Download Astronomy 311: Magnetism • Atoms consist of protons and neutrons

Astronomy 311: Magnetism
• Atoms consist of protons and neutrons in a nucleus and electrons in shells
orbiting the nucleus.
• The electrons and protons have negative and positive charge respectively.
• Electrons orbiting the nucleus constitues an electric current. This electric
current gives a small magnetic field to the atom, called the orbital magnetic
moment.
• Electrons also spin about their own NS axis and this again consititues an
electric current which gives the atom a magnetic field, called the spin magnetic moment.
• The magnetic moment is a vector with a direction.
• A lump of iron is magnetized if its atoms have magnetic moments which
line up in the same way.
• Generally orbital magnetic moments cancel out.
• The spin magnetic moments can only have directions up or down. Hence
it can be the case that a group of iron atoms have spin magnetic moments
which can be lined up.
• A group of Iron atoms with lined up spin magnetic moments is called a
domain.
• So a lump of liquid iron could by chance condens slowly so that it becomes
solid with all the atoms lined up in terms of spin magnetic moments. This
is a permanent magnet.
• Heating this lump to its Curie temperature destroys the alignment of the
spin magnetic moments.
• So a planetssimal forms, its hottest at the center with perhaps liquid iron
there. It starts to cool slowly from the inside out. This cooling slowly
grows small ”crystals” of iron aligned spin magnetic moments. This acts as
a seed.
• The outer core is still liquid and hot and starts to convect because of the
heat flowing up from the slowly solidfying core.
• The core is rotating because of conservation of angular momentum.
• Because the outer core is hot, it contains ions, which, in taking part in the
convection process, constitute an electric current.
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• Bob’s your Uncle.
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