# Wire in the sky

1. Jan 22, 2009

### crx

Lets say that we have a very long, vertical, grounded, electric conductor planted at the equator (something similar with the "space tether" satellite, but here on Earth). Now we can consider that the electric wire is moving through Earths magnetic field with the Earths rotational speed at that point. My question is if we could tap electric energy from the electromagnetic induction in the wire, considering that we can remove the accumulated electrical charge at the tip of the wire, lets say by spraying water mist....?

2. Jan 22, 2009

### zoobyshoe

Does a magnet's field rotate with the magnet, or does it stand still while the magnet rotates?

3. Jan 22, 2009

### crx

.... just like an homopolar generator, the magnetic field is considered uniform and homogeneous, so you cannot tel if its "moving" or not...

4. Jan 22, 2009

### zoobyshoe

Are you quoting something?

5. Jan 23, 2009

Staff Emeritus
If you want electrical energy, you need a circuit. If you have a circuit, you have a loop of wire. You get a current in the wire if you have a changing magnetic flux through it. I don't see how the flux is changing through this loop.

6. Jan 23, 2009

### zoobyshoe

You believe the earth's magnetic field rotates with the earth? Or you believe one can't tell?

7. Jan 23, 2009

### LURCH

Follow-up question; are you familiar with the "agonic line?"

8. Jan 24, 2009

Staff Emeritus
I have heard of the agonic line, and at one time could have defined it. I know it's related to grid magnetic angle, but couldn't define it today.

That's the reason I don't quite see how this would work. If the Earth's magnetic field were stationary and the earth rotated through it, the magnetic field on the earth's surface would change with a 24-hour periodicity. Instead, as viewed from the earth's surface, it is relatively static. (Yes, there are variations, but they happen on a longer timescale) So this loop has essentially a constant magnetic flux inside it.

9. Jan 24, 2009

### zoobyshoe

There is a minor controversy raging as to whether or not a magnet's field rotates with the magnet. This got started as a result of one of Faraday's meticulous experiments which seemed to indicate a magnet's field does not rotate with the magnet.

Here's the problem: even if we can prove by some other means the field stands still while the earth moves through it how can we generate current from that in a loop?

10. Jan 24, 2009

Staff Emeritus
To be honest, I am not 100% sure what that means, as a field is at its heart a set of numbers for each point in space that describe the results of a measurement, were you to make it. I can easily see how people can get tangled up in terminology while they are discussing what is "really" happening.

That's why I am trying to think in terms of flux. If the total flux is the same at t and $t + \delta t$, it doesn't really matter how it got that way, nor how many field lines entered and then exited. All that matters is the difference in flux is zero, so there's no emf.

11. Jan 24, 2009

Assume that there is a change of flux then you cannot tap energy by having a closed loop of conductor because in effect the whole loop becomes equivelent to a single conductor the emf induced on both sides of the loop being equal but opposing eachother making the net current zero.If we think of the space tether in terms of a conductive loop then power will be generated because the return circuit will be stationary with respect to the wire.

12. Jan 24, 2009

### zoobyshoe

Exactly. Whether or not the field is rotating with the magnet you can't "tap" that energy with a closed loop.

13. Jan 24, 2009

Staff Emeritus
So we're up to how many reasons why this won't work?

14. Jan 24, 2009

### crx

I'm thinking that the circuit would be made up by the wire between the Earth and the atmosphere. The return path would be the ionized particles that fall back on the Earth, like rain, light-bolts etc. Because the return path is so dispersed (basically the whole Earth surface), we could consider that Earth and the atmosphere have an "infinite" capacitance. In this case we would have a circuit made up by a wire connected to two infinite capacitance bodies, so we don't need a return path. My problem is that I'm not sure if we could "see" a separation of charges inside the wire and on the surface of "capacitors" if the "capacitors" themselves move in the same way as the wire does in the magnetic field. I'm very sure that this is the same problem as in the case of an "homopolar generator".
In other words... Can we observe a potential difference at the ends of an electrical conductor that is free, straight, not part of a circuit, using an electroscope attached to the each end of the wire, that moves with constant speed in an homogeneous, uniform and constant magnetic field , with B at 90"' to the wire?

15. Jan 24, 2009

### henxan

This would work in theory. The magnetic field with respect to a location on the ground would change with 24 hours interval.. But, would be almost impossible to harness.

16. Jan 25, 2009

Staff Emeritus
Why do you say this? Doesn't a compass needle point in the same direction no matter what time of day it is?

17. Jan 25, 2009

### henxan

look at image of earthsmagnetic field

18. Jan 25, 2009

### LURCH

Yep, that's what made me mention it. The anagonic angle is the difference between magnetic north and true north. The agonic line is the line around the surface of the Earth along which the magnetic and rotational poles line up (anagonic angle = 0). It passes close to Michigan (where I live) and Florida. It has been there since before I was born, and will be long after I die. Since this line doesn't go 'round the Earth every twenty-four hours, the magnetic field must rotate with the planet. So, the wire would be in a staic magnetic field, and no voltage would be produced.

An idea not too dissimilar from yours was attempted once on the Shuttle, as I recall. A long wire was to be dragged out behind the orbiter while it orbitted the Earth, making its sin-wave ground track. As the orbiter moves from north to south and back again, the alternating magnetic field would produce electricity to power some of the science equipment onboard. Unfortunately, once the tether had spooled out to its full length, it detatched from the orbiter and just kept going.

I don't know of they ever tried again.