# Magnetic propulsion

1. Jul 5, 2009

### physics-help

Can a magnetized ship go to the South Magnetic Pole on its own accord? Let's say it is currently located in the Southern Hemisphere so the attraction from the N-pole in the South Magnetic Pole is stronger than the attraction from the S-pole in the North Magnetic Pole. Let's also say that there is no friction and that as long as the ship accelerates slightly towards the South Pole, it is moving.

If it works, what is doing the work? Static magnetic fields cannot do work. Did we do work by magnetizing the ship in the presence of the Earth's magnetic field?

2. Jul 5, 2009

### HallsofIvy

Essentially yes. You are postulating a "ship" with a high potential energy with respect to the south magnetic pole. How was that potential energy created? By magnetizing the ship?

3. Jul 5, 2009

### jambaugh

Practically No!
Without as yet undiscovered magnetic monopoles magnetizing a ship will only make it act as a compass i.e. it will be north seeking on one end and south seeking on the other. You at best get the equivalent of a tidal effect where the S end of the ship is attracted to the south pole slightly more strongly than the N end is repulsed.

Now to magnetize the ship strongly enough to have a force stronger than random winds would exceed the material strength of any material we know. Imagine making a stronger magnet by stacking two bar magnets in parallel N next to N and S next to S.
The two pieces repel each other. Magnets tend to want to expand away from the axis of magnetism. An electromagnet for example will experience a tension along the coil of wires.

To get strong enough forces from the Earths magnetic field to even turn the ship like a compass much less attract the ship would require a magnetic field so strong the ship itself would fly apart.

Remember that the force is going to be proportional to both the strength of the ships magnetic field and of the Earth's which is quite weak. Imagine pulling a ship with a simple refrigerator magnet and then compare the effect that magnet has on a compass needle relative to the Earths.

Even with magnetic monopoles I doubt you could get enough close-enough together to effect propulsion on the ship.

4. Jul 6, 2009

### Nick Bruno

in addition to what jam said, I found this on wikipedia.

In summary, the earths magnetic field is changing constantly, and there are also fields going east/west.

Earth's magnetic field

A sketch of Earth's magnetic field representing the source of Earth's magnetic field as a magnet. The north pole of earth is near the top of the diagram, the south pole near the bottom. Notice that the south pole of that magnet is deep in Earth's interior below Earth's North Magnetic Pole. Earth's magnetic field is produced in the outer liquid part of its core due to a dynamo that produce electrical currents there.Main article: Earth's magnetic field
Because of Earth's magnetic field, a compass placed anywhere on Earth will turn so that the "north pole" of the magnet inside the compass points roughly north, toward Earth's north magnetic pole in northern Canada. This is the traditional definition of the "north pole" of a magnet, although other equivalent definitions are also possible. One confusion that arises from this definition is that if Earth itself is considered as a magnet, the south pole of that magnet would be the one nearer the north magnetic pole, and vice-versa. (Opposite poles attract and the north pole of the compass magnet is attracted to the north magnetic pole.) The north magnetic pole is so named not because of the polarity of the field there but because of its geographical location.

The figure to the right is a sketch of Earth's magnetic field represented by field lines. The magnetic field at any given point does not point straight toward (or away) from the poles and has a significant up/down component for most locations. (In addition, there is an East/West component as Earth's magnetic poles do not coincide exactly with Earth's geological pole.) The magnetic field is as if there were a magnet deep in Earth's interior.

Earth's magnetic field is probably due to a dynamo that produces electric currents in the outer liquid part of its core. Earth's magnetic field is not constant: Its strength and the location of its poles vary. The poles even periodically reverse direction, in a process called geomagnetic reversal.

Earths Field: 0.3 - 0.6 Gauss or 0.00003 Tesla
Microwave ovens typical 0.2 Gauss
Hair Dryers typical 0.3 Gauss
Fridge Magnets typically 50 Gauss
Electromagnets typically can get to about 1 tesla or 10000 gauss.

I think my units are correct.

5. Jul 6, 2009

### fleem

...and ferromagnetism with the ocean floor would give you a one-way ticket to Davy Jone's locker before anything else half as interesting happened.

6. Jul 6, 2009

### Lok

True facts from our real world do not matter here ....

This is a simple experiment as you would need a big magnet in a round object ... use a globe just for fun and a smaller magnet as the ship. Both should be quite powerful and several angles to the vertical should be experimented as gravity is a powerful *****.

My guess is that it should work in the conditions given.

7. Jul 6, 2009

### mgb_phys

It still wouldn't work because the north pole at one of the ship would be pulled to the south hemisphere with the same force that the north pole at the other end of the ship would be pulled back to the northern hemisphere.

8. Jul 6, 2009

### Lok

That would be true only if the ship would be exactly on the ecuator.

Please tell me that a system of two magnets is not stable and never will be someone bothered to write a theorem about it.

9. Jul 6, 2009

### fleem

Right:
http://en.wikipedia.org/wiki/Earnshaw's_theorem

Although when there are other constraints then stability is possible , but in this case the constraint of remaining on the water surface won't keep it at a single point on the Earth except at the magnetic poles (or thereabouts)