Properties of Electrical Materials

Question 1

conducting materials

Answer 1

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

Question 2

dielectric (insulating) medium

Answer 2

• 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)

Question 3

dielectric polarization

Answer 3

• 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

Question 4

semiconducting materials

Answer 4

materials though which charge easily flow more or less easily

Question 5

hole

Answer 5

• 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

Question 6

p-type material

Answer 6

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

Question 7

n-type

Answer 7

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

Question 8

permeability, μ

Answer 8

• 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

Question 9

nonmagnetic materials

Answer 9

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

types include:

• diamagnetic
• paramagnetic
• antiferromagnetic

Question 10

diamagentism

Answer 10

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

Question 11

paramagnetism

Answer 11

• due to circulation of unpaired electrons in their orbitals

- spins align with magnetic field

Question 12

antiferromagnetism

Answer 12

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

Question 13

magnetic materials

Answer 13

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

types include:

• ferromagnetic
• ferrimagnetic

Question 14

ferromagnetism

Answer 14

• 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

Question 15

ferrimagnetism

Answer 15

• 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)

Question 16

ferromagnetism

Answer 16

• fully aligned

- large increase in permeability

Question 17

ferrimagnetism

Answer 17

• neither fully aligned nor fully canceling

- moderate increase in permeability

Question 18

antiferromagnetism

Answer 18

• fully canceling

- tiny increase in permeability

Question 19

coefficient of thermal expansion

Answer 19

- 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

Question 20

electric flux Φ

Answer 20

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

Question 21

permittivity, ε

Answer 21

• 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

Question 22

dielectric constant (or relative permittivity), k

Answer 22

• 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

Question 23

electric flux density, D

Answer 23

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

Question 24

Capacitance, C

Answer 24

• 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

Question 25

relative electric susceptibility, Xe

Answer 25

- like relative permittivity, a dimensionless comparison to free space εo ε = K εo = (1 + Xe) εo

Question 26

conductivity,

Answer 26

- 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)

Question 27

resistivity, p

Answer 27

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

Question 28

conductivity and resistivity

Answer 28

- 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)

Question 29

conductance, G

Answer 29

- measure of ease of current flow in circuit or circuit element - units are siemens (S)

Question 30

Resistance, R

Answer 30

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

Question 31

resistance

Answer 31

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

Question 32

resistivity and resistance

Answer 32

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

Question 33

piezoelectric effect

Answer 33

- 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

Question 34

magnetic field strength, H

Answer 34

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

Question 35

magnetic flux density, B

Answer 35

- 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

Question 36

corrosion

Answer 36

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

Question 37

galvanic action

Answer 37

- 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

Question 38

anode reaction

Answer 38

- metal with higher potential acts as anode and will corrode

Question 39

cathode reaction

Answer 39

metal with lower potential acts as cathode and is unchanged

Question 40

fretting corrosion

Answer 40

- 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

Question 41

law of mass action (mass action law)

Answer 41

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

Question 42

concentration of holes

Answer 42

- 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