Welcome to the I Can't Sleep Podcast,
Where I help you drift off one fact at a time.
I'm your host Benjamin Boster,
And today's episode is about magnetism.
Magnetism is the class of physical attributes that occurs through a magnetic field,
Which allows objects to attract or repel each other.
Because both electric currents and magnetic moments of elementary particles give rise to a magnetic field.
Magnetism is one of two aspects of electromagnetism.
The familiar effects occurring in ferromagnetic materials,
Which are strongly attracted magnetic fields,
And can be magnetized to become permanent magnets,
Producing magnetic fields themselves.
Demagnetizing the magnet is also possible.
Only a few substances are ferromagnetic.
The most common ones are iron,
Cobalt,
Nickel,
And their alloys.
All substances exhibit some type of magnetism.
Magnetic materials are classified according to their bulk susceptibility.
Ferromagnetism is responsible for most of the effects of magnetism encountered in everyday life.
But there are actually several types of magnetism.
Paramagnetic substances,
Such as aluminum and oxygen,
Are weakly attracted to an applied magnetic field.
Diamagnetic substances,
Such as copper and carbon,
Are weakly repelled.
While anti-ferromagnetic materials,
Such as chromium,
Have a more complex relationship with the magnetic field.
The force of a magnet on paramagnetic,
Diamagnetic,
And interferomagnetic materials is usually too weak to be felt,
And can be detected only by laboratory instruments.
So,
In everyday life,
These substances are often described as non-magnetic.
The strength of a magnetic field always decreases with distance from the magnetic source,
Though the exact mathematical relationship between strength and distances varies.
Many factors can influence the magnetic field of an object,
Including the magnetic moment of the material.
The physical shape of the object.
Both the magnitude and direction of any electric current present within the object,
And the temperature of the object.
Magnetism was first discovered in the ancient world when people noticed that lodestones,
Naturally magnetized pieces of the mineral magnetite,
Could attract iron.
The word for magnet comes from the Greek term magnetos lithos,
The magnesium stone,
Lodestone.
In ancient Greece,
Aristotle attributed the first of what could be called a scientific discussion of magnetism to the philosopher Thales of Miletus,
Who lived from about 625 BCE to about 545 BCE.
In ancient China,
The earliest literary reference to magnetism lies in a 4th century BCE book named after its author,
Guiguizi.
The 2nd century BCE annals,
Lu Xuechunqio also notes,
The lodestone makes iron approach.
Some force is attracting it.
A lodestone attracts a needle.
The 11th century Chinese scientist Shen Kuo was the first person to write in the dream pool essays of the magnetic needle compass and that it improved the accuracy of navigation by employing the astronomical concept of true north.
By the 12th century,
The Chinese were known to use a lodestone compass for navigation.
They sculpted a directional spoon from lodestone in such a way that the handle of the spoon always pointed south.
Alexander Neckam,
By 1187,
Was the first in Europe to describe the compass and its use for navigation.
In 1269,
Peter Peregrinus de Maricourt wrote the Epistola de Magnete,
The first extant treatise describing the properties of magnets.
In 1282,
The properties of magnets and the dry compasses were discussed by Al-Shraf Umar II,
A Yemeni physicist,
Astronomer,
And geographer.
In 1600,
William Gilbert published his On the Magnet and Magnetic Bodies,
And On the Great Magnet,
The Earth.
In his work he describes many of his experiments with his model Earth called the Torella.
From his experiments,
He concluded that the Earth was itself magnetic,
And that this was the reason compasses pointed north,
Whereas previously,
Some believed that it was the pole star Polaris,
Or a large magnetic island on the North Pole that attracted the compass.
An understanding of the relationship between electricity and magnetism began in 1819 with work by Hans Christian Ørsted,
A professor at the University of Copenhagen,
Who discovered by the accidental twitching of a compass needle near a wire that an electric current could create a magnetic field.
This landmark experiment is known as Ørsted's experiment.
Jean-Baptiste Biot and Félix Savart,
Both of whom in 1820 came up with the Biot-Savart law,
Giving an equation for the magnetic field from a current-carrying wire.
Around the same time,
André-Marie Ampère carried out numerous systematic experiments and discovered that the magnetic force between two DC current loops of any shape is equal to the sum of the individual forces that each current element of one circuit exerts on each current element of the other circuit.
In 1831,
Michael Faraday discovered that a time-varying magnetic flux induces a voltage through a wire loop.
In 1835,
Carl Friedrich Gauss hypothesized,
Based on Ampere's force law in its original form,
That all forms of magnetism arise as a result of elementary point charges moving relative to each other.
Wilhelm Eduard Weber advanced Gauss's theory to Weber electrodynamics.
From around 1861,
James Clerk Maxwell synthesized and expanded many of these insides into Maxwell's equations,
Unifying electricity,
Magnetism,
And optics into the field of electromagnetism.
However,
Gauss's interpretation of magnetism is not fully compatible with Maxwell's electrodynamics.
In 1905,
Albert Einstein used Maxwell's equations in motivating his theory of special relativity.
Requiring that the laws hold true in all inertial reference frames.
Gauss's approach of interpreting the magnetic force as a mere effect of relative velocities thus found its way back into electrodynamics to some extent.
Diamagnetism appears in all materials,
And is the tendency of a material to oppose an applied magnetic field,
And therefore to be repelled by a magnetic field.
However,
In a material with paramagnetic properties,
That is,
With a tendency to enhance an external magnetic field,
The paramagnetic behavior dominates.
Thus,
Despite its universal occurrence,
Diamagnetic behavior is observed only in a purely diamagnetic material.
In a diamagnetic material,
There are no unpaired electrons,
So the intrinsic electron magnetic moments cannot produce any bulk effect.
In these cases,
The magnetization arises from the electron's orbital motions,
Which can be understood classically as follows.
When a material is put in a magnetic field,
The electrons circling the nucleus will experience,
In addition to their coulomb attraction to the nucleus,
A Lorentz force from the magnetic field.
Depending on which direction the electron is orbiting,
This force may increase the centripetal force on the electrons,
Pulling them in towards the nucleus.
Or it may decrease the force,
Pulling them away from the nucleus.
This effect systematically increases the orbital magnetic moments that were aligned opposite the field,
And decreases the ones aligned parallel to the field.
This results in a small bulk magnetic moment with an opposite direction to the applied field.
This description is meant only as a heuristic.
The Bohr-Van Leeuwen theorem shows that diamagnetism is impossible according to classical physics.
And that a proper understanding requires a quantum mechanical description.
All materials undergo this orbital response.
However,
In paramagnetic and ferromagnetic substances,
The diamagnetic effect is overwhelmed by the much stronger effects caused by the unpaired electrons.
In a paramagnetic material there are unpaired electrons,
I.
E.
Atomic or molecular orbitals with exactly one electron in them.
While paired electrons are required by the Pauli exclusion principle to have their intrinsic spin magnetic moments pointing in opposite directions,
Causing their magnetic fields to cancel out.
An unpaired electron is free to align its magnetic moment in any direction.
When an external magnetic field is applied,
These magnetic moments will tend to align themselves in the same direction as the applied field,
Thus reinforcing it.
A ferromagnet,
Like a paramagnetic substance,
Has unpaired electrons.
However,
In addition to the electron's intrinsic magnetic moments tendency to be parallel to an applied field,
There is also in these materials a tendency for these magnetic moments to orient parallel to each other to maintain a lowered energy state.
Thus,
Even in the absence of an applied field,
The magnetic moments of the electrons in the material spontaneously line up parallel to one another.
Every ferromagnetic substance has its own individual temperature,
Called the Curie temperature,
Or Curie point,
Above which it loses its ferromagnetic properties.
This is because the thermal tendency to disorder overwhelms the energy lowering due to ferromagnetic order.
The magnetic moments of atoms in a ferromagnetic material cause them to behave something like tiny permanent magnets.
They stick together and align themselves into small regions of more or less uniform alignment called magnetic domains or vice domains.
Magnetic domains can be observed with a magnetic force microscope to reveal magnetic domain boundaries that resemble white lines in the sketch.
There are many scientific experiments that can physically show magnetic fields.
When a domain contains too many molecules,
It becomes unstable and divides into two domains aligned in opposite directions so that they stick together more stably.
When exposed to a magnetic field,
The domain boundaries move so that the domains aligned with the magnetic field grow and dominate the structure.
When the magnetizing field is removed,
The domains may not return to an unmagnetized state.
This results in the ferromagnetic materials being magnetized,
Forming a permanent magnet.
When magnetized strongly enough that the prevailing domain overruns all others,
To result in only one single domain,
The material is magnetically saturated.
When a magnetized ferromagnetic material is heated to the Curie point temperature,
The molecules are agitated to the point that the magnetic domains lose the organization,
And the magnetic properties they cause cease.
When the material is cooled,
This domain alignment structure spontaneously returns,
In a manner roughly analogous to how a liquid can freeze into a crystalline solid.
In an anti-ferromagnet,
Unlike a ferromagnet,
There is a tendency for the intrinsic magnet moments of neighboring valence electrons to point in opposite directions.
Antiferromagnets have a zero net magnetic moment,
Because adjacent opposite moment cancels out,
Meaning that no field is produced by them.
Anti-ferromagnets are less common compared to the other types of behaviors,
And are mostly observed at low temperatures.
In varying temperatures,
Anti-ferromagnets can be seen to exhibit diamagnetic and ferromagnetic properties.
In some materials,
Neighboring electrons prefer to point in opposite directions,
But there is no geometrical arrangement in which each pair of neighbors is anti-aligned.
This is called a canted anti-ferromagnet or spin ice and is an example of geometrical frustration.
Like ferromagnetism,
Ferromagnets retain their magnetization in the absence of a field.
However,
Like anti-ferromagnets,
Neighboring pairs of electron spins tend to point in opposite directions.
These two properties are not contradictory,
Because in the optimal geometrical arrangement,
There is more magnetic moment from the sub-lattice of electrons that point in one direction,
Than from the sub-lattice that points in the opposite direction.
Most ferrites are ferromagnetic.
The first discovered magnetic substance,
Magnetite,
Is a ferrite and was originally believed to be a ferromagnet.
Louis Le Niel disproved this,
However,
After discovering ferromagnetism.
When a ferromagnet or ferromagnet is sufficiently small,
It acts like a single magnet spin that is subject to Brownian motion.
Its response to a magnetic field is qualitatively similar to the response of a paramagnet,
But much larger.
An electromagnet is a type of magnet in which the magnetic field is produced by an electric current.
The magnetic field disappears when the current is turned off.
Electromagnets usually consist of a large number of closely spaced turns of wire that create the magnetic field.
The wire turns are often wound around a magnetic core,
Made from a ferromagnetic or ferromagnetic material,
Such as iron.
The magnetic core concentrates the magnetic flux and makes a more powerful magnet.
The main advantage of an electromagnet over a permanent magnet is that the magnetic field can be quickly changed by controlling the amount of electric current in the winding.
However,
Unlike a permanent magnet that needs no power,
An electromagnet requires a continuous supply of current to maintain the magnetic field.
Electromagnets are widely used as components of other electrical devices,
Such as motors,
Generators,
Relays,
Solenoids,
Loudspeakers,
Hard disks,
MRI machines,
Scientific instruments,
And magnetic separation equipment.
Electromagnets are also employed in industry for picking up and moving heavy iron objects,
Such as scrap iron and steel.
The phenomenon of magnetism is mediated by the magnetic field.
An electric current,
Or magnetic dipole,
Creates a magnetic field,
And that field in turn imparts magnetic forces on other particles that are in the field.
Maxwell's equations,
Which simplify to the Biot-Savart law in the case of steady currents,
Describe the origin and behavior of the fields that govern these forces.
Therefore,
Magnetism is seen whenever electrically charged particles are in motion,
For example,
From movement of electrons in an electric current,
Or in certain cases from the orbital motion of electrons around an atom's nucleus.
They also arise from intrinsic magnetic dipoles arising from quantum mechanical spin.
All known magnets are dipoles,
Meaning that they have a north and south pole,
So named as the Earth's magnetic field applies a force to point a magnet's poles towards areas near the Earth's respective north pole and south pole,
Specifically the north magnetic pole and south magnetic pole.
A magnet's north pole is attracted to another magnet's south pole.
A magnetic field contains energy,
And physical systems move toward configurations with lower energy.
When diamagnetic material is placed in a magnetic field,
A magnetic dipole tends to align itself in opposed polarity to that field,
Thereby lowering the net field strength.
When ferromagnetic material is placed within a magnetic field,
The magnetic dipoles align to the applied field,
Thus expanding the domain walls of the magnetic domains.
