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FE ELECTRICAL > Properties of Electrical Materials > Flashcards

Properties of Electrical Materials Flashcards

1
Q

conducting materials

A

allow the movement of charge with quantum states for free electrons and electrons that can be liberated with very little energy input

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2
Q

dielectric (insulating) medium

A
  • an ideal dielectric medium contains no free charges
  • all electrons are bonded to atoms
  • the medium will not conduct electricity
  • in reality, a dielectric medium can be overcome by a strong enough electric field (called a breakdown)
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3
Q

dielectric polarization

A
  • the electric field polarizes the dielectric medium
  • atoms and molecules are aligned and stretched in the direction of the field
  • positive nuclei are pushed in one direction, electrons in the opposite direction
  • directions depend on direction of the field
  • this creates an induced electric dipole
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4
Q

semiconducting materials

A

materials though which charge easily flow more or less easily

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5
Q

hole

A
  • a hole is positively charged atom that attracts electrons
  • when voltage is applied, electrons are drawn to positive charge
  • hole attracts an electron from a neighboring atom, most likely from direction of negative charge
  • hole atom become neutral, neighboring atom becomes new hole
  • hole migrates toward negative charge, though atoms do not move
  • in this way, both electrons and holes carry current
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6
Q

p-type material

A

majority carrier: holes
valance electrons in dope atoms: 3 electrons (Al, B, ln, Ga)
no. of covalent bonds formed by dope atom: 3, resulting in one neighbor atom unbonded
dope atom charge: retains static negative charge, hole is free to move
dope atom name: acceptor
minority carrier: free electrons

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7
Q

n-type

A

majority carrier: electrons
valence electrons in dope atoms: 5 electrons ( Ph, As, Sb)
no. of covalent bonds formed by dope atom: 4, donates a free electron
dope atom charge: retains positive charge, electron is free to move
dope atom name: donor
minority carrier: holes

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8
Q

permeability, μ

A
  • a property of a material
  • measure how readily a magnetic field is created within the material when an external magnetic field is applied
  • due to behavior of electrons in presence of external magnetic field
  • examples of high permeability: iron and steel
  • examples of low permeability: wood and water
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9
Q

nonmagnetic materials

A

negligibly affected by presence of magnetic field (i.e. have low permeability)

types include:

  • diamagnetic
  • paramagnetic
  • antiferromagnetic
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10
Q

diamagentism

A
  • due to electrons circulation in their orbits
  • exhibited by all materials
  • cancels due to random orientation of the spins
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11
Q

paramagnetism

A
  • due to circulation of unpaired electrons in their orbitals

- spins align with magnetic field

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12
Q

antiferromagnetism

A
  • leads to tiny increase in permeability
  • due to magnetic dipole moments that align
  • moment of one atom has the opposite orientation of its neighbor
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13
Q

magnetic materials

A
  • significantly affected by presence of magnetic field
  • cause circulating currents in plane perpendicular to magnetic field

types include:

  • ferromagnetic
  • ferrimagnetic
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14
Q

ferromagnetism

A
  • leads to large increase in permeability
  • due to magnetic domains with fully aligned magnetic dipole moments
  • results in spinning electrons even without external magnetic field
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15
Q

ferrimagnetism

A
  • leads to increase in permeability, but less so than ferromagnetism
  • due to ordered spin structures that neither cancel fully (as in antiferromagnetism) nor add fully (as in ferromagnetism)
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16
Q

ferromagnetism

A
  • fully aligned

- large increase in permeability

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17
Q

ferrimagnetism

A
  • neither fully aligned nor fully canceling

- moderate increase in permeability

18
Q

antiferromagnetism

A
  • fully canceling

- tiny increase in permeability

19
Q

coefficient of thermal expansion

A 
- rate at which material expands and contracts as temperature increases and decreases
α = E/delta T 
- engineering strain: 
E = delta L/ Lo
-change in linear dimension: 
delta L = Lo α delta T

The coefficient of thermal expansion is a property of both electrical and magnetic materials
However, the change in dimensions is not considered in resistance, capacitance, and so on, because the change is so small

20
Q

electric flux Φ

A
  • measures electric field
  • does not flow equally well through all materials
  • units are volt-meters (V * m)
21
Q

permittivity, ε

A
  • determines the flux that passes through the medium
  • total electric flux generated by a point charge is proportion to charge
    ϕ = q/ E
    Eo is permittivity of free space
    for air E= Eo= 8.85x 10^-12 F/m
22
Q

dielectric constant (or relative permittivity), k

A
  • dimensionless comparison to permittivity of free space, Eo
    E= k Eo
  • informal definition: ratio of flux in a medium to flux in a vacuum
  • for a vacuum, k =1
  • formal definition: ratio of capacitances for a given voltage and separation
    k = C with dielectric/ C vacuum
  • The NCEES Handbook sometimes uses the symbol, Er for relative permittivity
23
Q

electric flux density, D

A
  • relationship between density and electric flux is
    D= εE
  • E is electric field intensity in (N/C)
24
Q

Capacitance, C

A
  • measures of capacitor’s ability to store electric charge
  • ratio of stored charge to applied voltage
    C = q/V
    -for a parallel plate capacitor,
    C = εA/d
25
Q

relative electric susceptibility, Xe

A
  • like relative permittivity, a dimensionless comparison to free space εo
    ε = K εo = (1 + Xe) εo
26
Q

conductivity,

A
  • measure of how easily current flows through a medium (how much current is created by an electric field)
  • ratio of current density to electric field strength
  • units are siemens per meter (S/m)
27
Q

resistivity, p

A

inverse of conductivity: p = 1/σ
small conductivity means large resistivity (and vice versa)
units are ohm-meters

28
Q

conductivity and resistivity

A
  • intrinsic properties of materials
  • small conductivity, large resistivity: even a strong field creates little current (for example, rubber)
  • large conductivity, small resistivity: strong field creates large current (for example, copper)
29
Q

conductance, G

A
  • measure of ease of current flow in circuit or circuit element
  • units are siemens (S)
30
Q

Resistance, R

A
  • measure of opposition to current flow in circuit or circuit element
  • inverse of conductance R= 1/G
  • units are ohms
31
Q

resistance

A
  • proportional to resistivity of material
  • proportional to length of element
    -inversely proportional to cross-sectional area of element
    R= pL/A
32
Q

resistivity and resistance

A
  • both depend on temperature

- as temperature increase, both p and R increase for most conductors, decrease for most semiconductors

33
Q

piezoelectric effect

A
  • certain crystals and ceramics respond to mechanical stress with change in electric charge
  • for example: quartz exhibits very stable piezoelectric effects
  • practical uses include sensors for sound, pressure, mechanical stress
  • also used in reverse: voltage applied between faces of certain crystals and ceramics to produce mechanical distortion
34
Q

magnetic field strength, H

A
  • measure of strength of magnetic field in free space
  • independent of medium
  • units are amperes per meter (A/m)
35
Q

magnetic flux density, B

A
  • includes magnetic response of material that field passes through
  • dependent on medium
  • units are teslas (T)
  • in strongly magnetic material, B-field is greater than in free space
36
Q

corrosion

A

degradation of a material ( usually a metal) by chemical reaction with its environment

four main types:

  • galvanic action
  • fretting corrosion
  • stress corrosion
  • cavitation

Stress corrosion and cavitation rarely apply to electrical materials

37
Q

galvanic action

A
  • caused when metallic ions differ in oxidation potential
  • if metals are similar in oxidation potential, corrosion can be slow or negligible
  • if metals are very dissimilar, corrosion is much faster
38
Q

anode reaction

A
  • metal with higher potential acts as anode and will corrode
39
Q

cathode reaction

A

metal with lower potential acts as cathode and is unchanged

40
Q

fretting corrosion

A
  • when metals rub and slide
  • combination of wear and chemical corrosion
    -happens on metals that depend on film of surface oxide for protection (aluminum, stainless steel)
    can be reduced by
  • lubricating rubbing surfaces
  • sealing surfaces
  • reducing vibration and movement
41
Q

law of mass action (mass action law)

A

The number of holes, p, multiplied by the number of free electrons, n, in a semiconductor, doped or not, is equal to the intrinsic carrier concentration, ni, squared

42
Q

concentration of holes

A
  • in a p-type material, is approximately the same as the concentration of the acceptor (p-dopant) atoms
  • in an n-type material, is approximately the same as the concentration of the donor (n-dopant) atoms

FE ELECTRICAL (17 decks)

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