🔋College Physics I – Introduction Unit 22 – Magnetism

What's Magnetism Anyway?

• Magnetism fundamental force of nature that causes certain materials to attract or repel each other
• Arises from the motion of electric charges within atoms
• Magnetic fields exert forces on other magnets and magnetic materials
• Magnets have two poles, north and south, which determine the direction of the magnetic field
• Like poles repel each other, while opposite poles attract
• Magnetic fields represented by lines of force that flow from the north pole to the south pole
• Strength of a magnetic field decreases with distance from the source
  • Follows an inverse square law relationship

Magnetic Fields: The Invisible Force

• Magnetic fields are invisible regions around magnets where magnetic forces can be detected
• Represented by magnetic field lines, which show the direction and strength of the field
  • Field lines are closer together where the field is stronger
  • Field lines never cross each other
• Magnetic field strength measured in teslas (T) or gauss (G)
  • 1 tesla = 10,000 gauss
• Moving electric charges, such as electric currents, create magnetic fields
• Right-hand rule used to determine the direction of the magnetic field around a current-carrying wire
  • Point your thumb in the direction of the current, and your fingers will curl in the direction of the magnetic field
• Magnetic fields can be uniform (constant strength and direction) or non-uniform (varying strength and/or direction)
• Earth's magnetic field acts like a giant bar magnet, with the south magnetic pole near the geographic north pole

Magnetic Materials: From Fridge Magnets to Earth's Core

• Magnetic materials contain atoms with unpaired electrons that align to create a net magnetic field
• Ferromagnetic materials (iron, nickel, cobalt) exhibit strong magnetic properties
  • Atoms align in the same direction, creating a strong net magnetic field
• Paramagnetic materials (aluminum, platinum) exhibit weak magnetic properties
  • Atoms align in the presence of an external magnetic field but lose their alignment when the field is removed
• Diamagnetic materials (copper, water) exhibit weak repulsion in the presence of a magnetic field
• Magnetization process of aligning the magnetic domains within a material to create a stronger magnetic field
• Curie temperature, above which a ferromagnetic material loses its magnetic properties
• Earth's core consists primarily of iron and nickel, creating a geodynamo that generates Earth's magnetic field
  • Convection currents in the outer core drive the geodynamo

Electromagnetism: When Electricity and Magnetism Hook Up

• Electromagnetism describes the relationship between electric and magnetic fields
• Moving electric charges create magnetic fields, and changing magnetic fields induce electric currents
• Ampère's law relates the magnetic field around a closed loop to the electric current passing through the loop
  • $\oint \vec{B} \cdot d\vec{l} = \mu_0 I_{enc}$, where $\vec{B}$ is the magnetic field, $d\vec{l}$ is an infinitesimal length element, $\mu_0$ is the permeability of free space, and $I_{enc}$ is the enclosed current
• Faraday's law of induction states that a changing magnetic flux induces an electromotive force (EMF) in a conductor
  • $\mathcal{E} = -\frac{d\Phi_B}{dt}$, where $\mathcal{E}$ is the induced EMF and $\Phi_B$ is the magnetic flux
• Lenz's law determines the direction of the induced current, opposing the change in magnetic flux that produced it
• Electromagnetic induction basis for transformers, generators, and motors

Magnetic Forces in Action

• Magnetic force experienced by a moving charged particle in a magnetic field
  • $\vec{F} = q\vec{v} \times \vec{B}$, where $\vec{F}$ is the force, $q$ is the charge, $\vec{v}$ is the velocity, and $\vec{B}$ is the magnetic field
• Direction of the force determined by the right-hand rule
  • Point your fingers in the direction of the velocity, curl them towards the magnetic field, and your thumb points in the direction of the force (for positive charges)
• Magnetic force on a current-carrying wire in a magnetic field
  • $\vec{F} = I\vec{L} \times \vec{B}$, where $I$ is the current and $\vec{L}$ is the length vector of the wire
• Lorentz force combination of the electric and magnetic forces on a charged particle
  • $\vec{F} = q(\vec{E} + \vec{v} \times \vec{B})$, where $\vec{E}$ is the electric field
• Cyclotron motion of charged particles in a uniform magnetic field, resulting in circular or helical paths
• Hall effect voltage difference across a conductor in a magnetic field due to the deflection of charge carriers

Applications: Magnets in the Real World

• Compasses use Earth's magnetic field for navigation
  • Magnetic needle aligns with the field, pointing towards the magnetic north pole
• Electric motors convert electrical energy into mechanical energy using magnetic fields
  • Current-carrying coils in a magnetic field experience a torque, causing rotation
• Generators convert mechanical energy into electrical energy using electromagnetic induction
  • Rotating coils in a magnetic field produce an alternating current (AC)
• Transformers use electromagnetic induction to change the voltage of AC
  • Primary and secondary coils wound around a common iron core
• MRI (Magnetic Resonance Imaging) machines use strong magnetic fields and radio waves to create detailed images of the body
  • Hydrogen atoms in the body align with the field and absorb and emit radio waves
• Maglev (magnetic levitation) trains use strong magnets to lift and propel the train, reducing friction
• Hard drives store data using magnetic domains on a spinning disk
  • Read/write heads use electromagnetic coils to access and modify the data

Mind-Bending Magnetic Phenomena

• Diamagnetism weak repulsion of materials in the presence of a magnetic field
  • Superconductors exhibit perfect diamagnetism, expelling magnetic fields (Meissner effect)
• Paramagnetism weak attraction of materials in the presence of a magnetic field
  • Caused by the alignment of unpaired electrons in the material
• Ferromagnetism strong, permanent magnetism in materials like iron, nickel, and cobalt
  • Caused by the alignment of magnetic domains within the material
• Antiferromagnetism magnetic moments of atoms align in opposite directions, canceling each other out
• Ferrimagnetism magnetic moments of atoms align in opposite directions but do not cancel out completely, resulting in a net magnetic field
• Spin magnetic moment intrinsic angular momentum of electrons and other particles
  • Contributes to the magnetic properties of materials
• Magnetoresistance change in electrical resistance of a material in the presence of a magnetic field
  • Giant magnetoresistance (GMR) used in hard drive read heads and magnetic sensors

Key Equations and Problem-Solving Tips

• Magnetic field strength: $B = \frac{F}{qv\sin\theta}$, where $\theta$ is the angle between the velocity and the magnetic field
• Force on a current-carrying wire: $F = ILB\sin\theta$, where $\theta$ is the angle between the wire and the magnetic field
• Magnetic flux: $\Phi_B = \int \vec{B} \cdot d\vec{A}$, where $d\vec{A}$ is an infinitesimal area element
• Faraday's law: $\mathcal{E} = -\frac{d\Phi_B}{dt}$
• Ampère's law: $\oint \vec{B} \cdot d\vec{l} = \mu_0 I_{enc}$
• Lorentz force: $\vec{F} = q(\vec{E} + \vec{v} \times \vec{B})$
• When solving problems, always identify the given quantities and the desired unknown
• Draw diagrams to visualize the problem and identify the relevant vectors (magnetic fields, currents, forces)
• Use the right-hand rules consistently to determine the directions of magnetic fields, forces, and induced currents
• Break down complex problems into smaller, manageable steps
• Double-check your units and ensure that your final answer is in the correct units
• Practice, practice, practice! Solve a variety of problems to reinforce your understanding of the concepts and equations