What is the reason for the Earth to behave like a bar magnet and have poles (North and South poles)?
Answer
The Earth's magnetic field is caused by eddy currents in the liquid parts of the planet's interior. We believe the field is not due to a permanent magnet because: (1) Its direction and strength change over time, and (2) the planet's interior is hotter than the Curie temperature of its elements, and so a permanent magnet would not retain its magnetism. However, saying the field is due to eddy currents is not sufficient. Simply swirling around a conductive fluid does not produce a magnetic field. There has to be some source of electric current to make the whole process work, and we don't know what that source is. Several possibilities have been proposed. Examples include: (1) Gravitationally separated elements produce a voltage difference, as in a battery; (2) Gravitationally separated elements combined with the temperature differential between the Earth's core and its mantle creates a thermocouple junction that produces voltage, and therefore current; and (3) Flowing electrons produced by Beta decay of radioactive material in the core. However, none of these explanations are accepted as proven, and some are thought to be highly unlikely. So the source of the electric current remains a mystery.
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