lines were really said.
PhillyGuy, I guess that you have worked out from the replies above that the short answer to your question is just ‘convention’.
We can say that a cloud has a potential of +10,000,000 V and the Earth a potential of 0 V. We could just as well say that the cloud has a potential of 0 V and the Earth has a potential of -10,000,000 V but that is not quite so convenient because most people will not have access to the particular cloud in question but most people have access to the Earth. As it happens, the Earth is a reasonable conductor (mostly) so the potential does not vary much from place to place, making it a convenient reference.