Conductivity - Fang

Question 1

what are the uses of electric properties of materials?

Answer 1

1 - computing (nanotech)
the processing and transmissions of information electronically
2 - fibre optic communication for insulators
3 - transportation locking devices semiconductor uses
4 - healthcare pacemakers which stimulate the hearts
5 - energy a lot of oxide semiconducting materials, e.g. for silicon in solar panels

Question 2

what did jj thomson do?

Answer 2

discovered the electron via
catthode rays in a device were streams of negatively charged particles
by varying the magnitude of e fields and magnetic fields found mass to charge ratio

electrons were basic constituent of all matter
1000 times lighter than hydrogen

Question 3

what are charge carriers?

Answer 3

an atomic scale species in which the conduction of electricity in materials occurs

Question 4

what are the types of charge carriers?

Answer 4

electrons - simplest with 1.6 x 10-19 of charge
holes - abstract concept, missin gelectron in an electron cloud with 1.6 x 10-19 of positive charge

Question 5

what are anions and cations?

Answer 5

they are charge carries in ionic materials
a serves as negative charge carries 1.6 x 10-19
c serves as positive charge carrier of 1.6 x 10-19

Question 6

fill in the blank
the valence of each ion indicates ____ or ______ charge in _____ of single electron charge

Answer 6

the valence of each ion indicates positive or negative charge in multiples of single electron charge

Question 7

how can we find the conductivity?

Answer 7

the reciprocal of resistivity is conductivity, sigma
sigma = 1/rho so per ohm per metre

respectively, to find conductance = 1/resistance per ohm

Question 8

what is joule heating?

Answer 8

the passage of electrons through a material
creates generation of heat
due to large number of collisions

Question 9

simple planetary model of atomic structure

Answer 9

electrons orbit around a nucleus (sun)

Question 10

pauli exclusion principle

Answer 10

no two electrons can occupy precisely the same state

Question 11

using the pauli exclusion principle, explain how the solid sodium has half an energy level

Answer 11

large number of atoms release the electron in the outer energy level which all lie on the 3s energy level. each electron slightly on a higher energy level. because the spacing is minimal, the valence energy band is half-filled permitting high mobility with outer electrons
the energy termed is the valence band

Question 12

what happens when the temperature is 0k?

Answer 12

the energy levels of the valence band are completely full up to the midpoint

Question 13

define fermi level

Answer 13

the energy of the highest filled state in the energy band at 0k

Question 14

define the fermi function

Answer 14

the probability that an energy level is occupied by an electron

Question 15

why are metals good electrical conductors?

Answer 15

thermal energy sufficient enough in promoting electrons above fermi level towards unoccupied energy levels
the accessibility of unoccupied levels in adjacent atoms yield high mobility of conduction electrons
known as free electrons through solid

Question 16

what happens to conductivity in metals as temperature increases?

Answer 16

decreases
due to the drop in electron mobility, as thermal agitation increases in a crystalline structure
wavelike electrons can move through crystalline structure effectively when structure is perfect

Question 17

what is drude’s first assumption

Answer 17

matter consists of
1 - lightly negatively charged ions which are mobile
2 - heavy static ions which are static

Question 18

drude’s second assumption

Answer 18

the only interactions is electron ion interactions which take place over a very short time
1 - the electron-electron interaction is neglected
2 - the electron-ion interaction is neglected

Question 19

drude’s third assumption

Answer 19

the probability of electron having a short collision time is dt/t

Question 20

drude’s fourth assumption

Answer 20

after each collision an electron emerges in a random direction and with a speed which is appropriate to the local temperature

Question 21

mean free path of an electron

Answer 21

the average distance travelled by an electron between two successive collisions inside the metal in the presence of the applied field

Question 22

why are electrons almost negligible in insulators compared to conductors?

Answer 22

as insulators have a large band gap, most electrons do not have have enough energy to be promoted to the conduction band

Question 23

what is they key factor for insulators density?

Answer 23

n carrier density, really low, therefore conductivity is really low

Question 24

capacitor

Answer 24

parallel plate insulator between two metal electrodes, the charge will build up on capacitor surface

Question 25

charge density

Answer 25

directly proportional to electric field strength

Question 26

dielectric strength

Answer 26

limiting voltage gradient, which appreciable current flow occurs and the dielectric fails

Question 27

semi conductor

Answer 27

matierals with conductivity intermidiate between those of conductors and insulators

Question 28

intrinsic elemental superconductors

Answer 28

both electrons and holes are charge carriers; the number of electrons is equal to the number of holes a hole is missing due to a valence electron

Question 29

in semi conductors why does the conductivity increase with temperature?

Answer 29

the number of electrons that are able to jump from valence band to conduction band increases as charge carrier increases exponentially

Question 30

hall effect

Answer 30

if current is affected by the magnetic field there should be a state of stress the electricity passing through one side of the wire

Question 31

how is the hall voltage generated?

Answer 31

during magnetic field charge carriers experience a force - lorentz charge carriers detour inducing an electric force which is equal and opposite to lorentz force seperation of charge creates a steady an electric field, and a voltage - hall

Question 32

extrinsic semi conductor

Answer 32

results from impurity additions such as dopants there is ntype (negative) and ptype (positive)

Question 33

what is the difference between an n type and p type semi conductor?

Answer 33

with the example silicon phosphorus in an n type dopant because it has 5 electrons, one extra in conduction band aluminium is a p type dopant as it has 3 valence electrons, the shortage of one electron can become a hole

Question 34

describe n-type si

Answer 34

4 out of 5 electrons in P is needed for bonding the extra electron is relative unstable and produces a donor level near the conduction band as a result the conduction band is Eg - Ed

Question 35

describe p-type si

Answer 35

a 3 electrons are used for bonding, but with 1 missing al's hole is in acceptor level near the valence band and an si electron is easily promoted to this level creating a hole the fermi level is shifted downwards

Question 36

what is the general observation of the p-n junction

Answer 36

when voltage is applied to the junction, the holes go to the negative electrode and the electrons go to the positive electrode only a minimal current can flow this reverse bias leads to the polarisation of the diode if voltage is reversed forwards bias occurs