Lecture Notes on Magnetism

Introduction to Magnetism

• Magnetic objects can attract at long distances due to magnetic fields extending beyond them.
• Key question: Where do magnetic fields come from?

Relationship Between Electricity and Magnetism

• Electricity and magnetism are two sides of the same coin, similar to mass-energy and space-time.
• Magnetic fields are essentially electric fields that manifest when charged objects move.

Microscopic Perspective on Magnetism

• Objects like bar magnets and compass needles are metals without visible electrical currents.
• On a microscopic level, electrons within atoms contribute to magnetism.

Quantum Mechanics and Magnetism

• Magnetism must be understood as a quantum mechanical effect.
• Particles like electrons have an intrinsic magnetic moment, a fundamental property like mass and charge.
• Since the 1920s, it's known that electrons and protons are tiny magnets.

Atomic Level Magnetism

• Atoms consist of positively charged protons and negatively charged electrons.
• Proton's magnetic effect is weaker than that of electrons.
• Electrons generate orbital magnetic fields, but these often cancel out in filled shells.
• In half-filled shells, unpaired electron magnets align and contribute to an atom's magnetic field.
• Atoms with nearly full or half-full electron shells exhibit magnetic properties (e.g., Nickel, Cobalt, Iron).

Crystal Level Magnetism

• Magnetic atoms in a solid can either align their fields or cancel each other out.
• Chromium is anti-ferromagnetic despite being made up of magnetic atoms.
• Iron is ferromagnetic, meaning it has aligned magnetic fields.

Domain Level Magnetism

• Magnetic materials have domains, regions where atoms' magnetic fields align.
• Domains can be misaligned, leading to no net magnetic field.
• Applying external force can unify domains, resulting in a magnetic field.

Conclusion

• Magnetism is a quantum property that can be amplified to macroscopic scales through alignment of domains and atoms.
• Limited materials fulfill the criteria for being magnetic.
• Running a current through a conductor can generate a magnetic field, hinting at ties to special relativity and the speed of light.