#41: Resistance is futile.

The Question

sander says:
2005-07-19 16:04:28

We all know lightning travels through the path of least resistance.

Let’s say we zoom in on the electricity that’s flowing downwards, and see what it does every 1 nanosecond (it travels about 1 foot in this time period). The flow of electrons gets to a point, and it has to find the path of least resistance to keep going. For simplicity, let’s say it has 1 of 3 options. How does it know which to choose? Clearly, it doesn’t have “senses” to figure this out. If you know your electricity, you know it doesn’t “probe” its different path options until it finds the best way to go. So, how does it know?

A semi-comparable system would be water flowing down a hill. If the water gets to a place where it must choose to go 1 of 3 ways, most of it will go down the steepest path. But some of it will still go down the other two.

Electricity is different. If +10V gets to an intersection, and has to choose between 1 of 3 equally resistive paths (let’s say 250 ohms for the heck of it), one going to +7V, one going to +5V, one going to ground, ALL of the current will go right to ground. Nothing will go down the other two paths. Voltage (potential) difference is just like “steepness” of a hill in this example.

So, how does it know?

The Answer

Sander, the problem is that neither your lightning description, nor your analogous “river” scenario, is representative of the system under question. If they were, you would have a valid point.

However, the truth is a bit more subtle than that.

I had to consult with some Electrical Engineer friends to come up with a way to explain this.

The idea that lightning flows downwards from the cloud to the ground, is the problem. What really happens is this: the storm creates a potential difference between the charge of the clouds and the charge of ground. When the potential becomes greater than the resistance of the air, the electrons flow OUT OF GROUND to the cloud. That’s why no electrons end up on either of the other paths (in the circuit example) – they didn’t come from there.

It’s more convenient to think of electricity flowing to ground, but it’s more accurate to describe it as flowing FROM the path of least resistance than DOWN the path of least resistance.

At least, that’s what my sources tell me.

2 thoughts on “#41: Resistance is futile.

  • 10/12/2005 at 9:00 am

    While Mr. McQuaid’s answer is good, I think it relies on the old “electrons go from earth to the cloud, not the other way around” rhetoric. Here’s the way I (EE graduate) explain it:

    Take a circuit with two resistors in parallel. The current will flow through each resistor, though more will flow through the one with less resistance. You can do the same thing with 100 resistors in parallel, the lowest resistance resistor will have the highest current.

    Lightning is an arc. It uses the air as a conductor. That’s like having a near infinite number of parallel paths. More current flows along the path with the least total resistance, usually the shortest one since air is somewhat uniform in conductance/resistance.

    BUT, here’s where the funny stuff comes in. When electricity creates an arc across air, the resistance of the air in the arc decreases. So, once the lightning bolt is started, the current increases at a ridiculous rate, and the proportion of electricity that was taking other paths is reduced nearly instantaneously to nearly 0. Kind of like what would happen if you just dropped a low-resistance conductor across a circuit that had only a high-resistance load.

  • 10/13/2005 at 11:15 am

    I think that the clearest and most accurate answer combines the two points raised here – both mine and Aaron’s. While I am not an EE graduate, I did counsel with one to get this answer as close as I could.

    And now, Aaron has exposed himself as an EE resource who can explain things clearly. Next time I have an electrical answer to provide, I may look you up…


Leave a Reply

Your email address will not be published. Required fields are marked *