Chapter 4 Lecture Notes

Electromagnetism

Magnetism

Orbital Magnetic Moment: when a charged particle is in motion, a magnetic field perpendicular to the motion will be created.

Spin Magnetic Moment or Electron Spin: The magnetic effect created by electrons spinning on their axes.

Magnetic Dipoles or Magnetic Domains: Groups of atoms with their net magnetic field moving in the same direction.

Magnetic Field: The force fields that are created when magnetic dipoles orient to create a magnet; also called lines of flux and lines of force.

Lines of force always flow from north to south outside the magnet and from south to north within a magnet.

Flux density: (1) A measure of the intensity of magnetization produced by a magnetic field. (2) A vector quantity representing the number of flux lines crossing a unit area at right angles. Measured in Tesla or Gauss
1 tesla = 10000 gauss, and
magnetic flux is a measure of the strength of a magnetic field over a given area, measured in Weber

Types of Magnets

Natural Magnets: Created when iron oxide remains in the earth's magnetic field for ages, slowly orienting the magnetic dipoles in the same direction.

Artificial Permanent Magnets: Manufactured from steel alloy called alnico, composed of aluminum, nickel, and cobalt.

Electromagnets: Temporary magnet produced by moving electric current.

Laws of Magnetism

1. Repulsion-attraction: Like poles repel; unlike poles attract.

2. The inverse square law: Directly proportional to the product of the magnitude and inversely proportional to the square of the distance between them.

3. Magnetic Poles: Every magnet has two poles.

Magnetic Classification of Materials

Permeability is the ease with which a material can be magnetized.
Retentivity is the ability of a material to stay magnetized.

1. Ferromagnetic: A material with high magnetic permeability (imposes little resistance to orientation in the presence of a magnetic field). Such materials as iron, steel, and nickel are ferromagnetic substances.

2. Paramagnetic: Weakly attracted to magnetic fields.

3. Diamagnetic: Diamagnetism is a very weak form of magnetism that is only exhibited in the presence of an external magnetic field. It is the result of changes in the orbital motion of electrons due to the external magnetic field. The induced magnetic moment is very small and in a direction opposite to that of the applied field. When placed between the poles of a strong electromagnet, diamagnetic materials are attracted towards regions where the magnetic field is weak.

4. Nonmagnetic: Materials not affected by a magnetic field and cannot be magnetized.

Electromagnetism

A moving charge produces a magnetic field.

Fleming's Hand Rule for Wires

Along a Conductor Conventional Current Flow Electron Flow
Thumb = Conventional Current or Electron Flow Right-Hand Thumb Rule Left-Hand Thumb Rule
Fingers = Magnetic Field

Solenoids and Electromagnets

Solenoid: A coil consisting of a series of loops, which serve to increase the flux density.

Fleming's Hand Rule for Solenoids and Electromagnet Poles

Solenoid Conventional Current Flow Electron Flow
Thumb = Direction of North Pole Right-Hand Thumb Rule Left-Hand Thumb Rule
Fingers = Conventional Current or Electron Flow

Electromagnetic Induction

Ways to create motion between lines of force:
1. Move the Conductor through a stationary, unchanging strength magnetic field.
2. Move magnetic lines of force through a stationary conductor with an unchanging strength magnetic field.
3. Vary the magnetic flux strength from a stationary magnet through a stationary conductor.

Induction Video 2

To regulate the strength of the induced current

1. the strength of the magnetic field.
2. the speed of the motion between lines of force and the conductor.
3. the angle between the magnetic lines of force and the conductor.
4. the number of turns in the conducting coil.

Lenz's Law: The current induced in a circuit due to a change in the magnetic field is so directed as to oppose the flux, or to exert a mechanical force to oppose the motion.

Mutual Induction: The result of two coils being placed in close proximity with a varying current supplied to the first coil, which then induces a similar flow in the second coil.

Primary Coil: The coil supplied with the current first.

Secondary Coil: The coil in which the current is induced.

Self-Induction: The ability of an alternating current to switch directions, causing an opposing potential difference to induce against the incoming supply of electrons; allows direct current to flow while at the same time hindering alternating current.

Inductive Reactance: Opposition to the flow of AC current produced by an inductor. Measured in Ohms and varies in direct proportion to frequency.