ELI5: How are we able to direct narrow bands of magnetic fields like utility lines if Earth's magnetic field emanates outward from the core towards the atmosphere?

Submitted by femmestem t3_127fk00 in explainlikeimfive

If Earth's magnetic field emanates from the interior of the earth outward, and magnetosphere is large enough and strong enough to deflect electrified particles thrown by solar flares, how are we able to direct narrow bands of magnetic fields while living within a giant magnetic field? E.g. transfer electricity along utility lines by directing magnetic fields around the wires?

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MercurianAspirations t1_jedycym wrote

The magnetosphere is very big because the earth is very big, but it isn't really that strong. The magnetic force exerted by the magnetosphere is something like 150-400 times weaker than that of a common magnet. It's just very big, and the charged particles hitting it in space are very small. Even a common magnet would be enough to deflect these particles if you brought it to space, it just wouldn't be very effective in doing so because of how tiny it's range is.

The other part of the question I'm not clear on. We don't use magnetic fields to direct energy through utility lines - that's just current flowing through wires. It does generate a magnetic field, but that's a consequence of electricity passing through the wires, not what is causing it to move.

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femmestem OP t1_jedz5qt wrote

> We don't use magnetic fields to direct energy through utility lines

Sorry, I said electricity when I meant electric energy. Electricity flows through the conductor, energy travels in the field around the wire and can be directed (e.g. from power station to a residence).

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MercurianAspirations t1_jedzagy wrote

I still don't understand what you mean - it's the current flowing through the conductor that is going from power station to a residence, not energy in a field

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PerturbedHamster t1_jeeic9v wrote

Yeah, OP is actually correct that it's the field. If you have say a 700 kV power line, that means the voltage difference between the two lines is 700 kV. The laws of E&M mean that there has to be an electric field between the two lines, so if you took a charged particle from one line to the other, it would pick up a ton of energy. Incidentally, this is why working on high voltage lines is kind of intense, and the lines themselves have to be incredibly smooth (I think surface imperfections are micron scale or smaller to avoid coronal emission). It's the electric current moving through the field, both of which are provided by the generating station, which carry the energy. It's easy to forget that Maxwell's equations still apply to transmission lines, but they do!

To answer what I think is OP's question, classical magnetic fields don't do any work because the force is always perpendicular to the direction of motion, so the Earth's magnetic field doesn't do anything for power transmission. The Earth doesn't have a large scale electric field, because ions in the atmosphere would rapidly adjust to cancel it out. There aren't a lot of ions in the lower atmosphere (and again, transmission lines are very carefully designed to not create new ones), so the electric field doesn't get cancelled out and we can send power down the lines.

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femmestem OP t1_jef4th6 wrote

This makes a lot of sense, thank you! I'm not specialized in physics, just a curious layman who learned about fields and then had a "hey, wait a minute..." moment. You made this explanation very accessible to me.

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PerturbedHamster t1_jefn2sx wrote

Glad you found it useful! You might enjoy looking at pictures of corona emission from power lines, which really let you see how strong the fields next to power lines are. The fields get so strong that they just rip electrons off of atoms, and you can see the glow as they combine back together. Wikipedia has some nice pictures here.

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GalFisk t1_jedyjqi wrote

The magnetosphere is strong because it is huge. It can deflect particles by nudging at them constantly over a distance of several earth diameters. It won't nudge the electrons out of our puny wires that span a tiny fraction of the planet's surface at most.

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Chromotron t1_jef1ni1 wrote

The main question has been answered in comments, I think, but to further put things into perspective:

The total energy of Earth's magnetic field is very roughly our current energy consumption over a thousand years. This on one side sounds seriously huge, it's enough to boil away the Caspian sea (twice, actually). On the other hand, it is not at the absurd scales most other cosmic things are (e.g. dismembering the entire planet, solar energy output, or worst, supernovae), we could get there if we very very seriously want it even with current tech.

(I hope I remembered the total energy correctly, as modern Google is utterly useless when trying to verify them; all other numbers have been re-calculated.)

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femmestem OP t1_jef55f8 wrote

> we could get there if we very very seriously want it even with current tech.

Well that's equally fascinating and terrifying. lol

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