Physics Viva

RCC Institute of Information Technology Paper Code: PH 391/491

Sem: odd/even

Eg = 2 x 8.6 x 10 -5 x (slope of the graph log 10 ρ vs. T-1) Discussion: 1) The probes should be just touching the wafer 2) The temperature should be taken at intervals of ~ 100C 3) The current should be kept constant at ~ 4 mA 4) The maximum temperature should be about 120 K 5) When the current starts to vary, the data should no further be taken.

Viva voce questions: 1. What is energy band gap? The gap between the bottom of conduction band and the top of valence band is called Energy gap. To move the electrons from the valence band to conduction band the supplied external voltage must be equal to energy band gap. 2. What is valence band? Ans: The range of energy which is possessed by valence electrons is known as valence band. Here the electrons which are situated at outer most orbits are called valence electrons. The valence band consists of valence electrons which are having highest energy. 3. What do you mean by conduction band? The range of energies possessed by conducting electrons is known as conduction band. The conduction electrons are responsible for the conduction of current in a conducting material. So, these electrons are called as conduction electrons. 4. How energy bands are generated in i n a semiconductor? A semiconductor remains in crystalline form. In such a crystal, the constituent atoms are orderly arranged., so the unfilled energy levels of the crystal atoms merge together to form an energy band called the conduction band and the filled and partially filled energy levels merge together to form valence band. In a semiconductor there remains gap between conduction and valence band called band gap. 5. Classify the solid materials on the basis of energy gap. Ans: Based on the energy gap the solid materials are classified into 3 types they are: conductors, insulators and semi conductors. 6. Define conductors, insulators and Semi conductors.

Page 4 of 29


Sem: odd/even

Conductors:

Those substances whose atoms have their outermost orbits incomplete are known as conductors (e.g. Cu, Ag, Au etc.). In conductors, valence and conduction bands are found overlapped into each other i.e. the energy gap is zero. Insulators:

Those substances which have large energy gap between their valence and conduction band, are called insulators (e.g. diamond, wood etc.). Semi conductors: Those

substances which have conductivity and resistivity properties in between conductors and insulators are called semi conductors (e.g. Si, Ge). Energy gap of these semiconductors lies between 0.5 to 1.1eV (Foe Ge it is 0.5 – 0.7eV). 7. How many types of semi conductors are there? Two types of semi conductors are there (i) Intrinsic or pure semi conductors and (ii) Extrinsic or impure semi conductors. 8. Define intrinsic and extrinsic semi conductor? Intrinsic semi conductor: A

pure semiconductor is known as intrinsic semi conductor. In these semi conductors, if the temperature increases then the conductivity is also increases. At higher temperatures due to collisions some electrons absorb energy and raises to conduction band then in their places in valence band holes are created. In intrinsic semiconductor number of holes is equal to number of electrons. Extrinsic semi conductor: A

pure semiconductor after doping is called extrinsic or impure semi conductor. Trivalent and penta-valent impurities are added to form P-type and N-type semiconductors respectively. 9. What do you mean by Fermi energy level? The level upto which all the energy states are filled by electrons is known as Fermi level. The average energy of charge carriers is calculated by Fermi energy level. In pure semi conductors Fermi energy level is at the centre of the valence and conduction bands. In extrinsic/impure P-type (N-type) semiconductor Fermi energy level is near to the valence (conduction) band. 10. Define Doping and Dopant? The process of adding impurities to a pure semi conductor is called doping The material added as impurity is called as Dopant. 11. What are P-type and N-type semi conductors?

Page 5 of 29


Sem: odd/even

If we add trivalent impurities such as Aluminum to a pure semi conductor then the material is called P-type semi conductor. If a pentavalent impurity such as Arsenic is added to a pure semi conductor then the material is called N-type semi conductor 12. Why P-type (N-type) semi conductor is called Acceptor (Donor)? In P-type material 3 electrons of trivalent atom makes covalent bonds with Semiconductors such as Si or Ge and there is a need of one more electron to make the system stable because Si or Ge has 4 electrons in their outermost orbits. For this reason P-type material is also known as Acceptor. On the other hand, in case of N-type of material 4 electrons of pentavalent atom makes covalent bonds with Semiconductors such as Si or Ge which have 4 electrons in their outermost orbits and hence there is one free or excess electron remains present in the structure. For this reason N-type material is also known as Donor. 13. What is P-N junction diode? If P-type and N-type semi conductors are combined to each other then the resultant structure is called P-N junction diode. This means if trivalent impurity is doped to one end of the pure semi conductor and pentavalent impurity to other end, a P-N junction diode can be formed. 14. What is 4 probe method ? In this method a wire or a small structure is contacted at 4 locations.

A

V Probe

15. What for 4 probe method is used ?

Semiconductor

It is used to determine the specific resistivity (Ωm) of metal line during electrical characterization of metallic deposition of thin metal line.

S

S

S

16. What is the principle used in 4 probe method ? Current is forced through outer pins 1 & 4 & a drop voltage across pins 2 & 3 is measured using a very high through pin 2 & 3 is nearly 0. In these case the individual n additional contact resistance does not play a role as it cancels out of the equations. 17. What is an ohmic resistor ? If the behavior of the structure of I/V curve is a straight line. Then the structure behaves as ohmic resistor. 18. What is the advantage of 4 probe method over other methods of measuring resistivity?

Page 6 of 29


Sem: odd/even

In most of other methods, the current carrying contacts injects minority carriers which ultimately modify the resistance of the material. 19. What is meant by electrical resistivity (  )? It is the property of the material by the virtue of which it opposes the flow of current. It is also defined as the reciprocal of electrical conductivity (Ωm)-1 . Its unit is Ωm i.e.,  = l/  = RA/L. 20. What are the values of band gap in the case of germanium and silicon? For Ge the band gap value is 0.785 eV, for Si the band gap value is 1.21 eV at 0K.

Page 7 of 29


Sem: odd/even

5) Care should taken to limit the current through the probe to a value less than that mentioned by the manufacturer.

Viva voce questions:

1. Define Hall Effect? When a current carrying specimen is placed in a transverse magnetic field then a voltage is developed which is perpendicular to both, direction of current and magnetic field. This phenomenon is known Hall Effect. 2. What causes Hall Effect? Whenever a charge moves in a mutually perpendicular electric and magnetic field it experiences Lorentz force due to which it deflects from its path and Hall voltage is developed. 3. What is Lorentz force? If charge ‘q’ moves in a magnetic and electric field ‘B’ &’E’ respectively with velocity v then force on it is given by F= qE+ Bqv.sinө 4. What is Hall Coefficient? It is the electric field developed per unit current density per unit magnetic field 5. What are the uses of Hall Effect? To determine the sign of charge carrier and char ge carrier concentration 6. Define Charge carrier concentration. No. of charge carriers per unit volume. 7. Why Hall voltage differ for different type of charge carrier? Because direction of Lorentz force is different for different type of charge carrier. 8. What is unit Hall coefficient? Ohm-meter/Tesla. 9. What is the unit of charge carrier concentration Per Cubic-centimeter.

Page 10 of 29


Sem: odd/even

10. Which type of magnet is used in the experiment, temporary or permanent? Temporary. 11. Mention some uses of the Hall effect (i) Determination of the semiconductor type. Since R H is positive for a p-type semiconductor (ii) Determination of the carrier concentration .Since R H

1 

ne

or

1

pe

(iii) Measurement of unknown magnetic field. The hall voltage VH is proportional to the magnetic field H for a given current I through the specimen. Thus knowing the sample dimensions and R H, the magnetic field can be determined by measuring I and V H.

Page 11 of 29


Sem: odd/even

3. From the below table of R/Rg and T draw a graph (T vs R/R g ). Use this graph to determine the temperature; T of the filament from the experimentally observed value of R/Rg obtained in step 1 and 2. Calculate 1/T.

T  150.141  634.007( R R g )

-------- (1)

4. Draw a graph of ln vs. 1/T.

5. Determine h from the slope of this curve and from the equation

Where, -10 -23  = 6000 X 10 m, k = 1.38 X 10 J/K, c = 3 X 10 8 m/s Standard value of Plank constant is 6.6 X 10 -34 Js

Discussions: 1. The setup should be initialized as follows: the light source should be turned away from the photocell and the ammeter dial adjusted so that it reads 0 for zero input voltage in presence of laboratory light. 2. The source light is then turned towards the photocell and the value of I for V = 0 recorded. 3. For small increments of V, I is recorded. After I saturates (or even before that, as all we are interested in is the stopping potential), we stop taking readings for increasing V. V is returned to the value V=0, the polarity of the connections to the photocell reversed by switching the wires, and V is again changed in small steps until I=0. The value of V obtained now is the stopping potential V S.


1. Define Photoelectric effect? When light falls on metal surface, an electron is emitted from a metal if the energy of the photon is greater than the work function of the metal.

Page 13 of 29


Sem: odd/even

2. What is Reverse Photoelectric effect? If an electron of sufficient voltage is passed across a material then a photon is emitted whose energy is equivalent to the work function of that material. The voltage at which this effect observed is the ‘turn on voltage’. In case of LED reverse photoelectric effect works. 3. Can we observe reverse photoelectric with Metal surface? This effect is not normally observed in metals and other typical substances because the photons emitted are usually outside the range of visible light, usually somewhere in the infrared Range. 4. What is the full form of LED? Full form of LED is Light Emitting Diode. 5. What is Planck’s constant (h)? What is the standard value of h. It is a fundamental physical constant. It gives the order of energy exchange in case of quantum mechanical action. It is the ratio of energy of a photon of its frequency. h= 6.6x10 -34 Js. 6. What is LED? A light-emitting diode (LED) is a semiconductor device that emits visible light when an electric current passes through it. 7. What is photo voltic cell? It is a p-n junction which can convert light energy into electrical energy. 8. In which factor the stopping potential of a particular colour of light depends? The stopping potential of a particular colour of light depends on its frequency and the stopping potential is directly proportional to its frequency. 9. In which factor the maximum value of the the photo current depends? The maximum value of the photo current depends on the intensity of the incident light.The photo current is directly proportional to the intensity of the incident light. 10. Why the green light has large stopping potential than red light? The energy of green wavelength is more than that of red. So the frequency of green is more than red. Since stopping potential is directly proportional to the frequency of the particular colour incident light. Thus due to high value of frequency green has large stopping potential than red. Page 14 of 29


Sem: odd/even

1. In taking readings between V f and I f every reading should be taken after getting steady state or the time difference between each reading should be approximately 3 to 4 minutes. 2. In plotting the graph between log P and log T the experimental point at the lower end of temperature state lies outside the straight line graph, since corrections due to heat power loss are neglected. At high temperature these losses are not negligible and so in fig.-2 the straight line is drawn through such points. 3. It should be necessary to determine the slope of the straight line as accurately as possible to verify the Stefan’s law within experimental errors.


1. What are meant by black body? Black body is the one which absorbs all radiation which incident on it. On heating black body stats emitting radiations called black body radiation which are independent of nature of body and depends on the temperature of black body. 2. Why black body is called as black body? Due to the fact that whatever may be the color of incident radiation the body appears black. 3. How does this law differ from Newton’s law of cooling ? Newton’s law of cooling is applicable only when the difference of temperature between the body and the surroundings is very small. This law , in fact, can be deduced from Stefan‟s law assuming the temperature difference as small. 4. Can the value of Stefan’ s constant be determined from this method ? Yes, taking the value of  as 4, the value of C can be obtained from E = σ (T 4 – T 4 0) or from the value of the intercept of the graph also, the value of C can be obtained from P = log C + nlog T, if the radiating body is not assumed as a black body. Assuming this to be a black body, this value of C so obtained will correspond to the Stefan‟s Law. 5. Is this method superior to the conventional thermal method ? This method id though not very precise and accurate. However it has some points of advantages. The bulb is never truly a black body and at steady state, the power radiated is never equal to V . I exactly. The working theory in this method is to some extent approximate, nevertheless, the method is very simple and the accessories are easy to procure.

Page 17 of 29


Sem: odd/even

It gives an approximate idea about Stefan‟s Law, Stefan‟s constant and the verification of the law. 6. State Stefan’ s law. It states that total radiant energy emitted per second from the unit surface area of a perfectly black body is proportional to the fourth power of its absolute temperature. 7. What is Stefan’ s constant ? If E denotes the total energy emitted per second from unit surface area of a black body then by Stefan‟s law, we have E = T 4 . 8. Do you know about Kirchoff‟s law of black body radiat ion? It states that at any temperature, the ratio of emissive power of the absorptive power of a given wavelength is same for all bodies. 9. What is emissive power and absorptive power? Emissive power: At a particular temperature and for a given wavelength, it is defined as the radiant energy emitted per unit time per unit surface area of the vody within a unit wavelength range. Absorptive power : At a particular temperature and for a given wavelength, it is defined as the ratio of the radiant energy absorbed per second per unit surface area of the body to the total energy falling per unit time on the same area. 10. State Wein’ s law? The wavelength corresponding to the maximum energy is inversely proportional to the absolute temperature. 11. Explain the distribution of black body radiation spectrum? The amount of energy radiated by a black body id not uniformly distributed over all the wavelength emitted by the body but it is maximum for a particular wavelength. The value of wavelength is different for different temperature and varies with temperature. 12. Define solar constant? Solar constant defined as the amount of energy received / sec/unit area of a perfect black surface at a mean distance of the earth from the sun in the absence of earth‟s atmosphere, the surface being perpendicular to the direction of sun rays. (1.34 KW/m).

Page 18 of 29


Result: Hance the Rydbarg constant

Sem: odd/even

R H



1





1

 1 1     n  n   2

1

= …………… cm – 1.

2

2

Discussion:

1. Before performing the experiment, the spectrometer should be adjusted. 2. Grating should not be touched by fingers. 3. Grating should be set normal to the incident light. 4. Both verniers should be read. 5. While taking observations, telescope and prism table should be kept fixed. Verification of R H: (Don’t write it in lab. Note Book)

R H=22me4/ch3 Where,

m = mass of an electron e = electronic charge h = Planck’s constant c = speed of light in vacuum

= 9.106 x 10-28 gm = 4.8025x 10-10 e.s.u = 6.625 x 10 -27 ergs-sec = 2.998 x 1010cm/sec.


1. What is diffraction? The process by which a beam of light or other s ystem of waves is spread out as a result of passing through a narrow aperture or across an edge, typically accompanied by interference between the wave forms produced. 2. What is diffraction grating ? It is an optically flat glass plate on which large number of equidistant parallel lines are ruled by a fine diamond pen. 3. Mention the two types of diffraction? i) Fresnel diffraction ii)Fraunhofer diffraction 4. What is the type of diffraction in the diffraction grating experiment?

Page 21 of 29


Sem: odd/even

Fraunhofer diffraction is involved because the source and the sc reen are effectively at infinite distance. 5. What is grating element? It is the distance between the centers of any two successive ruled lines or transparent stripes. 6. What is the difference between prism and grating spectrum? In grating spectrum violet color is least deviated and red color is most deviated but in prism the reverse is true. 7. When will the even order spectra disappear? They will disappear if the size of opaque lines and transparent stripes is made equal. 8. Why does red color deviate the most in case of grating? This is so because in case of grating sin θ=n λ/(e+d) i.e angle of diffraction is proportional to the wavelength and the wavelength of red is maximum. 9. What gives a more intense spectrum – prism or grating? A prism gives more intense spectrum because in prism entire light is concentrated into one spectrum while in the case of grating light is distributed in the grating spectra of different orders. 10. Why is light incident on the side of grating which has no rulings? To avoid refraction of diffracted light. 11. Define dispersion of light. The process of splitting of white light into it’s constituent colors is called dispersion of light. 12. Describe essential parts of spectrometer . Collimator, prism table, telescope. 13. Why do we need two vernier scales? To remove the error in reading due to not coinciding the axis of pris m table and telescope. 14. Name two types of spectra. Emission spectra, Absorption spectra

Page 22 of 29


Sem: odd/even

15. What is Rydberg constant ? It is a physical constant relating to atomic spectra in the science of spectroscopy. This constant R is named after Swedish physicisi Johannes Rydberg. The value of Rydberg constant is given by 1.097 X 10 7 m-1 16. What does Rydberg constant represents? This constant represents the limiting value of the highest wave number of any photon that can be emitted from the hydrogen atom or alternatively the wave number of the photon capable of ionizing the hydrogen atom from its ground state. 17. What is wave number? It is the inverse of the wavelength of a photon. Sometimes inverse of wavelength is multiplied by 2 . 18. What is spectrometer? It is an instrument used for analyzing the spectrum of a source of light.

Page 23 of 29


Sem: odd/even

6. Draw graph showing the variation of current as a function of accelerating voltage. 7. Turn the ‘manual/ auto’ switch to auto. 8. Connect the instrument’s Y, G, X sockets to the corresponding ports of the oscilloscope. Set the oscilloscope to x-y mode and the trigger to external x. 9. Adjust the shift and the gain switches to obtain a clear waveform. Apply the maximum scan range through the instrument. To measure the excitation potential from the CRO - NA No. Distance between the Gain factor Excitation potential of peaks (volts/div) g ng (eV) obs (no. of divisions) n

Average excitation potential (eV)

10. Measure the average horizontal distance between the pe aks. This would give the value of Argon atom’s first excitation potential in eV. To measure the excitation potential from the graph No. of Distance between peaks Average distance Obs to peaks (V) between peaks to peaks (V) 1 2 – 1 = 2 3 – 2 = 3 4 – 3 = 4 5 – 4 = 5 6 – 5 = 6 7 – 6

Average excitation potential (eV)

Viva voce questions

1. What is the use of setting V G1K at 1.5 volts? It is used to accelerate the electrons emitted from cathode towards anode. 2. What is the use of setting V G2A at 7.5 volts? This grid acts as a sort of retarding potential for the electrons if the electrons have more than 7.5 volts then they will pass and reach anode and we will get current otherwise they will return and then re-accelerate towards anode. 3. What is used to fill the glass tube? Name its suitable replacement.

Page 25 of 29


Sem: odd/even

It is filled with mercury vapors because they don’t react with free electrons any inert gas can be its suitable example.e.g-neon,argon etc. 4. What kind of collision occurs in between electrons and mercury vapors? Both elastic and inelastic collision occurs.During elastic collision electrons gain energy while during inelastic collision they give all their e nergy to mercury vapors. 5. How do we prove the existence o f discreet energy levels? As long as the collision between electrons and mercury vapors is elastic the electron will gain energy but after a certain time it reaches a thresh hold and it gains enough energy to ionize Hg vapors by inelastic collision.Thus,it loses its energy and jumps in the immediate next energy level i.e s,p,d,f. When plotting graph it is seen that after an inelastic collision the graph falls rather abruptly but it should also be noted that every time the graph falls its value will always be greater than the previous fall this along with the rather haphazard way the graph gets plotted contributes to the idea that electrons exist at different energy levels. 6. What do you man by Discreet energy levels? A quantum mechanical system or particle that is bound — that is, confined spatially — can only take on certain discrete values of energy. This contrasts with classical particles, which can have any energy. These discrete values are called energy levels. It means that atoms can have only certain definite amount of energy state as light is emitted and absorbed by atoms. 7. What is Bohr’s theory? A.Bohr’s theory states that electrons orbiting nucleus can exist only in certain energy levels. A jump from one energy level to another is usually accompanied by absorption or emission of a quantum of radiation.

Page 26 of 29


Sem: odd/even

5. Insert each of the material inside the parallel plate capacitor. Take Vs(pp) = 20 V. For each frequency find out the Vo(p). Material 1.

Frequency (kHz) F1 F2 F3 F1 F2 F3 F1 F2 F3 F1 F2 F3

2.

3.

4.

6. Find out C T and find out C T



1

Vo(p) (volt)

from the formula

V S ( PP ). .T

8 RiV 0 ( PP )

,



1



C T C 0


1. What is a capacitor? Capacitor is a device used to store charge. 2. What is meant by capacitance? Ability to store charge in a capacitor is called capacitance and it is measured in farad, F. 3. What is the relation between Q, C & V? Q=CV, Q, charge stored(coulomb), C capacitance(farad), V(voltage). 4. Define one farad. It is the amount of charge required to raise the potential by 1 volt. 5. What is dielectric? Dielectric is an insulator which is used to increase the capacitance of the capacitor. 6. Classify dielectrics? Page 28 of 29


Sem: odd/even

It is classified into polar and non-polar dielectrics. 7. The dielectric constant depends on what factors? It depends on frequency, material and temperature. 8. State coulombs law of charges. It states that force of attraction or repulsion between two charges is directly proportional to the product of the charges and inversely proportional to the square of the distance between them. 9. What is an electric field? It is the region of space in which a charged body experience force. It is measured in volt per meter. 10. Describe the phasor diagram for pure resistor, I & V will be in phase, phasor I leads V by 90 degree bur in case of a inductor V lead I by 90 degree. 11. When does the body get charged? When a body rubbed with another body it gets charged due to loss or gain of electron. 12. What are electric lines of forces? It is the path travelled by a unit positive charge from positive charge to negative charge.

Page 29 of 29

Physics Viva

Recommend Documents