# Earth magnetic field energy harness

1. Nov 9, 2014

### Stanley514

If we take in account that magnetic and geographical Earth poles are not completely identical and therefore magnetic poles suppose to rotate around geographical with some speed, could we harness at least a bit of energy from it? For example if you are on the South geographical pole, the South magnetic pole suppose to rotate around you and therefore Earth magnetic field lines suppose to change under some (not a big angle) regarding your position. Is it true?

Last edited: Nov 9, 2014
2. Nov 9, 2014

### Staff: Mentor

They aren't at the same place, but they are roughly stationary with respect to each other.

3. Nov 9, 2014

### Stanley514

That's true. However they rotate at a different speed regarding Earth center. For example at equator speed of Earth rotation will be the fastest and the pole the slowest. If we put a wire conductor from the Earth magnetic pole to geographic pole, then one end of it suppose to rotate faster than the other? I suppose Earth magnetic field suppose to penetrate entire Earth globe, not exist only over the surface?

Last edited: Nov 9, 2014
4. Nov 9, 2014

### Staff: Mentor

The magnetic pole in the northern hemisphere moves with approximately 50 kilometers per year.
Yes in principle it would be possible to get energy out of this, but the energy density of the field is tiny (~1mJ/m^3) and even after a year the fraction you could get out is tiny (let's say 1%). Completely impractical.

You cannot get energy out of the constant field component.

5. Nov 9, 2014

### Stanley514

I was talking not about that. Earth rotates daily pretty fast.

6. Nov 9, 2014

### Staff: Mentor

Yes, but that does not give you any method to get energy out of the field.

7. Nov 9, 2014

### Staff: Mentor

Yes, but we're rotating along with the earth and its magnetic field, so the magnetic field isn't changing relative to us - and that's what we would need to extract power.

At some point, you are either going to have calculate the amount of power that could in principle be extracted in this way, or take the word of people who have done the calculation.

8. Nov 9, 2014

### Staff: Mentor

No. Rotation is in angular speed, not linear speed. In the frame of reference of Earth's surface, they are not moving with respect to each other.

If what you suggest were true, you could spin an entire generator (frame and all) and generate electricity. You can't: you have to hold the generator's frame stationary whill spinning the shaft.

9. Nov 9, 2014

### Stanley514

Sorry, maybe my mistake. I just briefly thought about following things:
1) Different points on Earth surface have different speed of rotation:
http://classroom.synonym.com/earth-rotate-slower-faster-top-17525.html
2) Earth magnetic and geographical poles do not match each other. And therefore suppose to have different speed of rotation.
3) Earth magnetic field doesn't originate at the poles. And instead:
http://en.wikipedia.org/wiki/Earth's_magnetic_field

So, I thought there should be at least slight change in position to magnetic lines from point of view of stationary observer. But I agree that according to logic there should be mechanically separated stator and rotor and therefore it isn't going to work.

10. Nov 9, 2014

### Staff: Mentor

Different linear speeds, but the same angular velocity.

You could get energy out if you had some magical "anchor" in space, not rotating with earth (or slower than one orbit per day). A practical realization would look similar to a space elevator. And if we can build that, there are easier ways to get some energy out of it.

11. Nov 9, 2014

### zoki85

Even with satellites orbiting Earth it is not practical.