Ece 209: Previous Final Questions

Front 1

What is the Meissner effect and its origin?

Back 1

Meissner effect is the levitation of superconductors placed into magnetic field. It is due to diamagnetism of superconductors.

Front 2

What is the polarizability of a dielectric? What components does the polarizability generally consist of?

What is the susceptibility of a dielectric that shows no polarizability?

Back 2

Polarizability – proportionality constant between dipole moment and internal electric field, or the average dipole moment per unit of internal field (1 point)

Polarizability consists of:

i) electrical polarizability;

ii) molecular polarizability (which, in turn, consists of atomic and orientational polarizabilities) and

iii) interfacial polarizability. (1 point)

Susceptibility of a dielectric that shows no

polarizability is ZERO (

Front 3

What would be the dielectric constant of silicon dioxide in a capacitor driven by 10^17Hz frequency?

Back 3

The dielectric constant of silicon dioxide at 10^17Hz would be 1.

Front 4

What is the value of the heat capacity in aluminum above Debye temperature?

Back 4

Above Debye temperature, the heat capacity in metals is about 25 J/mol K, which originates from 3kNA : Dulong – Petit Law. (2 points)

Front 5

Formulate Wiedemann-Franz law. What materials is it applicable to?

Back 5

L = K/(σ T)

is known as Wiedemann-Franz Law – electrons carry both electrical and thermal energies. (2 points)

Applicable to metals and semiconductors. (2 points)

Front 6

Assume a ferromagnetic material becomes superconductive if cooled below Tc.

Would its magnetic properties change in that temperature range and if yes, then how?

How would they change if material is heated above Curie temperature? Explain, why?

Back 6

Superconductors below Tc are all diamagnetics. - 1 points

Above the Curie temperature, ferromagnetic materials become paramagnetic. - 1 points

It happens because atomic thermal vibrations (phonons) destroy the spin-spin interactions of electrons in the neighboring atoms.

Front 7

Why do materials composed of atoms with incomplete outer electron shells show zero net magnetic moment though the atomic magnetic moment is not zero?

Back 7

Due to the bonding, outer shells become completed by shared electrons.

Front 8

Sketch a typical B-H diagram of a ferromagnetic material and mark on it:

• the remanence,

• the coercive field.

Back 8

Front 9

What is the difference between hard and soft magnetic materials in terms of material structure, coercive field, and permeability?

Back 9

Permeability is the ease which the material can be magnetized

Permeability=u=B/H

Front 10

Calculate the refractive index of a non-magnetic material with dielectric permittivity of 11.9.

How does the refractive index in a crystal dielectric change if material is under compression and why?

Back 10

For non-magnetic material, n = (εr)1/2 = 3.45 (2 points).

If a crystal dielectric is under compression, dielectric constant and refractive index increase (2 points) due to an increase in its concentration of dipoles

Front 11

What is the penetration depth? How does it differ in metals in comparison with insulators? Why?

Back 11

Penetration depth is 1/α, where α – absorption coefficient. At penetration depth, 67% of incident radiation is absorbed. (1 point)

In metals, penetration depth is much shorter than in insulators due to high concentration of free electrons absorbing photons of any energy.

Front 12

Explain the difference between the photo-, electro-, and cathodoluminescence. Name an example of practical application for each of these effects.

Back 12

In photoluminescence, a photon-photon conversion takes place, i.e., electrons in a material are excited by photons (1 point) (example – fluorescent bulbs (1 point)).

In electroluminescence, an electron-photon conversion takes place (1 point); example – light emitting diode, LED (1 point).

In cathodoluminescence, the energy of electrons emitted from a cathode is converted to photons (1 point); example – CRT TV (1 point).

Front 13

the interatomic distance in diamond crystal is a angstroms. what will happen to the band gap if the crystal is (i) stretched so that a is increase, (ii) compressed so that a is decreased?

Back 13

(i) if a increases, then band gap decreases.

(ii) if a decreases, then band gap increases

Front 14

why is the effective mass of electron different from the mass of an isolated electron?

Back 14

the effective mass takes into account the interaction of the electron with with an applied electric field to the solid. in the solid, electron interacts with the crystal lattice atoms and experiences internal forces. if speed decreases, we say the effective mass increased. and vice versa

Front 15

with increasing temperature, how does the electrical conductivity change in (i) an intrinsic semiconductor, (ii) and extrinsic semiconductor, (iii) metals? explain

Back 15

(i) intrinsic semiconductor. as temp increases, carrier concentration increases linearly --> conduction increases linearly

(ii) in extrinsic semiconductors: as temp increases, you get electrons from both donors as well as valence band. therefore conduction increases faster than intrinsic material, until the intrinsic concentration is greater than the extrinsic concentration

(iii). in metals, conductivity decreases with increased temp. Carriers does not change (all electrons already in conduction band. It mobility decreases with increased temp

Front 16

why in non-superconducting metals, a non-zero resistivity exists at very low temperatures close to absolute zero?

Back 16

in non-superconductiong metals, the structural disorder gives rise to a resistivity. resistivity of most materials tends to a constant non-zero value.

Front 17

taking the photoelectric effect into consideration, explain the particle nature of light

Back 17

The photoelectric effect is the an effect that was discovered experimentally by scientists that found when certain wavelengths of light were shined on a metal surface, electrons would be ejected from the surface. It was discovered that regardless of the intensity of light, as long as it was the same frequency it would not change the rate that the electrons were being emitted at. Since during the experiment it was discovered that the rate of electron emission was dependent on frequency rather than intensity, it proves the existence of discrete packets of light energy, or photons. Thus light has wave-particle duality

Front 18

the dielectric constant of a dielectric material measured at a frequency of 10^5Hz was found to be due to electronic polarization. when measured at 50Hz, a difference dielectric constant was found. what are the possible polarization mechanisms that could have caused this change? explain each mechanism.

Back 18

10^5Hz, found to be due to electronic polarization. that means there was no atomic polarization. (atomic is around 10^10Hz)

at 50Hz, a different dielectric constant was found. means there was orientational polarization. (around 10^4Hz)

Electrical polarizability:

when electric field is applied, electron cloud is distorted. makes electric dipole.

molecular polarizability:

arises when molecules of the material naturally form dipoles.

when electric field applied, 2 other things can happen

--> atomic polarizability: when aligned with electric field, distance between dipole changes

--> orientational polarizability: when not aligned with electric field, angle changes.

interfacial polarizability:

due to imperfections in materials. as these , charges migrate through the dielectric, they can build up at the interface or the surface. when external field applied, the interface charges like a dipole.

Front 19

what do you understand by lossy dielectric? define and explain the term "loss angle"

Back 19

loss angle. caused by non-ideal capacitors, there will be a leakage current. makes the capacitance a real component. then the capacitance has 0 degree and 90 degree components, thus angle is not exactly 90 degrees. the amount this is off by is the loss angle. for a good dielectric, you want a small loss angle.

Front 20

what do you understand by breakdown of a dielectric? what are the different mechanisms that cause breakdown?

Back 20

cannot apply an infinately large voltage across capacitor without it breaking down. same with electric field.

mechanisms of breakdown:

1. avalanche breakdown: occurs if the electric field across insulator is high enough.

2. thermal breakdown: occurs if leakage current (loss angle) is large enough to cause significant heating

3. discharge breakdown: occurs if small bubbles are present in the material

Front 21

define the terms "specific heat capacity" and "molar heat capacity". in what units are they measured?

Back 21

heat capacity is indicative of a materials ability to absorb heat from the external surroundings.

specific heat capacity is heat capacity per unit mass. [Joules/kg*K]

molar heat capacity is heat capacity per mole of molicules[Joules/mol*k]

Front 22

in the classical treatment, the theoretical heat capacity of a metal should be around 25 Joule/mol*K. however in reality, there is a deviation from this a low temperatures. explain why.

Back 22

classical treatment implies that heat capacity is independent of temp. this is not true.

Front 23

what is dulong petit law?

Back 23

heat capacity of metals saturates at 25 Joule/mol*K

Front 24

how do you define the term Debye temperature?

Back 24

debye temperature is the temp at which CV reaches 96% of its final value (theta_D)

Front 25

define the terms "magnetic field strength" and "magnetic flux density". give their units

Back 25

magnetic field strength:

H = [A/m]

magnetic flux density:

B

Front 26

why do we find at least one transition element in any magnetic material?

Back 26

transition elements have an inner electron shell incomplete. when atoms bond together to form a solid, their electrons fill their outer shells. only materials with incomplete inner shells after bonding show magnetic properties.

Front 27

why do paramagnetic materials not show a net magnetization at room temperature?

Back 27

In paramagnetic materials, atoms have a permanent non-zero net magnetic moment due to the sum of orbital and spin magnetic moments. However at room temperature, in paramagnetic materials, thermal energy causes random distribution of magnetic moments and hence net magnetization appears to be zero for the whole material