Chapter 4-Electricity and Magnetism

the origin of magnetic force

any moving charged particle creates a magnetic field perpendicular to the motion of the particle

spin magnetic moment

magnetic field created from spin of electrons (or any charged particle) on their own axes

Magnetic dipoles

materials that have a spin in a dominate direction, exhibit magnetic domain. In other words, groups of atoms with their net magnetic field moving in the same direction

nonmagnetic dipoles

pairs of electrons (spin-up and spin-down) cancel each others force

flux lines

lines of force or induction, force field between N and S poles of a magnet. Stronger the magnet, more lines of flux, always flow north to south

flux density

concentration of number of lines of magnetic force (flux). Greater density, stronger magnetic field

SI unit for flux density

measured in Weber

SI unit for magnetic flux density

1 tesla = 10,000 Gauss

1 T = 1Wb/m2

Laws of magnetism

Repulsion and attraction

Inverse square law-Magnetic force is directly proportional to the product of the magnitude of the field strength, and inversely proportional to the square of the distance between them

Magnetic poles-break a bar magnet and they each still have two poles

Types of magnets

natural magnets, artificial permanent magnets, electromagnets

Natural magnets

Created when iron oxide remains in the Earth’s magnetic field for ages, slowly orienting 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

Characteristics of magnetic materials

Permeability: ease with which a material is magnetized

Retentivity: the ability to retain magnetization

magnetic classifications of materials

ferromagnetic

Paramagnetic

Diamagnetic

Nonmagnetic

ferromagnetic

a material with high magnetic permeability

Paramagnetic

materials weakly attracted to magnetic fields

Diamagnetic

Diamagnetic materials are weakly repelled by magnetic fields and do not retain magnetism when the field is removed

Nonmagnetic

materials not affected by a magnetic field and cannot be magnetized

Solenoids

a coil consisting of a series of loops, which serve to increase flux density.

Electromagnets

solenoids with an iron core to enhance flux density (ferromagnetic core)

Requirements for generating electrical energy (3)

1. Conductor

2. Magnet field

3. Motion between the conductor and the magnet

electromagnetic induction

a magnetic field is induced by current flow, and likewise, current is induced when a conductor passes through a magnetic field. Presence of magnetic field isn’t enough, must have motion

Ways to create motion between lines of force

1. move 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

Mutal induction

two coils of wire placed in close proximity. AC in the primary coil “induces” a similar current flow in the secondary coil. Used in transformers

Primary coil

coil supplied with the current first

Secondary coil

the coil in which the current is induced

Self induction

Self-induction is the phenomenon where a changing current in a coil induces an electromotive force (EMF) in the same coil that opposes the change in current. Allows direct current to flow while at the same time hindering alternating current

Inductive Reactance

Opposition to the flow of alternating current produced by an inductor. measured in ohms and varies in direct proportion to frequency

Flemmings right hand rule

The direction of the current and its associated magnetic field

Faradays law: 1st law of electromagnetics

Four factors influence induced currents:

-Strength of magnetic field

-Speed of motion

-Angle between rotating conductor and lines of force. Max at perpendicular

-Number of turns in the conductor

Generator

Converts mechanical energy (from motion) to electrical energy (current)

Armature

the rotating coil of wire at the core of an electric generator

Slip rings

To conduct electricity of an electric generator, slip rings provide contact, while turning on brushes. Whole slip rings are used for AC generators

Brushes

In contact with slip rings, current is conducted from the armature of a generator through brushes

commutator ring

a slip ring cut in half, which each half connected to one end of the armature wire. The gaps in the commutator ring create a change in the direction of current in a motor or generator. The gap eliminates reversal of polarity. used for DC generators

Motors

opposite of a generator. Same principles, simply in reverse. Motors use electrical energy to produce mechanical motion

DC motor

commutator rings provide the change in polarity for a DC power supply

AC synchronous motor

rotates at the same frequency as the AC generator

AC induction motor

an electromagnet in the stator produces the magnetic force that spins the rotor 

Rotor

Spins inside the motor

Stator

The stationary part, stationary magnets around the rotor. Supplied with multiphase current, energized in sequence to turn rotor, no slip rings and brushes

Motor force principle

describes opposing magnetic fields. When a magnetic field is produced by a conductor moving in a magnetic field, the two fields will repel each other. This is what turns the rotor in an induction motor

Oscilloscope

electronic test instrument that displays voltage signals as waveforms. Root is oscillate

Ammeter

Measures current in a series circuit, in amps

volt meter

measures potential difference (voltage) across a parallel circuit

Revolutions and pulses per second

60 Hz, 120 pulses per second

Transformer

take energy and transfers energy between circuits, changing the voltage levels, either step-up or step-down.

Mutual induction

two coils of wire placed in close proximity. AC in the primary coil “induces” current flow in secondary coil. Current must be AC to induce current in secondary coil.

Primary Coil

The energized coil in an AC transformer. If number of turns in primary coil is greater than number in the secondary, it is a step-down transformer

Secondary coil

The coil in which current flow is induced in an AC transformer. If number of turns in secondary coil is greater than primary coil, it is a step-up transformer. 

Transformer Law

Voltage and number of turns (N) are directly proportional, whereas volts and amps are indirectly proportional. Number of turns are inversely proportional to amps.

Vs/Vp = Ns/Np

(s = secondary, p = primary)

Autotransformer

single coil of wire with multiple taps. Controls low voltage

I2R losses (copper losses)

Impedance in a transformer due to resistance

Hysteresis losses

loss due to remagnetization of the core. Constant changing of the magnetic field causes lag in the dipoles aligning in the proper direction

Eddy Current

Impedance in a transformer due to the reversal of current. Current is working against itself as it changes direction. Eddy currents cause power loss because much of the energy is wasted as heat. Laminated cores lessen loss

Air core transformer

simple primary and secondary coil

Open core transformer

simple primary and secondary coil with iron core in coils

Closed core transformers

coil wrapped on sides of square core

Shell type transformer

primary and secondary coil share an iron core in the middle of a double square configuration. Most efficient, used in X-ray generators

Rectification

changing of AC to pulsed DC, converting alternating current into a unidirectional current, by removing or inverting that part of the wave laying on one side of the zero amplitude axis. Used in high voltage circuit to maintain current flow from the cathode to the anode

Thermionic emission

When mA heats a filament (cathode) excited electrons rise to higher orbits, forming a space charge cloud

Space Charge

a cloud of electrons surrounding the cathode, formed when mA heats a filament causing electrons to raise higher orbitals

diode

semiconducting rectifier used in the high-tension circuit of an Xray machine. Replaced vacuum (valve) tube rectifiers to produce half wave

p-n junction

p (positive) side of the diode rectifier crystal contains electron holes receptive to electron flow. n (negative) side provides electrons when half of AC cycle is in n-p direction, current flows. The other half of the cycle, in p-n direction , opposes the current flow

Thyristor (silicon controlled rectifier, SCR):

extra p-n junctions provide faster switching of current flow

Capacitors

Stores electric charges temporarily on two insulated foil strips, one strip positively charged, one negatively charged. Used in capacitor discharge portables. Unit of capacitance is the farad, which is 1 coulomb/volt