The Earth acts like a very big, weak magnet -- that's why compasses point north! In this article, we explore a few interesting facts about Earth's magnetic field, including a few ways to play with it using neodymium magnets.
Why is the earth like a magnet?
Earth's magnetic field is mostly caused by electric currents in the liquid outer core, which is composed of conductive, molten iron. Loops of currents in the constantly moving, liquid iron create magnetic fields.
From afar, the Earth looks like a big magnet with a north and south pole like any other magnet. (As described in Which Pole is North, the pole located up in northern Canada is really the magnetic south pole.) If we consider it like a magnet, how does it compare to a magnet you might hold in your hand?
As described in our article on Magnetic properties of sintered NdFeB magnets( see Technical support), we often describe the strength of a magnet in two different ways: the strength of the magnetic field around it (surface field, expressed in gauss) and the force it takes to remove a magnet from either a steel plate or another magnet (pull force, expressed in pounds). How would we specify the Earth?
For fun, we made a rough estimate of what the pull force of an Earth magnet would be, by making a big list of questionable assumptions. If we assume the Earth is a permanent magnet instead of a ball of half-molten iron with electrical currents running through it, we might figure Brmax is about 1 gauss. That's a lot weaker than the Brmax of 13,200 gauss for a grade N42 magnet .
If you could find the largest refrigerator door in the universe to stick Earth to, we made some more rough calculations to figure it might have a pull force of about 19 tons. That sounds like a lot, but not so much when you consider the Earth has a mass of about one thousand trillion tons. The forces from gravity pulling it towards this huge refrigerator door would probably completely overshadow any magnetic forces.
The Compass
Standing on the surface of the Earth, we can use a magnet or magnetized object to act as a compass. Measuring the direction of the magnetic field has been a great way to tell the direction for almost a thousand years.
How does it work? The needle of a compass is actually a magnetized piece of metal. Carefully balanced, it tends to move to align itself with the local magnetic field. When it isn't pointed north, magnetic forces tend to push it towards that direction. The forces are very weak, but with a low-friction compass, they are enough to get the job done.
Note that a compass tends to point to the magnetic north pole, which is different than the geographical north pole where all the lines of longitude meet. The difference between the direction to the geographical north pole and where a compass is pointing is called declination.
Not only can a magnetized needle work as a compass, but a magnet can as well.