Electromagnetic Theory

What defines a magnetic material and how is magnetism created?

Magnetic materials are attracted to magnets Magnetism arises from a moving electrical charge Magnetic moment occurs when electrons align in one direction
Magnetic domain: group of atoms with aligned magnetic moments A material with many aligned domains forms a magnet

What are the two types of magnets and how do they differ?

Temporary: Aligned only under a magnetizing force Used in gas valves, relays, contactors, motors
Permanent: Always magnetic (e.g., magnetite) Used in meggers, speakers, analog meters, motors

What is the behavior of magnetic poles and lines of force?

North pole attracts geographic north

Outside: lines go N → S

Inside: lines go S → N

Lines:

Form closed loops

Directional

Do not intersect

Try to shorten

Repel laterally

Flow through any material

Seek least resistance

What is magnetic flux and what are key terms related to magnetism?

Magnetic flux (ϕ): invisible lines forming a closed loop
Flux density: flux per unit area
Permeability (μ): ability to concentrate flux High: soft iron
Low: air Reluctance: resistance to flux Flux may take longer path if reluctance is lower

What are residual magnetism, retentivity, and saturation?

Residual magnetism: magnetism remaining after force removed
Retentivity: ability to retain magnetism Soft iron: low nickel alloys: high
Saturation: all domains aligned, further force has no effect

What causes electromagnetism and how is it used in design?

Moving electrical charge induces a magnetic field

Field strength ∝ current

Field collapses if current stops (low retentivity in copper)

What is the left hand rule and how does polarity work in conductors?

Left thumb = electron flow
Fingers = magnetic lines around conductor

Two nearby conductors:

Same direction = combine
Opposite = cancel

Coil winding direction determines polarity, thumb points to north pole (flux travels from north to south pole external to magnet)

What factors influence electromagnet strength and how is it managed?

Factors:
Current in coil
Number of turns
Core permeability
Core length (longer = weaker)


Hysteresis loss: Friction from magnetic field reversal (AC), Reduced with silicon steel
Eddy currents: Right-angle flow from changing field Heat loss in solid core - Laminations of the solid core reduce loss

How is induced voltage (EMF) generated according to Faraday’s Law?

Conductor cutting magnetic field induces EMF Rate of cut ∝ voltage 100 million lines/sec = 1V
Affected by: Flux density Turns of conductor Cutting angle (90° best)

What is self induction and how does it relate to electromagnetic induction?

Each turn induces field into next turn Limited by coil resistance (Ohm’s Law)
Voltage opposes current (Lenz’s Law)
Mutual induction: same effect but voltage goes to a second circuit

What does Lenz’s Law say about induced voltage and how is it calculated?

Induced voltage opposes change in magnetic field, Greater rate of change = higher induced voltage

What is inductance and how is it measured?

Opposes change in current (not flow itself)

Symbol: L, Unit: Henry (H)

1 Henry = 1V induced per 1A/sec change

Only occurs with changing current

How is electromagnetism used in gas controls?

Used in relays with max contact amperage 15A

Magnets attract ____ or _____materials

Ferrous or magnetic

Left-hand rule thumb = ___ pole of coil

North pole

-2 factors affecting strength of a magnetic field around a current carrying coil?

-Current flow

-# of turns

-Eddy currents produce ____ and become an ____ in a solid core, this is why transformers are laminated instead of solid at the core.

-Eddy currents produce heat and become an energy loss in a solid core, this is why transformers are laminated instead of solid at the core.

-EMF is directly proportional to:

-# turns in conductor

-rate of change in signal

-Flux density