Introduction to Magnetic Fields and Magnetic Forces

Overview

• Topic: Magnetic fields and forces, part of General Physics playlist for university-level physics.
• Lecturer: Chad from Chad's Prep.
• Other available subjects on Chad's Prep: General chemistry, organic chemistry, high school chemistry, and preparation courses for DAT, MCAT, OAT.

Key Concepts

Bar Magnet and Magnetic Dipole

• Magnetic Dipole: Bar magnet with two poles (North and South).
• Cutting a magnet results in smaller magnets each with their own North and South poles.
• Magnetic Field Lines: Trace around the magnet from North to South outside the magnet, and from South to North within the magnet.

data:image/png;base64,iVBOR --> Visual to show magnetic field lines based on whether directions are within or outside the bar magnet.

Comparisons with Electric Dipole

• Electric Field Lines: Originate from a positive charge (or infinity), terminate at a negative charge (or infinity).
• Magnetic Field Lines: Behave similarly to electric field lines, but specific to magnets.

Interaction Between Bar Magnets

• Like poles repel (North-North, South-South). Opposite poles attract (North-South).
• Naming conventions due to a magnet's alignment with Earth's magnetic field (North Pole points approximately toward Earth’s northern magnetic pole).
• Geographic and Magnetic Poles: Geographic North Pole = Magnetic South Pole & Geographic South Pole = Magnetic North Pole.

Earth's Magnetic Field

• Not due to a solid bar magnet due to molten state of Earth’s iron core.
• Possible cause: electric currents within Earth’s core leading to a magnetic field.
• Earth's magnetic poles move and can reverse polarity over millions of years.

Forces on Charges in Electric vs. Magnetic Fields

Electric Fields

• Requires charge to experience force (positive charge along electric field direction, negative charge against electric field direction).
• Force Formula: F = qE.

Magnetic Fields

• Requires both charge and motion (velocity) to experience force.
• Force Formula: F = qvB sin(θ).
• Key Factor: Force is zero if velocity is parallel to the magnetic field.

Right-Hand Rule

• For Positive Charge: Fingers in direction of B, thumb in direction of v, force comes out of palm.
• For Negative Charge: Same as positive, but force is in opposite direction.
• Representations: *Dot (*