Magnetism

Regents Physics

What is a Magnet?

Historical Context

• Ancient Navigation: Naturally occurring magnets were used for navigation hundreds of years ago
• Pole Names: When freely suspended, one end pointed toward Earth's geographic north pole (called "north pole"), the other toward the south pole (called "south pole")
• Dipoles: All magnets have two poles - north and south

Magnetic Interactions

Fundamental Rule

Like poles repel, opposite poles attract

This is similar to electric charges!

Examples:

• North North = Repel
• South South = Repel
• North South = Attract

What Happens When You Cut a Magnet?

Think About It:

If you cut a bar magnet in half, what do you get?

A) One north pole and one south pole
B) Two smaller magnets, each with north and south poles
C) The magnet loses its magnetism

Cutting Magnets

You get TWO smaller magnets!

• Each piece still has both north and south poles
• Cut them again → four smaller magnets
• Continue to atomic level → atoms are tiny magnets
• No magnetic monopoles exist (unlike electric charges)

Magnetic Domains - The Atomic Explanation

Electrons are tiny magnets:

• Every spinning electron creates a small magnetic field
• Electrons spinning in same direction → stronger magnet
• Electrons spinning in opposite directions → cancel out

Magnetic Domains:

• Groups of aligned atoms pointing in same direction
• All atoms in a domain act like one larger magnet

Magnetic Fields

Fields as "Force at a Distance"

Just like gravity and electric forces, magnets affect other magnets without touching.

Scientists explain this through invisible fields that surround magnets

We can make these fields visible using iron filings!

Visualizing Magnetic Fields

Purpose: Show both strength and direction of magnetic field

Field Strength:

• Dense lines = strong field
• Spaced out lines = weak field

Field Direction:

• Direction a compass needle's north end would point
• Lines point away from north poles
• Lines point toward south poles

Reading Magnetic Field Diagrams

Bar Magnet Field Pattern

• Strongest field: At the poles (lines closest together)
• Weakest field: Sides of magnet (lines far apart)
• Field lines: Form continuous loops from north to south
• Never cross: Field lines never intersect

The Compass - Nature's Field Detector

How It Works

• Tiny suspended magnet that can freely rotate
• North end aligns with local magnetic field direction
• Universal tool used by sailors, hikers, and physicists

Compass behavior reveals field direction at any point in space

Earth's Magnetic Field

Our Planet is a Giant Magnet!

Source: Movement of molten iron in Earth's liquid outer core

• Moving iron → electric currents → magnetic field

Important Fact:

• Geographic north pole = Magnetic south pole
• Geographic south pole = Magnetic north pole
• Compass north points to magnetic south!

Magnetism vs Electric Charges

Similarities

• Both have two types (N/S poles, +/- charges)
• Like repels like, opposites attract
• Both create fields around them
• Field lines point away from positive/north
• Field lines point toward negative/south

Magnetism vs Electric Charges

Key Differences

Oersted's Discovery (1820)

The Connection Between Electricity and Magnetism

Hans Christian Oersted's Observation:

• During a lecture, closed a switch to pass current through wire
• Nearby compass needle deflected!
• Electric current created a magnetic field

Revolutionary Discovery:

"Electricity causes magnetism"
"Moving electric charges create magnetic fields"

Impact of Oersted's Discovery

Birth of Electromagnetism

This discovery led to:

• Electric motors
• Speakers and headphones
• Power plants and generators
• Wireless communication
• MRI machines
• And much more!

Key Insight: All magnetism comes from moving electric charges

Summary

Key Concepts to Remember

1. Magnets always have two poles (dipoles) - no monopoles exist
2. Like poles repel, opposite poles attract
3. Magnetism comes from aligned atomic domains
4. Magnetic fields show strength (line density) and direction
5. Earth is a giant magnet with magnetic poles opposite to geographic poles
6. Electricity and magnetism are connected - moving charges create magnetic fields parallel to path