Short Questions and Answers [PDF]

SHORT ANSWER TYPE QUESTIONS

Engg. Physics – II Unit- I (Wave Mechanics and X-rays) 1.

What is the concept of wave-particle duality? Ans: On the basis of existing experimental facts like interference, diffraction and polarization it is clear that EM radiation possess wave nature. On the other hand there are experimental evidences like photoelectric effect, emission and absorption spectra, black body radiation etc., which shows that EM radiation consists of discrete indivisible packets of energy (hν), i.e., photons. Hence, we can conclude that EM radiation has dual character, i.e., in certain situations it exhibits characteristics of wave, while in other; it shows its particle characteristics. However, both the characteristics can never be observed simultaneously.

2.

Why the de-Broglie wave associated with a moving car is not observable? Ans: We know that  ∝ 1/m . Since m is very large far a car therefore λ is very small. Consequently, the de-Broglie wave associated with moving car is not visible.

3.

4.

5.

6.

7.

What is the rest mass of a photon? Write down relation for de-Broglie wavelength of photon. hc Ans.: Zero. Since  = h and for hoton p = E , therefore,  = . p c E What is de’Broglie hypothesis? Ans.: According to de’Broglie hypothesis, every fast moving particle has a wave associated with it. The wavelength of the wave depends upon the momentum of the particle. The de-Broglie wavelength is given by:  = h or  = h . mv p Are matter waves electromagnetic? Ans.: No. This is because electromagnetic waves are produced by accelerated charge. On the other hand, the de-Broglie wave is independent of the charge of a particle. Write down the relation between the energy and momentum of a photon. E Ans.: p = c What is the frequency of a photon whose energy is 66.3 eV? Given: h = 6.63x10-34Js. E 66.3 ×1.6 ×10−19 = Hz = 16 × 1015 Hz = 1.6 × 1016 Hz. −34 h 6.63 ×10 What is the basic principle of electron microscope? Ans.: It is based on de-Broglie hypothesis. A beam of accelerated electrons behaves like wave. This wave can be handled by electric and magnetic fields in exactly the same way as electromagnetic waves are handled by lenses. Thus, de-Broglie waves associated with electron can be used in electron microscope to obtain magnified image with atomic resolution. An electron and a photon have got the same KE. Which of the two has greater de-Broglie wavelength? Ans.: E = h ,  =

8.

9.

Ans.: Photon

-1-

10. Prove that de-Broglie wavelength λ of electrons of kinetic energy E is given by  =

Ans.:

Kinetic energy = work done over the electron



=

1 2 mv = eV or 2

v=

h 2meV

2eV m

h h m h = = mv m 2eV 2meV

11. An electron and a proton have the same amount of kinetic energy. Which of the two possesses

greater wavelength? Ans.: E =

1 2 m2v 2 p2 h2 mv = = = 2 2m 2m 2m 2

1 m 2 But, E is Constant in the given problem

∴

E∝

1 m Since m of electron is less than that of proton, therefore λ of electron is greater than that of proton. 12. What is the dimensional formula of h/mv Ans.: We know that = h/mv. So, the dimensional formula of h/mv is the same as that of λ, i.e., [M0LT0]. ∴

m 2 = Cons tan t ⇒  ∝

13. What useful information is derived from Davisson Germer Experiment?

Ans.: Particles (electron) possess wave nature. 14. Show that the wavelength of electromagnetic radiation is equal to de-Broglie wavelength of its

quantum (photon). h h c Ans.:  = = = p h c  15. A photon of energy 6x10-20J has linear momentum 2x10-30 kg m s-l. Verify this statement.

Ans.: It is not correct. Momentum of photon, p =

h c

6 ×10−20 kg m s −1 = 2 × 10−28 kg m s −1. 3 ×108 16. “de-Broglie hypothesis supports the Bohr's model of stationary orbits”. Comment on this statement. h Ans.: We know that  = mv For stationary orbit, the circumference of the orbit must be integer multiple of λ. i.e., 2π r = n λ or 2 r λ= , where r is the radius of the orbit. n h 2 r h ∴ = or mvr = n , mv n 2 h which is in accordance with Bohr's concept of stationary orbits ( J = n ). So, the given 2 statement is correct. ∴

p=

-2-

17. What is the difference between light waves and matter waves?

Ans.: The velocity of light waves in vacuum is a constant quantity. On the other hand, the velocity matter waves in vacuum depends upon their wavelength 18. What led de-Broglie to think that the material particles may also show wave nature?

Ans.: Nature loves symmetry. Since matter and energy are inter-convertible therefore matter and energy are two aspects of the same entity. 19. What is complementary principle

Ans.: According to complementary principle, the wave and particle aspects of matter and light are complementary rather than contradictory. That is, both the aspects are necessary to have a complete picture of the same system. However, both the aspects of matter (or light) never appear simultaneously in the same experiment. 20. Why is the wave nature of matter not noticeable in our daily observations?

Ans.: In our daily life, we experience the motion of macroscopic objects e.g. car, cricket ball etc. We know that the value of de-Broglie wavelength is very small as compared to the size of the body. This is why wave nature of matter is not noticeable in our daily life. 21. Crystal diffraction can be studied by electron diffraction as well as by neutron diffraction. What

should be the ratio of the velocities of electron and neutron for the de-Broglie wavelength to be the same? Ans.: Since,  = ∴

h ; Clearly, mv should be constant. m

velocity of electron mass of neutron = velocity of neutron mass of electron

22. What is wave packet?

Ans.: When several progressive waves of slightly different wavelength travel along a straight line in one direction, the resultant wave obtained due to the superposition is in the form of group of waves called wave packet. Indeed the wave packet is a group of several waves of slightly different velocity and different wavelengths. The amplitude and phase of the component waves are such that they interfere constructively in the limited region of space, where the probability of existence of particle is maximum, and outside this region they interfere destructively. 23. What is the significance of wave packet? OR What is the need for wave packet representation

Ans.: Form the result of de’Broglie hypothesis; it was proved that material particle in motion is equivalent to a group of waves or a wave packet. Thus, motion of a particle can be described in terms of motion of wave group associated with it. The necessary formulation for motion of pilot wave group was given by Schrödinger in 1927. 24.

State the principle of uncertainty. Ans.: Heisenberg’s uncertainty principle states that, “ It is impossible to measure precisely and simultaneously both the members of a pair of canonically conjugate variables describing the behaviour of an atomic system”. The canonically conjugate pairs are position and momentum, energy and time, angular momentum and angular position etc. In a simplified manner, uncertainty principle may also be stated as: “The product of uncertainties in determining the position and momentum of a particle at the same instant is a best of the order of  The uncertainty in momentum ∆p and uncertainty in position ∆x are related by: ∆x ⋅ ∆p ≥  . The Heisenberg’s uncertainty principle is universal and hold for all pairs of conjugate variables. -3-

25.

Comment on the statement “Heisenberg’s uncertainty principle is valid for all kinds of particles” Ans.: Heisenberg’s uncertainty principle is valid for all kinds of particles. For the atomic particles, there is always some uncertainty in the measurement of two conjugate quantities (whose product has unit joule-sec), like position- momentum, angular position-angular momentum, energy-time etc) but for the particles of large size, this uncertainty is very small as compared to the value of h, the Planck’s constant. Hence, uncertainty is not observable.

26. What is wave function?

Ans.: The properties of a system comprised of particles moving in a conservative force field are described by a function ψ , where ψ is a function of position coordinates and time i.e. ψ = ψ(x, y, z, t), or ψ = ψ(r, t). This function is called the wave function of the system. Hence we can say that “The quantity whose variations make up the matter waves is called the wave function”. The value of the wave function associated with a moving body at the particular point x, y, z in space at time t is related to the likelihood of finding the body there at that time. The wave function should be finite, continuous and single valued. 27.

What do you understand by the stationary state? Ans.: The state of the system for which the probability density  ∗ =  ..is independent of time, is called stationary state. 2

28. What is the physical significance of Schrödinger wave equations?

Ans.: Form the result of de’Broglie hypothesis it was proved that a material particle in motion is equivalent to a group of waves or a wave packet. Thus, motion of a particle can be described in terms of motion of wave group associated with it. The necessary formulation for motion of pilot wave group was given by Schrödinger in 1927. The physical significance of Schrödinger wave equations is that it relates the amplitude of the guiding wave to the probability of finding a material particle at a point. 29. The extent of localisation of a particle is determined roughly by its de-Broglie wavelength. If an

electron is localised within the nucleus (of size about 10-14 m) of an atom, what is its energy ? Compare this energy with the typical binding energies (of the order of a few MeV) in a nucleus, and hence argue why electrons cannot reside in a nucleus. Ans.: For λ= 10-14 m, E ~ 10 MeV, this is much much large compared to the experimentally observed value of β- decay. Therefore, electrons do not reside in a nucleus. 30. What are the operators?

Ans.: The physical quantities obtained by measurements or observations on a physical system are called the observable. For example position, momentum, velocity etc. are the observable. In quantum mechanics, all these observable can be represented by operators. Generally, “An operator is a mathematical rule which when operates on a function, changes that function to some another function. For example, if A is an operator which when operates a function f(x), changes that function to some another function B(x) then: A f(x) = B(x). 31. How is expectation value of an observable quantity obtained for a normalised wave function?

Ans.: For a normalised wave function the expectation value of an observable quantity f (operator f(r)] is given by < f > = ∫  * f (r ) dv 32. What is Compton’s effect?

Ans.: When a monochromatic beam of high frequency radiation (such as X-rays or γ-rays) falls on a target containing free electrons ( e.g., Graphite), the scattered radiation consists of two components, one having same wavelength as that of incident beam and other of slightly greater wavelength than the incident radiation. The observed change in wavelength is known as Compton’s effect. -4-

33. What is Compton wavelength?

Ans.:

The change in wavelength of scattered beam in Compton’s experiment is given by h h is known as Compton wavelength. It has dimensions of length. ∆ = (1 − cos  ) , where m0 c m0 c For scattering with free electrons in the target, its numerical value is 0.02426 Å. 34. How does x-rays differ from y-rays? Ans.: X-rays consists of photons of electromagnetic radiation and are distinguished form gamma rays only by their origin. Whereas gamma rays arise from transitions in the nuclei radioactive atoms, x-rays are produced from extra nuclear process involving electrons 35. Why Compton shift is not observed for visible light?

Ans.: Compton shift is not observed for visible light because the m aximum value Compton shift ∆λ

= 0.04852 Å (when θ = 180°) is very small (about 0.001%) as compared to the mean value of wavelength of visible light ((~ 5000 Å ). Compton effect is observable only with X-rays (or EM waves of much shorter wavelength, e.g., γ rays) and not with visible light. 36. Explain in brief the presence of unmodified radiation along with modified radiation for non-zero scattering angles. Ans.: In Compton’s scattering X-ray photons are scattered by free electrons as well as electrons tightly bonded to the atoms in the target. In the case of scattering with tightly bonded electrons, the atom as whole recoils. Since mass of atom is very high in comparison to rest mass of electron, the value of the term

h m0 c

be very small. Therefore, change in wavelength ∆λ will be negligible for all values of θ and cannot be detected. Hence, both modified and unmodified radiations are present for every non-zero scattering angle.

-5-

SHORT ANSWER TYPE QUESTIONS

Engg. Physics – II Unit- II (Dielectrics, Magnetic Properties of Materials and Ultrasonics)

1.

What do you mean by dielectric constant of a material? Ans. Dielectric constant (k) or relative permittivity (εr) is defined as the ratio of capacitance of a capacitor with dielectric to the capacitance of same capacitor without dielectric. It generally describes the ability of a material to polarize and store a charge. Mathematically, k or εr = C/C0 . [Since C = q/V, we have εr = E0/E = V0/V = ε/εr ].

2.

Define dielectric medium. Ans. Dielectric is a material in which energy can be stored by the polarization of the molecules. It is a material that increases the capacitance or charge storage ability of a capacitor. Ideally, a dielectric is an insulator and does not contain free charge. However, in the presence of external field it exhibits a relative displacement of opposite bound charges and hence the polarization of the medium. Due to polarization induced surface charges tend to weaken the original field within the dielectric. [The resultant field in the presence of dielectric is given by E = E0 – E’ where E’ is the field due to induced charges and E0 is the field in the absence of dielectric.]

3.

What is the difference between insulators and dielectrics? Ans. When electrically non-conducting materials like ceramics, polymers and paper are used for electrical insulation, they are called insulators. On the other hand, when these non-conducting materials are placed in an electric field, they modify the electric field due to induced charges on the surface of the material. As a result these material act as a storage of electrical charges and materials are known as dielectrics. For a material to be a good dielectric, it must be an insulator. Thus all insulators are dielectric.

4.

5.

Give relation for electric field strength due to spherical distribution of charge.   Ans. Electric field ( E ) around a charge q at a point r in a medium of absolute permittivity ε is given by   1 q 1 q. r E= or E = 4 r 2 4 r 3  What is electric displacement vector ( D )? Ans. Electric displacement vector or electric flux density is defined as the number of line of forces received by unit normal surface area in a charge distribution. For spherical distribution of   q charge, D = . Thus, D is independent of medium and its direction is taken normal to the 2 4 r     surface enclosing charge. [From above definition of E and D , we get D =  E ].

6.

Define polarization of dielectric medium. Ans. The induced electric dipole moment per unit volume of dielectric is called dielectric polarization. Polarization vector (P) represents a measures of the extent of polarization in a unit volume of dielectric material. It is equal to the vector sum of dipole moments per unit volume. If the dipoles are randomly oriented then the vector sum is zero and so is the polarization vector. If p is the average dipole moment per molecule and n is the number of molecules per unit volume then P = np. -6-

7.

What are various types of polarization mechanisms? Ans. Polarization in dielectric materials occurs due to following atomic mechanisms: (a) Electronic polarization charge polarization.

8.

(b) Ionic polarization

(c) Orientational polarization

(d) Space

What do you mean by space charge polarization? Ans. In some materials the ions diffuse towards the electrodes in response to the electric field. Therefore, there is a redistribution of charges in the medium. The tendency of redistribution of charges in multiphase dielectric medium (or due to electrode polarization) in the presence of external electric field is known as space charge polarization.

9.

What is Clausius-Mossotti relation? Ans. Clausius-Mossotti equation relates the dielectric constant (εr), a macroscopic property, to the polarizability (α), a microscopic property of a dielectric material by the following relation

 r − 1 n . =  r + 2 3 0 10.

What do you mean by electric susceptibility? Ans. Electric susceptibility (χe) is a material property that measures the extent of polarization in the material per unit field, i.e., χe = P/E. It relates the amount of polarization, P at a point in the dielectric to the field, E at that point via P = χeE = ε0(εr-1)E, here ε0 is the permittivity of free space and εr is the relative permittivity.

11. What do you mean by internal field or local field?

The electric field acting at the location of a given atom is called as local field (Ei). It is the sum of applied electric field and the electric field due to surrounding dipoles, i.e., Ei = E + E’ 12.

What are ferroelectric materials? Ans. A ferroelectric material develops a spontaneous polarization in response to an external electric field. The polarization does not reduce to zero when the external field is removed. In addition, the direction of the polarization is reversible in ferroelectric material.

13.

What are the characteristics of ferroelectric materials? Ans. . 1. They are non-centro-symmetric. 2. Ferroelectric materials exhibit spontaneous polarization without the presence of external electric field. 3. Their dielectric constants are many orders of magnitude large than dielectric materials 4. If exposed to a strong electric field E, its permanent electric dipoles become increasingly aligned until eventually all dipoles are parallel to E and saturation of polarization Ps is achieved. When external field is withdrawn, a remanent polarization Pr remains until a certain reverse electric field is applied. On reversal of electric field, dipoles reorient in opposite direction. As a result they exhibit hysteresis loop (P-E loop)

14.

What is ferroelectric Curie temperature? Ans. This is the critical temperature above which the ferroelectric effect is destroyed and material becomes Para-electric.

15.

What are piezoelectric materials? What is piezoelectricity? Ans. Materials that develop voltage upon the application of a stress and develop strain on an application on electric field are known as piezoelectric materials. The phenomenon of conversion of mechanical (strain) energy in to electrical energy and vice-versa is known as piezoelectricity. -7-

16.

What is relaxation time? How does it vary for different types of polarization? Ans. The time required by dipoles to reorient in response to an external electric field known as relaxation time (τ). The relaxation time is different for each of the various polarization mechanisms contributing to the polarization of a dielectric. For electronic polarization, response is fast and relaxation time is small, for Ionic polarization response is slower, for dipolar polarization response is still slower and finally for space charge polarization response is quite slow and relaxation time is large.

17. What is dielectric loss ? Why does dielectric loss occur?

Dielectric loss is the loss of energy in the form of heat due to internal friction of dipole in aligning themselves in the direction of electric field during charging and discharging of a capacitor. When the relaxation time and the time period of the applied field are similar, a phase lag occurs and energy is absorbed leading to loss of energy. 18. Define loss factor.

The product of real part of relative permittivity and tangent of loss angle is known as loss factor or dissipation factor of a dielectric. It is given by : Dissipation factor = r tan , where tan  = r /r 19. What is electrostriction?

Ans. When a material undergoes polarization, its ions and electronic clouds are displaced, causing the development of a mechanical strain in the material. This effect is seen in all materials subjected to an electric field and is known as the electrostriction. 20.

What do you understand by spontaneous magnetization? Ans. The spontaneous magnetization is the net magnetization that exists inside a uniformly magnetized microscopic volume in the absence of field. The magnitude of this magnetization, at 0 K, is dependent on the spin magnetic moments of electrons.

21.

List two distinct characteristics of ferromagnetic materials. Ans. Tow distinct characteristics of ferromagnetic materials are their : (a) Spontaneous magnetization and (b) Existence of magnetic ordering temperature. (known as Curie temperature T C below which material is ferromagnetic).

22. What is the relation between magnetic field, magnetic induction and magnetisation?

Ans. B = µ0(H+M), where B is the magnetic induction, H is the magnetic field and M the magnetisation. 23. What is the difference between diamagnetism and Para magnetism?

Ans. In diamagnetic materials, the susceptibility χ< 0, while in paramagnetic materials, the susceptibility χ> 0. 24. Why are not all ferro- and ferrimagnetic materials magnetized to their saturated states, even in zero

field? Ans. According to domain theory of ferromagnetism, ferromagnets are subdivided into many small sub volumes, called domains. Each domain is spontaneously magnetized to saturation, but the direction of magnetization varies from domain to domain. The net vector sum of magnetization of all the domains therefore produces a total magnetization near to zero. 25. Why do magnetic domains form?

Ans. There are two competing energies in a ferromagnet: the exchange interaction between neighbouring spins which favours parallel spins, and the magnetic energy which favours small magnetisation. Domains of opposite magnetisation allow the magnetic energy to be reduced without creating too many neighbouring spins that are not parallel. -8-

26.

What is the importance of Hysteresis loop? Ans. The lagging of magnetic flux density B with respect to the magnetising field H called Hysteresis. The complete cycle of magnetization for ferromagnetic material leads to formation of Hysteresis loop, commonly known as B-H curve. The area under this curve is proportional to the energy lost per unit volume per cycle of magnetization. The thin loop corresponds to soft magnetic materials and large one for hard magnetic materials. Thus, one can select a material suitable for making transformer core, permanent magnet etc. on the basis of hysterisis loop of the material.

27. What informations can be drawn from such loops?

Ans. Following important informations about a magnetic material can be drawn from hysterisis loop: (i) Value of saturation magnetization (ii) Retentivity or residual magnetization (iii) Coercive field or coercivity (iv) Reluctance (it is opposition that a ferromagnetic material disfavours the establishment of a magnetic field). (v) Permeability (wider B-H curve shows lower permeability and narrower B-H curve shows higher permeability) (vi) Energy lost per unit volume per cycle of magnetization. 28. What are the characteristics of a material to be used for making permanent magnets?

Ans. (a) High coercive force so that magnetism is retained even at strong external magnetic field and at high temperature and (b) The retentivity should high. Steel possesses above characteristics. 29. What are the characteristics of a material to be used for electromagnets?

Ans. (a) High initial permeability Low coercive force

(b) Low hysteresis loss

(c) High retentivity

(d)

Soft iron possesses such characteristics. 30. What are the characteristics of a material to be used for transformer cores, armatures and chokes?

Ans. (a) Low hysteresis loss

(b) High specific resistance

(c) Bigh initial permeability

Soft iron possesses above characteristics. 31. What are ultrasonic waves?

Ans. Ultrasonic waves are sound waves having frequencies above audible limit (i.e. > 20 kHz). They have small wavelength, higher frequency and higher energy. 32.

Define magnetostriction effect? Ans. Magnetostriction effect is defined as change in dimension of the ferromagnetic material when they are placed parallel to an external magnetic field.

33.

What are the methods to generate ultrasonic waves? Which one is superior? Ans. Ultrasonic waves are produced by piezoelectric and magnetostriction methods. The piezoelectric method of generation of ultrasonic wave is superior to the magnetostriction method in a number of ways, i.e., one can generate ultrasonic waves of frequencies op to 500 MHz, the generation is not affected by environmental changes such as temperature and humidity.

34.

What are the methods of detection of ultrasonic waves? Ans. The ultrasonic waves can be detected by Piezoelectric detector, Kundt’s tube method, Sensitive flame method and thermal detector method. -9-

35.

What does the term SONAR stands for. The term SONAR stands for “Sound Navigation and Ranging”. As ultrasonic waves are very energetic and highly directional, they can be used for locating objects such as submarines, measuring their distance in the sea, depth of sea, lakes etc.

36. Give five applications of ultrasonic waves.

Ans. Ultrasonic waves have many applications. Some important of them are: (i) Non destructive testing and study of structure of materials, (ii) Detection of submarine, iceberg, and pther objects under water (iii) Detection of cracks or flaws in metals (iv) Cleaning and clearing and mixing (v) medical diagnostic such as detection of abnormal growth, treatment of neuralgic pain, detection of blood flow and movement of heart etc.

- 10 -

SHORT ANSWER TYPE QUESTIONS

Engg. Physics – II Unit- III (Electromagnetics) 1. Define a uniform plane wave. Ans. In an uniform plane wave the electric and magnetic field vectors both lie in a plane and all such planes are parallel to each other. Also the amplitude and phase of vectors E and H are constant over the planes & they are always normal to the direction pf propagation. 2. State Gauss divergence theorem. Ans. This theorem states that the net flux of a vector field F over any closed surface S is equal to the volume integral of the divergence of that vector field over the volume enclosed by the surface S. Mathematically it is expressed as:

∫

S

 F .ds =

∫

V

 Div F .dV

3. State Stoke’s theorem. Ans. This theorem states that the line integral of vector field A around the closed curve forming the periphery of any surface S is equal to surface integral of the curl of that vector field taken over surface S bound by the curve forming periphery of the surface. Mathematically it is expressed as:

∫

l

 F .dl =

∫

S

 Curl F .dS

4. What do you understand by displacement current? Ans. Maxwell suggested that it is not only the current in a conductor that produces a magnetic field around it, but a time varying electric field in vacuum or in a dielectric also produces a magnetic field. This implies that a changing electric field in vacuum or a dielectric is equivalent to a current which flows as long as the electric field is changing. This equivalent current is known as displacement current. 5. Compare Gauss's law and Ampere's law Ans. S.No. Gauss Law 1. It is used to determine electric field due to stationary symmetric charge distributions. 2. It involves a surface integral. 3. For applying this law, we need to construct a Gauss's an surface.

Ampere’s Law It is used to evaluate magnetic field due to symmetric steady current distributions. It involves a line integral. For applying this law, we need to consider a path.

6. Which equation shows that isolated magnetic poles do not exist? Ans. The Maxwell’s second equation ∇ ⋅ B = 0 shows that isolated magnetic poles do not exist. 7. Name the work-energy theorem of electrodynamics. Ans. Poynting's theorem. 8. What do you understand by electromagnetic waves? Ans. Electromagnetic waves are coupled to electric and magnetic field oscillations that moves with the speed of light and exhibit typical wave behaviour, i.e., (a) they travel with speed of light (b) they are transverse in nature. (c) the ratio of electric to magnetic field vector (E/B) in an electromagnetic wave equals to the speed of light (d) they carry both energy and momentum.

- 11 -

9. Write down Maxwell's equations? Ans. . Maxwell's equations are a set of equations that describe the space and time dependence of the electric and magnetic fields in a medium through partial derivatives. a) ∇ ⋅ E =

 , 0

known as Gauss's law electrostatics

b) ∇ ⋅ B = 0 ,

known as Gauss's law in magnetostatics and states that no magnetic monopole exists.

∂B , ∂t ∂E   d) ∇ × B =   J +  0 , ∂t   c). ∇ × E = −

known as Faraday's law known as Ampere's law

10. What is continuity equation? Ans. Continuity equation represents law of conservation of charge. It is expressed as:

∇⋅ J = −

∂ ∂t

Which states that current diverging from an infinitely small volume element is equal to the rate of decrease of charge within that volume. 11. Are all Maxwell’s equations independent? Ans. No. All four Maxwell’s equations are not independent. In fact all four equations are interlinked. 12. What is represented by Poynting vector? Ans. Poynting vector represents the energy transported by the EM fields per unit time per unit area. 13. What do you mean by intrinsic impedance? Ans. The intrinsic impedance or characteristic impedance is the ratio of electric field to the magnetic field intensities, i.e.,

z= For free space,

z0 =

E or H

   ohm  

 0    = 120  0 

or 376.72 Ω

14. State Poynting theorem. Ans. Poynting theorem states that the vector product of electric and magnetic field intensity at any point is a measure of rate of flow of energy per unit area at that point, i.e., P = E × H . 15. What is Poynting vector? Ans. Poynting vector is defined as the rate of flow of energy carried by EM waves per unit area of the medium. It is a vector product of electric field and magnetic field vectors. The direction of Poynting vector is in the direction of wave propagation. 16. Define Propagation Constant. Ans. Propagation constant is a complex number and it is given by:

 =  + j  ; where α is attenuation constant and β is phase constant. 17. How do you classify a good dielectric and a good conductor based on conduction and displacement current densities? Ans. For a good dielectric, the ratio of conduction current density (Jc) to displacement current density (Jd) is always less than one in radio frequency range, i.e., tangent. For good conductor

Jc   = 1  - 12 -

SHORT ANSWER TYPE QUESTIONS

Engg. Physics – II Unit- IV (Superconductivity and Science & Technology of Nano Materials) 1. What is superconductivity? Ans. The phenomenon of sudden disappearance of electrical resistance when a substance is cooled below critical temperature (TC) is known as superconductivity. 2. Define critical temperature of a superconductor. Ans. The temperature at which a normal material turns into a superconductor is known as critical temperature. 3. What are the two essential requirements for a superconducting material? Ans. A superconducting material should have zero resistance to current flow and should also exhibit perfect diamagnetic character (i.e., shows Meissner effect). 4. Define critical magnetic field for a superconductor. Ans. Critical magnetic field is the maximum field that can be applied to a superconductor without destroying its superconducting behavior. Alternatively, the minimum magnetic field required to destroy superconductivity is called critical magnetic field. 5. How superconductivity can be destroyed? Ans. The superconducting state can be destroyed by passing an excessive current through the material. 6. Why ferromagnetic materials (such as Fe, Co, Ni etc) do not show superconductivity? Ans. It is due to the presence of strong molecular magnetic field inside the ferromagnetic materials. 7. Define Meissner effect? Ans. The phenomenon of exclusion of magnetic flux (or ejection of lines of magnetic induction) from the interior of bulk superconductor when they are cooled below the transition temperature, is called Meissner’s effect. 8. What is the physical mechanism behind Meissner effect? Ans. It occurs as a result of surface currents in the superconductor that produce magnetic field that exactly opposes the applied field in superconductor. 9. Why type-II superconductors are more important? Ans. They have great technological importance because type –II superconductors can carry very high current densities (because of large value of HC2). 10. What is cooper pair? Ans. Cooper pair is a bound pair of electrons, formed below a critical temperature, due to attraction between two electrons of opposite spins and opposite linear momenta under phonon field. 11. What is physical mechanism that generates the superconducting electrons? Ans. Superconducting electrons exists in bound pairs (called cooper pairs). This is because of an effective attractive force between electrons of opposite spin and momentum. When an electron comes near positive ion core of the lattice, it experiences an attractive force. Due to this ion core is slightly displaced (called lattice distortion). Distorted lattice attracts second electron in this region leading to an effective attraction between electrons and formation of cooper pair. 12. What is Josephosn effect? Ans. The The tunneling of Cooper pairs through a thin insulating layer placed between two superconductors is called Josephson effect. - 13 -

13. What is isotope effect in superconductors? Ans. The critical temperature for many superconductors depends on the isotopic mass. This shows the relevance of lattice vibration in the superconductivity. The dependence of isotopic mass follows the relation: TC  M-1/2 . i.e., TC decreases with increasing isotopic mass. 14. What is flux quantization? Ans. The magnetic flux enclosed by a ring of superconducting material is quantized and it is an integral multiple of fundamental quantum of flux. The magnetic flux quantum is known as “fluxon” h and is given by 0 = = 2.07 ×10−15 Weber. 2e 15. What is SQUID? Ans. SQUID means Superconducting Quantum Interference Device. It is very sensitive to magnetic field and used to detect very weak magnetic field changes in the human body to diagnose problems with various organs in the body. 16. Why are the Type-I superconductors poor current carrying conductors? Ans. The Type-I superconductor has low critical magnetic field and hence can carry current density. 17. What is vortex state? Ans. The region where both superconducting and normal state co-exist. 18. What do you mean by high temperature superconductors? Ans. The superconductor which have critical temperature more than 40K. 19. How BCS theory accounts for superconductivity?. Ans. The BCS theory accounts for occurrence of superconductivity on the basis of formation of cooper pairs due to attractive interaction between two electrons of equal and opposite momenta. 20. What is Nanoscience? Ans. Nanoscience deals with the study of phenomena at a very small scale (1 nm to 100 nm) where properties of matter differ significantly from those of bulk. 21. What are nanotechnologies? Ans. Nanotechnologies are new approaches to develop the ability to control or manipulate the fundamental structure and behavior of matter at the level of atomic and molecular level and to manufacture devices and systems that have novel properties and functions. 22. What are nanomaterials? Ans. The materials with average particle size less than 100 nm are known as nanomaterials. Nanomaterials can be of nano scale in one dimension (e.g. thin films), two dimensions (e.g. strands or fibers), or three dimensions (e.g. particles). 23. Why properties of nano materials are different from their bulk form. Ans. The properties of nano materials are different at the nanoscale for two main reasons : (i) Nanomaterials have relatively larger surface area as compared to the same mass of material in the bulk form. This can make materials more chemically reactive and affect their strength or electrical properties. (ii) Quantum effects (e.g., quantum confinement dominates for particle size < 10 nm) can begin to dominate and affects the behaviour of matter at the nanoscale leading to drastic change in the optical, electrical and magnetic properties of materials. 24. What is quantum confinement? Ans. Confinement of exciton (electron or hole) occurs in a semiconductor when size of the semiconductor, at least along one dimension, is of the order of deBroglie wavelength of the exciton. 25. Define semiconductor quantum well, quantum wire and quantum dot. Ans. Semiconductor quantum wells are quasi-two-dimensional nanostructure (e.g. monolayer) of semiconductor and electrons have sharper density of state than bulk semiconductor because of confinement along thickness of the film. It is formed by sandwiching a semiconductor like GaAs - 14 -

between two layers of wider band gap material (e.g. AlAs). The thickness of the film is down to momolayer. A semiconductor Quantum wire is a semiconductor nanostructure in which confinement of electrons is along the direction transverse to the length of the wire. Transverse energy of electrons is quantized into a series of discrete values. A semiconductor Quantum dot is a semiconductor whose excitons are confined in all three spatial dimensions. As a result they have the properties in between those of bulk semiconductor and discrete molecules. 26. Why the color of suspension of a semiconductor nanocrystal depends on the size of nanocrystals? Ans. Change in color is observed due to increase of energy band gap on decreasing the size of nanocrystals. 27. How nanomaterials can be formed? Ans. Nanomaterials can be made from “top-down” or “bottom-up” approaches. 28. Name examples of a carbon nanoscale structure and describe its interesting properties. Ans. Examples are: (1) Carbon nanotubes which are 100 time stronger than steel, yet very flexible. (2) Carbon buckyballs which can pass through cell membranes and be used for drug delivery 29. What is Buckyball?. What are its applications? Ans. Buckyball is a hollow cluster of 60 carbon atoms shaped like a football. It is the roundest and most symmetrical large molecule known in the world. The buckyball has 60 carbon atoms at chemically equivalent vertices which are connected by 32 faces, 12 of which are pentagonal and 20 are hexagonal. It has variety of applications, e.g., Drug delivery and treatments, as Gadolinium carriers in MRI, in nano STM, as lubricant, as catalyst and in developing organic superconductors. 30. How nanoscale can be visualized? Ans. One can visualize the nanoscale with the help of scanning electron microscope (SEM), scanning tunneling microscope (STM) and atomic force microscope (AFM). 31. What is the smallest size (in meters) that the human eye can see? Ans. The naked eye can see down to about 20 micrometers, i.e., 20×10-6 meters.

- 15 -