As you may recall from your IP physics tuition in Singapore, magnetic objects like iron lose their magnetism when they reach high temperatures. Despite that, Earth’s core, which generates a powerful magnetic field that holds the planet together, is made out of iron (and nickel) so hot that it is in a molten, liquid state. This raises the question: how can this molten iron within the Earth produce a magnetic field? Read on to get to the bottom of this mystery.
An Overview of Ferromagnetic Materials
Ferromagnetic materials are objects which are greatly magnetised in an external magnetic field. Even after this field is removed, they still somehow maintain their magnetic moment. Iron is one such example of a ferromagnetic material.
To better understand ferromagnetism, we must first delve into atoms called atomic moments. Iron is made up of these ‘things’ and acts like microscopic magnets, each having its own north and south pole. When you hold up a magnet to an iron material, the small magnets within the latter line-up or align themselves, making an object magnetic. Any object that behaves similarly in the presence of an external magnetic field is thus classified as a ferromagnetic material. When materials like iron are heated, however, things start to change.
What Happens When Ferromagnetic Materials Get Heated?
Heating iron means supplying more thermal energy to the material, promoting the tiny magnets within it to a high-energy state as a result, and then pointing them in the opposite direction with respect to their neighbours. As a result, they become less aligned than before and now have a decreased ‘combined’ magnetism. This reduced magnetic strength continues as the iron gets heated until it reaches a point called the Curie temperature, where an object permanently loses its magnetic properties, which is around 770°C /1417°F/1043K for this particular metal.
Therefore, it is a fact that iron loses its ferromagnetic properties beyond 770°C. Yet, we also know that Earth’s molten iron core (nearly 6000°C) produces a powerful magnetic field despite its extremely high temperature. What is the reason for this contradiction, then?
How The Earth’s Core Generates Its Magnetic Field
Several theories have been proposed to elucidate the origin of Earth’s magnetic field, yet the most widely accepted and credible explanation is the dynamo theory. According to this theory, the Earth’s core functions as a dynamo, producing a self-sustaining magnetic field.
A dynamo is essentially a device that turns mechanical energy into electrical energy. As a refresher, the Earth’s core is divided into an inner and outer core, with the former being solid due to extreme pressure while the latter is in a liquid and constantly moving state due to Earth’s convection and rotation.
Therefore, the fluid motion in the outer core moves the molten iron (a conducting material) across an existing and weakened magnetic field, causing magnetic induction and generating an electric current. This electric current creates a magnetic field that further interacts with the fluid motion to produce a second magnetic field. This secondary field reinforces the initial, weaker one and ultimately creates a self-sustaining process. Thus, the core will continue producing a magnetic field if the fluid motion in the outer core continues unabated.
Conclusion
In summary, the Earth’s molten iron core does not directly generate a magnetic field because of its extremely high temperature. Rather, it produces an electric current that creates an electromagnetic effect, resulting in the strong magnetic field that holds the planet together.
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