Electromagnetics

Chapter 4: Electromagnetism

Magnetism

• Magnetism is one of the fundamental forces in physics.

• Orbital Magnetic Moment: Refers to the perpendicular magnetic force generated by electrons in their orbitals.

• Spin Magnetic Moment: Caused by the intrinsic spin of electrons, creating an effect due to their movement.

Magnetic Properties

• Magnetic Dipoles: Groups of atoms with aligned magnetic fields known as magnetic domains. Each dipole contains approximately 10^15 electrons.

• Lines of Force: Also called lines of flux, these represent the magnetic field created by the alignment of magnetic dipoles.

• SI Unit: The unit for measuring magnetic flux is the Weber (Wb), where 1 Wb equals 10^8 lines of flux.

Units of Measurement

• Flux Density: Measured in Tesla (T) or Gauss (G), with 1 T equal to 10,000 G or 1 Wb/m².

• Earth's Magnetic Field: Approximately 0.0001 T (or 1 G).

Classifications of Magnets

• Natural Magnets: Created when iron oxide (magnetite) remains in the earth's magnetic field for an extended time, aligning dipoles.

• Artificial Permanent Magnets: Made from steel alloys like alnico, which contains aluminum, nickel, and cobalt.

• Electromagnets: Temporary magnets formed by electric current flow.

Laws of Magnetism

• Repulsion-Attraction: Fundamental interactions between magnets.

• Inverse Square Law: The magnetic force between two fields is proportional to their magnitudes and inversely proportional to the square of the distance between them.

Magnetic Poles

• Every magnet has two poles: north and south; these poles always exist as a dipole, regardless of the size.

Material Responses to Magnetic Fields

• Permeability: The ability of a material to be magnetized.

• Retentivity: The capacity of a material to retain magnetization after the external magnetic field is removed.

Magnetic Classifications of Materials

• Ferromagnetic: Strongly attracted to magnets.

• Paramagnetic: Weakly attracted.

• Diamagnetic: Repelled by magnets.

• Nonmagnetic: Not affected by magnetic fields.

Electromagnetism Overview

• Oersted's Experiment: Demonstrated that a moving charge creates a magnetic field; a current passing near a compass caused it to move.

Electromagnetic Induction

• Faraday's Discovery: A magnetic field alone cannot induce electron movement; movement must occur between the wire and the magnetic field's lines of force.

Methods to Induce Current

• Move a conductor through a stationary magnetic field.

• Move magnetic lines of force across a conductor.

• Vary the strength of a magnetic field around a conductor.

Primary Laws of Electromagnetism

• Faraday's Law (1st Law): Relates to electromagnetic induction.

• Lenz's Law (2nd Law): States that induced currents will generate a magnetic field opposing the change that caused them to induce.

Factors Influencing Induction (Faraday's Law)

1. Strength of the magnetic field.

2. Speed of motion between conductor and magnetic lines.

3. Angle between lines of flux and conductor (ideal at 90 degrees).

4. Number of turns in the conductor.

   • Factors 1 and 4 are interconnected.

Mutual Induction vs Self-Induction

• Mutual Induction: Occurs in two coils placed near one another; the primary coil induces current in the secondary when current varies.

• Self-Induction: Present in coils with alternating current; the changing current creates varying fields affecting the coil's own turns, causing inductive reactance.

Generators and Motors

• Generators: Convert mechanical energy into electrical energy. Key components include:

  • Armature (conductor)

  • Magnets

  • Slip rings

  • Brushes

• Motors: Convert electrical energy into mechanical energy, using similar components as generators but in reverse. The armature is supplied with current.

Motor Configurations

• DC Motors: Use commutator rings to switch current flow.

• AC Motors: Rely on slip rings for operation.

Types of Motors

• Synchronous AC Motors: Conducting coils rotate in sync with the generator armature, generating more electricity than induction motors.

• Induction AC Motors: Create strong magnetic fields, commonly used in rotating anode x-ray tubes.

Measurement Instruments

• Ammeter: Measures current in amperes when connected in series.

Controlling Electrical Current

• Instruments include transformers, autotransformers, and capacitors.

Transformers

• Utilize mutual induction and Ohm's Law. Comprise two coils—current supplied to the primary induces current in the secondary.

• Can be structured as step-up or step-down transformers based on design.

Transformer Laws

• Voltage Relationship: Voltage induced in the secondary coil is related to the number of turns in each coil, represented by the equation:

  • Vs/Vp = Ns/Np

• Current Relationship: The current in the secondary coil inversely correlates with voltage and the number of turns in the primary coil, expressed as:

  • Is/Ip = Vp/Vs = Np/Ns

Transformer Inefficiencies

• I²R Loss: Caused by resistance in copper wires, generating heat. Known as winding loss.

• Hysteresis Loss: Resistance caused by the frequent reversal of magnetic fields in alternating current.

• Eddy Current Loss: Currents opposing their source, wasting energy.

Transformer Configurations

• Air Core: No magnetic core present.

• Open Core: Has an open magnetic structure.

• Closed Core: Contains a closed loop of magnetic material.

• Shell Type: A combination of closed and air core structures.

Key Transformers in X-Ray Circuits

1. Autotransformer

2. High Voltage Step-Up Transformer

3. Filament Step-Down Transformer

Autotransformers

• Two designs: (1) Primary and secondary coils in series, and (2) a single coil on a central core allows selection of active turns.

Capacitors

• Function as storage for electrical voltage, consisting of two oppositely charged plates separated by a dielectric.

• The unit of capacitance is the Farad.

Rectification

• The process of changing AC into pulsating DC, using devices like diodes and vacuum-tube rectifiers.

• Types of Rectification:

  1. Full-wave rectification

  2. Half-wave rectification