1 Basics of Magnets
A Magnet is an object that produces its own magnetic field and has the property to attract certain magnetic materials such as iron, cobalt, and nickel.
Every magnet always has 2 poles (Magnetic Dipole): a North pole (N) and a South pole (S). Even if a magnet is broken in half, each piece will still have both poles. In physics, isolated magnetic monopoles do not exist in nature.
Iron filings (Real World)
Vector representation (Ideal)
As shown above, external magnetic field lines always point outwards from the North (N) pole and inwards toward the South (S) pole. Areas where lines are densely packed indicate a stronger magnetic field.
2 Magnetic Materials
Whether an object is attracted or repelled by a magnet depends on its atomic and electronic structure. Based on their response to external magnetic fields, materials are classified into 3 main groups:
Slightly repelled by a magnet
Cause: These materials have completely paired electrons. Their internal magnetic fields cancel out perfectly, resulting in zero net magnetic moment.
When exposed to external B-field: According to Lenz's Law, the induced changing electron motion creates a small induced magnetic field that opposes the external field.
Result: A very weak repulsion occurs.
Examples: Water (H₂O), Copper (Cu), Gold (Au), Silver (Ag), Plastics.
Slightly attracted by a magnet
Cause: These materials have unpaired electrons, meaning each atom acts like a tiny magnet (has a net magnetic moment). However, thermal agitation randomly misaligns them.
When exposed to external B-field: The external field forces some of these atomic magnets to align with it.
Result: A weak attraction. Once the external field is removed, thermal motion quickly randomizes the alignment, and magnetism is lost.
Examples: Aluminum (Al), Platinum (Pt), Magnesium (Mg), Liquid Oxygen.
Strongly attracted (Can become permanent magnets)
Cause: Extremely strong quantum exchange interactions force neighboring atoms to align their magnetic moments together in clusters called Magnetic Domains.
When exposed to external B-field: Domains aligned with the field grow rapidly, and other domains abruptly rotate to align.
Result: Very strong attraction. Upon removal of the external field, they often retain their alignment (Hysteresis), becoming permanent magnets.
Examples:Iron (Fe)Nickel (Ni)Cobalt (Co)
