INEL 15100 Magnetism

Lodestones

Natural magnets that became known as “leading stones” aligning themselves north and south in the first compasses.

Permanent Magnets

Magnets that don’t require any power force to maintain their field.

Basic Law of Magnetism

Energy is required to create a magnetic field, but no energy is required to maintain a magnetic field.

Electron Spin Patterns:

The magnetic moment of an electron being proportional to its inherent spin orientation. Electron spins contribute to the magnetic properties of materials, which is evident through the interaction between magnetic moments of electrons and protons.

Magnetic Domains/Molecules

a region within a magnetic material in which the magnetization is in a uniform direction. This means that the individual magnetic moments of the atoms are aligned with one another and they point in the same direction.

Ferromagnetic

Materials that are easily magnetized. Ex: Iron nickel, cobalt, manganese.

Paramagnetic

Can be magnetized but not as easily. Ex: Platinum, titanium, chromium.

Flux

Magnetic lines of force, symbol: φ

Whenever an electric current flows through a conductor, a magnetic field is _______.

formed around the conductor.

Electromagnets

Depend on electric flow to produce a magnetic field.

Ampere-Turns

unit of magnetomotive force, represented by a direct current of one ampere flowing in a single-turn loop. Turns refers to the winding number of an electrical conductor composing an electromagnetic coil.

Formula = (I) x (# of turns)

Permeability

Measure of a material’s ability to become magnetized.

Reluctance

Resistance to magnetism

Saturation

Occurs when all the molecules of a magnet are lined up.

Residual Magnetism

the amount of magnetism left in a material after the magnetizing force has stopped.

Flux denisity

Measured in lines per square inch, B used to represent it.

Formula: B = φ/A

A=Area φ = flux lines

Magnetomotive Force

Used to describe the total force producing a magnetic field or flux.

mmf = φ (reluctance)

strength of a magnet = B(A) / 72,000,000

CGS System

One magnetic line of force is a Maxwell. A Gauss equals a magnetic force of 1 Maxwell per centimeter. A Gauss is equal to 6.45 lines of flux per inch.

1 Gilbert = 1.256 ampere-turns

1 ampere turn = 0.7905 gilberts

Dynes

Weak unit of force (1/27,800 oz). When two magnets are separated by 1 cm expert one dyne, they are considered.

Force in dynes = M1(M2) / D²

Principle of Electromagnetic Induction

Whenever a conductor cuts through magnetic lines of flux, a voltage is induced into the conductor.

Fleming’s Left Hand Generator Rule

Used to determine the relationship of the motion of the conductor in a magnetic field to the direction of the induced current.

THumb = THrust Forefinger = Flux Center Finger = Current

The 3 factors that determine the voltage induced in a conductor:

Number of wire turns

Strength (flux)

Speed of cutting action

Weber

100,000,000 lines of flux are equal to 1 weber (Wb)

Lenz’s Law

States that an induced voltage or current opposes the motion that causes it. Inductors always oppose a change of current.

henry (H)

Unit of measurement for inductance. A coil has an inductance of 1 henry when a current change of 1 amp results in an induced voltage of 1 volt.

L = 0.4πN²µA / I

eddy current

a swirling electrical current that is created in a conducting metal such as copper or steel by moving the metal through a magnetic field or a magnetic field over the metal.

Hysteresis

dependence of the state of a system on its history. A magnet may have more than one possible magnetic moment in a given magnetic field, depending on how the field changed in the past

R-L time-constant

Time necessary for current in an inductor to reach it’s full Ohm’s law value.

T = L / R

Voltage Spike

Occurs when the current flow through an inductor stops.

The amount of induced voltage can be determined if inductance of coil, amount of current change, and time is known by using this formula.

EMF = -L ( ∆I / ∆t )

Metal Oxide Varistor

Device that can be used for spike suppression in DC or AC circuits.

An exponential curve is divided into __ constants.

5 time. Each time constant is equal to 63.2% of some value.

Air Core Inductor

Inductor wound on cores of non-magnetic material.

Iron-Core Inductor

Wound on cores of magnetic material.

Two devices used to prevent large spike voltages:

Diode and Resistor