310102iA 1 Magnetism, Electromagnetism and Electromagnetic Induction Part A 2017 (TF)

Magnetism, Electromagnetism, and Electromagnetic Induction

Objectives

• Describe the properties of magnetic materials.

• Define terminology related to magnetism.

Historical Context

Earliest References to Magnetism

• 4th Century BC: The earliest literary reference to magnetism found in the Book of the Devil Valley Master (China).

• 20-100 AD: Mention of lodestone's attraction to a needle; significant progression towards navigation.

• 12th Century: Chinese utilization of lodestone compass for navigation.

Definition of Magnetism

• Magnetism is a force acting at a distance, caused by a magnetic field.

• The magnetic force:

  • Attracts opposite poles of two magnets.

  • Repels like poles.

• The magnetic field shares similarities and differences with an electric field.

Types of Magnets

Permanent Magnets

• Retain magnetism without external influence.

• Example: Inducing magnetism in steel creates a permanent magnet.

Temporary Magnets

• Materials like iron are effective for electromagnets.

• Can lose magnetism if subjected to heating, sharp impacts, or placed in a reducing current solenoid.

Properties of Permanent Magnets

• Magnetite:

  • Naturally occurring magnetic material.

  • Relatively weak as a permanent magnet.

• Most permanent magnets are synthetic, often made from an alloy of iron, nickel, and cobalt.

• Rare-earth magnets: Special category known for extreme strength.

Electromagnets

• Construction: Wire wrapped around a metal core (iron or steel) carrying electrical current.

• Characteristic: If core is soft iron, magnetism diminishes once current is turned off.

Properties of Magnets

• All magnets possess two poles: North-seeking (N) and South-seeking (S).

• Various shapes: bar, square, spherical, horseshoe, and donut.

• In a compass, the N-marked side points toward Earth's North magnetic pole.

Attraction and Repulsion

• Magnets attract materials like iron, nickel, and cobalt.

• Interactions:

  • Unlike poles attract (N to S).

  • Like poles repel (S to S).

Effects of Shapes on Magnetism

• Short-Circuiting Magnetism:

  • Placing an iron plate across N and S poles of a horseshoe magnet reduces strength momentarily but restores when removed.

• Cutting a Magnet:

  • Each cut piece will maintain both N and S poles.

Naming Conventions

• Pole Designation:

  • North-seeking pole (N), South-seeking pole (S) relates to Earth's poles.

• Magnetic Flux: Defined as moving from N to S.

Magnetic Flux

• Magnetic Lines of Flux:

  • Represented by Greek letter phi (ϕ).

• Flux Density (B):

  • Concentration of lines of flux per unit area.

• Relationship: ϕ = B * A (where A = area).

Measurements of Flux Density

• Measured in Tesla (T):

  • 1 Weber (Wb) = 1 line of flux per m².

Magnetic Field Strength and Permeability

• Magnetic Field Strength (H): Amount of magnetizing force.

• Permeability (µ): Ability to concentrate lines of flux; higher values signify better magnetism.

  • Notation: o (vacuum) and r (relative permeability).

Reluctance and Residual Magnetism

• Reluctance (R): Opposition in a magnetic path, measured in A/Wb.

• Residual Magnetism: Magnetic effect remaining after removal of magnetizing force.

Saturation and Coercivity

• Saturation: Domains aligned to maximum extent, no further magnetic increase possible.

• Coercivity: Resistance of a material to demagnetization from an external field.

B-H Curve

• Displays characteristics of coercivity and residual magnetism in various magnetic fields.

Creating a Magnet

• Technique: Rubbing a magnet along steel to align domains, creating temporary magnetism.

• Permanent magnets retain strength longer than temporary magnetism in steel.

Compass

• First true application of a magnet, aiding in navigation and detecting magnetic fields.

• A compass needle, a thin magnet balanced on a pivot, rotates to point toward magnetic poles.