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Q & A: Earth's magnetic field

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Most recent answer: 12/31/2010
Q:
hello........ i got to ask about earth magnetism..... my teacher explained that its cause is that the core's outer part has iron ion( without its valence electrons) in molten form due to high temperature , and due to rotation of earth ...........current is set up..... in the ring of the core ..and there magnetic field is produced....... can you please tell me why is it said that the theory of earth magnetism is open or incomplete....... what is the problem with this explanation......?? .....AND one of my other teacher said that earth magnetism is due to rotating molten iron pillar..... so which explanation is more accurate??? .................... ......... ........... i have to ask one more thing related to this that i just noticed..... assuming that the molten iron rotates in almost circular path then it undergoes acceleration motion ,and hence time varying magnetic field should be produced and hence light( em wave....)....so this means all the rotating celestial bodies should emit light........ but even for earth it does not.....why not?? ............ . . .... ..... thanks for your time.........
- apurva (age 17)
rajkot
A:

Let's take a look at the Earth's magnetic field ():

The yellow lines are the magnetic field lines coming from N (close to the geographical south pole), and the blue lines are those going toward S (close to the geographical north pole). At first glance, it looks like a magnetic dipole field, so people suspected the origin of the field is something like a loop current (i.e. the ring model you mentioned). However, when you take a closer look, you will notice that the field lines are not perfectly North-South oriented. There are some yellow in the northern hemisphere and the lines seems tangling. This implies that the current loop approximation does not tell the whole story, and better models are needed. The current pillars model (  , figure below) is one of them, but again, the mechanism is not that simple.

Scientists have been relying on computers to solve this problem. Computers can simulate the magnetic field generated by complicated current flow forms in the core of the Earth. If the simulated magnetic field turns out to be similar to what is observed, the current is likely to be the reality.

Even though the current in the Earth's core is widely believed to be the origin of the Earth's magnetic field, there are problems with the model. For example, it is not very clear how the current is generated and sustained, and how it evolves over centuries. Also, although lots of evidence suggests that the core of the Earth is made of melted iron and nickel, the detailed composition and its property under such high pressure and temperature are not well-understood.

For the other question, you are correct that any accelerating charges emit EM waves. (There's an exception to this rule. If a smooth current loop doesn't change at all in time, there's no radiation. Essentially the radiation from all the different parts of the loop ends up cancelling. /mw) For instance, if you swing a charged ruler, it will radiate EM waves. But since the radiated energy is proportional to the square of the acceleration, the radiation from the current in the core is too weak to detect at the surface of the Earth.

- Tsung


(published on 12/31/2010)

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