Lecture 24: Magnetism
Introduction to Magnetism
Definition: Magnetism describes the properties of magnetic objects, which possess the capability of attracting and repelling other magnets.
Ubiquity of Magnetism: Magnetism plays a significant role in daily life, with applications ranging from:
Using refrigerator magnets to secure artwork and photographs.
Navigating geographic locations using a compass.
The planetary-scale protection provided by the Earth’s magnetic field.
Scope of Study: The core concepts in this field include magnetic fields, types of magnets, electromagnets, the deflecting force, and Earth’s magnetic field.
The Atomic and Structural Basis of Magnetism
Atomic Structure: Atoms contain a central nucleus composed of protons and neutrons, with electrons orbiting around this nucleus.
Electron Spin: In addition to their orbital motion, electrons possess a property called "spin." This spin is the fundamental source that generates magnetism.
Magnetic vs. Non-Magnetic Substances:
In most substances, the electron spins are aligned randomly. This random orientation leads to the cancellation of magnetic effects, resulting in an overall non-magnetic material.
When the majority of atoms in a substance have the same spin alignment, the material becomes a permanent magnet.
Magnetic Domains: These are specific regions within a material where the magnetism is uniform. Each domain functions essentially as an individual, microscopic permanent magnet.
Demagnetization and Remagnetization
Loss of Alignment: Magnetic domains can lose their uniform alignment through external stressors:
Heating: Increasing thermal energy randomizes the orientation of the domains.
Mechanical Damage: Repeatedly dropping a magnet or subjecting it to physical shock can disrupt the domain alignment.
Case Study: Gadolinium:
Gadolinium is a metal that functions as a permanent magnet only at temperatures lower than approximately .
At standard room temperature, Gadolinium remains magnetic enough to adhere to surfaces.
When heated with a device such as a heat gun, the domains become randomized, the attraction is lost, and gravity causes the metal to fall.
The domains re-align and magnetic properties return once the metal cools back below its threshold temperature.
Remagnetization: Permanent magnets demagnetized by physical use can be restored by placing them in the presence of a strong external magnetic field.
Example: At the College of DuPage, bar magnets that lose strength through heavy use are restored using a dedicated device that applies a strong magnetic field to re-align the domains.
Magnetic Fields and Visualization
Definition: A magnetic field describes the sphere of influence a magnet exerts on other magnets and charged particles. This is conceptually similar to gravitational fields (acting on mass) and electric fields (acting on charges).
Visualizing Fields:
Iron Filings: Because iron filings are magnetic, they align themselves along the magnetic field lines. For a single bar magnet, these lines form loops around the object.
Compass: A compass consists of a magnet suspended in fluid, allowing it to rotate freely. It will always align with the strongest magnetic field in its immediate vicinity.
Directional Conventions:
Outside the Magnet: The magnetic field points from the North Pole to the South Pole.
Inside the Magnet: The magnetic field points from the South Pole to the North Pole.
Compass Interaction: When a compass is positioned near a permanent magnet, the magnet's field is strong enough to overcome the Earth’s magnetic field, causing the needle to point toward the magnet’s poles rather than geographic north.
Interactions of Magnetic Poles
Dipole Nature: All magnets must have both a North and a South pole. Currently, physicists have no evidence that a magnetic monopole (a single isolated pole) can exist; poles always exist in pairs.
Breaking a Magnet: If a bar magnet is broken in half, the result is not two isolated poles. Instead, each individual piece becomes its own complete magnet with its own North and South poles. This can be verified by observing attraction and repulsion between the two halves or using a compass.
Fundamental Laws of Interaction:
Repulsion: Like poles ( or ) repel each other, experiencing a force that pushes them apart.
Attraction: Unlike poles () attract each other, experiencing a force that draws them together.
Field Mapping:
Between attracting poles, field lines flow directly from one pole to the next.
Between repelling poles, field lines are seen to flow away from one another.
Material Types and Geometric Shapes of Magnets
Material Compositions:
Iron-oxide Ceramics: Frequently used for low-cost, common magnets.
Rare-earth Metals: These elements are used to manufacture extremely strong, small permanent magnets. Because of their strength, they require careful handling.
Common Shapes:
Bar Magnets: Rectangular prisms with poles at opposite ends.
Horseshoe Magnets: A bar magnet bent into a U-shape. This configuration brings the North and South poles closer together than in a standard bar magnet.
Disk Magnets: Short cylindrical magnets where the poles are located on the flat top and bottom surfaces. Many rare-earth magnets take this shape.
Ferrofluids: These are specialized liquids consisting of tiny magnetic particles suspended in a fluid carrier, allowing the entire liquid to be attracted to magnetic fields.
Electromagnetism and Controlled Magnetism
History: In the early 1800s, scientists discovered that electric current flowing through a wire generates a magnetic field.
Field Geometry: The magnetic field generated by a straight wire forms concentric circles around the wire. Consequently, the field points in opposite directions on the top vs. the bottom of the wire.
Strength Augmentation:
Coiling: Coiling a wire increases the field strength because the field is the sum of the contributions from hundreds of individual loops of current.
Iron Core: Placing an iron core inside a coil creating an electromagnet significantly increases the field. The current magnetizes the iron core, and the core's magnetism adds to the overall field strength.
Advantages of Electromagnets:
Controllability: Unlike permanent magnets, electromagnets can be turned on and off by connecting or disconnecting the power source.
Applications: Industrial electromagnets are used to lift and move massive objects like scrap metal or cars.
The Deflecting Force and the Right-Hand Rule
The Phenomenon: An electric charge moving through a magnetic field experiences a deflecting force. This force occurs when the current flow is perpendicular to the magnetic field.
Perpendicularity: Unlike gravity or the Coulomb force, which cause motion along field lines, the magnetic deflecting force acts perpendicular to both the direction of the current and the direction of the magnetic field.
The Right-Hand Rule: Used to determine the direction of the force:
Index Finger: Point in the direction of the electric current.
Middle Finger: Point in the direction of the magnetic field (North to South).
Thumb: The direction the thumb points indicates the direction of the deflecting force.
Orientation Examples:
Current pointing toward the viewer (out of screen) + Field pointing down = Force to the right.
Current pointing toward the viewer (out of screen) + Field pointing up = Force to the left.
Applications of the Deflecting Force
Electric Motors: In DC motors, a battery provides current to a coil of wire, which sits in a magnetic field created by a permanent magnet. The deflecting force causes the coil to rotate, converting electrical energy into mechanical work.
Particle Accelerators: To move charged particles in a circular path, a centripetal force is required. This is provided by the magnetic deflecting force.
Advanced Photon Source: Located at Argonne National Lab in Lemont, Illinois.
Large Hadron Collider: Located in France and Switzerland; currently the world's largest particle accelerator.
Earth’s Magnetic Field and Global Protection
Generation: The Earth's magnetic field is generated by currents of molten iron and nickel in the planet's core.
Navigation: Compasses align with this global field, pointing toward the magnetic North Pole.
Atmospheric Protection:
The field protects Earth from the "solar wind," which is a stream of charged particles emitted by the sun.
Without this field, the solar wind would destroy the ozone layer.
Loss of the ozone layer would expose the surface to harmful UV rays, increasing rates of skin cancer.
Auroras: The interaction of the magnetic field deflecting the solar wind produces the visible light phenomena known as auroras.