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Q & A: Why does a magnet have north and south poles?

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Most recent answer: 10/31/2015
Q:
Why magnet have north and south poles instead of east & west poles?
- Usman Khan (age 25)
Peshawar KPK Pakistan
A:

Hi Usman,

Moving charged particles create a magnetic field, which exerts a force on other moving charged particles. Electrons have a property called 'spin' which causes magnetic fields to exert a force on them. (The electron 'spin' is analagous to the everyday spin we're accustomed to but is actually a quantum property, so it's not the same. The difference is another discussion). In a magnet, the spins of the electrons line up so that the small magnetic field produced by each electron's spin adds up into a large magnetic field which can be felt by other charged particles far away.  ( is a more in depth discussion of how magnets work.) The force of a magnetic field on a particle with spin causes the particle to rotate it's spin to align with the magnetic field. Hence a smaller magnet inside a large magnetic field will try to align with that larger magnetic field.

The core of the Earth is made of metal. It has a hot solid inner core and a cool liquid outer core. The difference in temperature causes currents in the liquid metal, just as hot air rises above cold air. At the same time, the Earth is spinning, and causes the liquid metal current to also spin. The total effect is to make a huge magnetic field that points (approximately) in the northern direction.

Taking these two ideas together, then, the Earth's magnetic field will cause the magnet to align north to south. That's why people named them the north and south poles. 

But, you might still ask, why does the earth's magnetic field point north? Is it a coincidence that magnetic north is the same as polar north? In other words, why should the Earth spin in the same plane as it rotates around the Sun?

As it happens, it's not a coincidence! To understand why it's this way, we have to go back to the formation of the solar system. The solar system, like all other stars and and their planets, started as a ball of dust floating around in space. Each particle had a random position and velocity. Gravity caused the dust to contract. Because some portion of their motion was in a direction perpendicular to the direction gravity was pulling them, the particles would have started to spin. (This is analogous to the way your arm rotates when you catch a ball.) Since they were moving around randomly, most of the spinning throughout the entire cloud would cancel out, but there would have been a little bit left in one direction just by chance. As the dust ball condensed more and more, the speed of the spinning ball would grow, because of the conservation of angular momentum. The ball would also eventually form a disk. (To understand this, imagine a spinning globe. Notice that the top and bottom points of the globe aren't moving.) The particles on the top and bottom of the dust ball don't have enough speed to prevent them from being sucked into the center plane, so the whole thing squashes into a disk.

From the intial cloud of dust, we're left with a disk of material spinning around a preferred axis. From this the Sun and planets formed, each continuing to spin along the direction of the disk. The planets all rotate more-or-less in the same direction the Sun does (with the exception of Uranus and Venus which had their rotations changed by other effects).

The spinning of the Earth creates the magnetic field, but it doesn't specify which direction is north or south... you can imagine that the north pole could be the south pole and vice versa. In fact, the direction of the Earth's magnetic field does switch from time to time. We know this because we can look at the orientation of magnetic materials in the ground and can see how the direction of the field has changed throughout time.

-Courtney K


(published on 10/31/2015)

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