100 M0ST IMPORTANT QUESTION ON JKSSB JE AND JKPSC AE EXAM 2019

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100 M0ST IMPROTANT QUESTION ON JKSSB JE AND JKPSC AE EXAM 2019

SEMICONDUCTORS – PN JUNCTION Theory Questions :-

1. What is Fermi level?

The maximum energy that an electron in a metal has at the absolute zero temperature is called the Fermi level of energy.

2. What is the basis for classifying a material as a conductor, semiconductor, or a dielectric? Define what is the conductivity of perfect dielectric?

Conductors possess high conductivity whereas the characteristic property of insulating materials (or dielectrics) is poor conductivity. Semiconductors occupy an intermediate position between conductors and insulators. Though there is no rigid line separating the conductors from semiconductors and semiconductors from insulators, but still according to resistivity the materials of resistivity of the order from 10-8 to 10-3 , 10-13 to 106 and 106 to 1018 ohm-meters may be classified as conductors, semiconductors and dielectrics respectively.

Another classification is based on temperature coefficient of resistivity. Metals have positive temperature coefficient of resistivity. Semiconductors have small negative temperature coefficient of resistivity and insulators have large negative temperature coefficient of resistivity.

3. Differentiate semiconductors, conductors and insulators on the basis of band gap.

The distinction between conductors, insulators and semiconductors is largely concerned with the relative width of the forbidden energy gaps in their energy band structures. There is a wide forbidden gap (more than 5eV) for insulators, narrow forbidden gap (about 1eV) in case of semiconductors and no forbidden gap in case of conductors.

4. Define what is the importance of valence shell and valence electrons?

The outermost shell of an atom is called valence shell and the electrons in this shell are called valence electrons. Formation of energy bands occur owing to overlapping of energy levels of these valence electrons in valence shells. With the decrease in interatomic distance between the atoms in a crystal, the energy levels of electrons in outermost shells of atoms overlap to form energy bands.

5. What is the forbidden energy gap? How does it occur? Define what is its magnitude for Ge and Si?

The energy gap between the valence band and conduction band is known as forbidden energy gap. It is a region in which no electron can stay as there is no allowed energy state. Magnitude of forbidden energy gap in germanium and silicon is 0.72 eV and 1.12 eV respectively at 300 K and 0.785 eV and 1.21 eV respectively at absolute zero temperature.

6. Is a hole a fundamental particle in an atom?

Hole is not a fundamental particle in an atom. Holes may be thought of as positive particles, and as such they move through an electric field in a direction opposite to that of electrons.

7. Define a hole in a semiconductor.

When an energy is supplied to a semiconductor a valence electron is lifted to a higher energy level. The departing electron leaves a vacancy in the valence band. The vacancy is called a hole. Thus, a vacancy left in the valence band because of lifting of an electron from the valence band to conduction band is known as a hole.

8. Define what is hole current?

The movement of the hole (positively charged vacancy in the valence band) from positive terminal of the supply to negative terminal through semiconductor constitutes hole current.

9. Define what is intrinsic semiconductor ?

An intrinsic semiconductor is one which is made of the semiconductor material in the extremely pure form (impurity content not exceeding one part in 100 million parts of semiconductors).

10. Why silicon and germanium are the two widely used semiconductor materials?

Because the energy required to release an electron from their valence band (i.e. to break their covalent bonds ) is very small (1.12eV for Si and 0.72eV for Ge).

11. Which of the two semiconductor materials Si or Ge has larger conductivity at room temperature? Why?

Since energy required in transferring electrons from valence band to conduction band is more in case of Si than that in case of germanium , the conductivity of Ge will be more than that of Si at room temperature.

12. Why does a pure semiconductor behave like an insulator at absolute zero temperature?

For a pure semiconductor at a temperature of absolute zero (-273.15oC)the valence band is usually full and there are may be no electron in the conduction band and it is difficult to provide additional energy required for lifting electron from valence band to conduction band by applying electric field. Hence the conductivity of a pure semiconductor at absolute zero temperature is zero and it behaves like an insulator.

13. Define what is the main factor for controlling the thermal generation and recombination?

Temperature, because with the increase in the temperature, concentrations of free electrons and holes increase and the rate of recombination is proportional to the product of concentration of free electrons and holes and also the rate of production of electron-hole pairs (thermal generation) increases with the rise in temperature.

14. Define mean life of a carrier.

The amount of time between the creation and disappearance of a free electron is called the life time. It varies from a few nanoseconds to several microseconds depending how perfect the crystal is and other factors.

15. In which bands do the movement of electrons and holes take place?

Free electrons move in valence band while holes in valence band.

16. Define what is the mechanism by which conduction takes place inside the semiconductor?

Conduction occurs in any given material when an applied electric field causes electrons to move in a desired direction within the material. This may be due to one or both of two processes, electron motion and hole transfer. In case of former process, free electrons in the conduction band move under the influence of the applied electric field. Hole transfer involves electrons which are still attached to the atoms i.e. those in valence band.

17. Define what do you mean by drift velocity and mobility of a free electron?

The average velocity of an electron is known as drift velocity whereas mobility of an electron is defined as the drift velocity per unit electric field.

18. Define mobility of a carrier. Show that the mobility constant of electron is larger than that of a hole.

Mobility is defined as the average particle drift velocity per unit electric field.

The mobility of electrons is more than that of holes because the probability of an electron having the energy required to move to an empty state n the conduction band is much greater than the probability of an electron having the energy required to move to the empty state in valence band. The mobility of electron is about double that of a hole.

19. Define diffusion current in a semiconductor.

The diffusion of charge carriers is a result of a gradient of carrier concentration (i.e., the difference of carrier concentration from one region to another). In this case concentrations of charge carriers (either electrons or holes ) tend to distribute themselves uniformly throughout the semiconductor crystal. This movement continues until all carriers are evenly distributed throughout the material. This type of movement of charge carriers is called diffusion current.

CURRENT ELECTRICITY Multiple Choice Questions :-

1. The S.I. unit of power is

(a) Henry

(b) coulomb

(d) watt-hour

Ans: c

2. Electric pressure is also called

(a) resistance

(b) power

(d) energy

Ans: c

3. The substances which have a large number of free electrons and offer a low resistance are called

(a) insulators

(b) inductors

(d) conductors

Ans: d

4. Out of the following which is not a poor conductor ?

(a) Cast iron

(b) Copper

(d) Tungsten

Ans: b

5. Out of the following which is an insulating material ?

(a) Copper

(b) Gold

(d) Paper

Ans: d

6. The property of a conductor due to which it passes current is called

(a) resistance

(b) reluctance

(d) inductance

Ans: c

7. Conductance is reciprocal of

(a) resistance

(b) inductance

(d) capacitance

Ans: a

8. The resistance of a conductor varies inversely as

(a) length

(6) area of cross-section

(d) resistivity

Ans: b

9. With rise in temperature the resistance of pure metals

(a) increases

(b) decreases

(d) remains constant

Ans: a

10. With rise in temperature the resistance of semi-conductors

(a) decreases

(b) increases

(d) remains constant

Ans: a

11. The resistance of a copper wire 200 m long is 21 Q. If its thickness (diameter) is 0.44 mm, its specific resistance is around

(a) 1.2 x 10~8 Q-m

(b) 1.4 x 10~8 Q-m

(d) 1.8 x 10?8 Q-m

Ans: c

12. Three resistances of 10 ohms, 15 ohms and 30 ohms are connected in parallel.

The total resistance of the combination is

(a) 5 ohms

(b) 10 ohms

(d) 55 ohms

Ans:

13. An instrument which detects electric current is known as

(a) voltmeter

(b) rheostat

(d) galvanometer

Ans: d

14. In a circuit a 33 Q resistor carries a current of 2 A. The voltage across the resistor is

(a) 33 V

(b) 66 v

(d) 132 V

Ans: b

15. A light bulb draws 300 mA when the voltage across it is 240 V. The resistance of the light bulb is

(a) 400 Q

(b) 600 Q

(d) 1000 Q

Ans: c

16. The resistance of a parallel circuit consisting of two branches is 12 ohms. If the resistance of one branch is 18 ohms, what is the resistance of the other ?

(a) 18 Q

(b) 36 Q

(d) 64 Q

Ans: b

17. Four wires of same material, the same cross-sectional area and the same length when connected in parallel give a resistance of 0.25 Q. If the same four wires are connected is series the effective resistance will be

(a) 1 Q

(b) 2 Q

(d) 4 Q

Ans: d

18. A current of 16 amperes divides between two branches in parallel of resistances 8 ohms and 12 ohms respectively. The current in each branch is

(a) 6.4 A, 6.9 A

(b) 6.4 A, 9.6 A

(d) 4.6 A, 9.6 A

Ans: b

19. Current velocity through a copper conductor is

(a) the same as propagation velocity of electric energy

(b) independent of current strength

(d) nearly 3 x 108 m/s

Ans: c

20. Which of the following material has nearly zero temperature co-efficient of resistance?

(a) Manganin

(b) Porcelain

(d) Copper

Ans: a

CABLES Objective Type Questions :-

1. The insulating material for a cable should have

(a) low cost

(b) high dielectric strength

(d) all of the above

Ans: d

2. Which of the following protects a cable against mechanical injury ?

(a) Bedding

(b) Sheath

(d) None of the above

Ans: c

3. Which of the following insulation is used in cables ?

(a) Varnished cambric

(b) Rubber

(d) Any of the above

Ans: d

4. Empire tape is

(a) varnished cambric

(b) vulcanised rubber

(d) none of the above

Ans: a

5. The thickness of the layer of insulation on the conductor, in cables, depends upon

(a) reactive power

(b) power factor

(d) current carrying capacity

Ans: c

6. The bedding on a cable consists of

(a) hessian cloth

(b) jute

(d) none of the above

Ans: c

7. The insulating material for cables should

(a) be acid proof

(b) be non-inflammable

(d) have all above properties

Ans: d

8. In a cable immediately above metallic sheath _____ is provided.

(a) earthing connection

(b) bedding

(d) none of the above

Ans: b

9. The current carrying capacity of cables in D.C. is more thanthat in A.C. mainly due to

(a) absence of harmonics

(b) non-existence of any stability limit

(d) absence of ripples

(e) none of the above

Ans: c

10. In case of three core flexible cable the colour of the neutral is

(a) blue

(b) black

(d) none of the above

Ans: a

11. cables are used for 132 kV lines.

(a) High tension

(b) Super tension

(d) Extra super voltage

Ans: d

12. Conduit pipes are normally used to protect _____ cables.

(a) unsheathed cables

(b) armoured

(d) all of the above

Ans: a

13. The minimum dielectric stress in a cable is at

(a) armour

(b) bedding

(d) lead sheath

Ans: d

14. In single core cables armouring is not done to

(a) avoid excessive sheath losses

(b) make it flexible

(d) none of the above

Ans: a

15. Dielectric strength of rubber is around

(a) 5 kV/mm

(b) 15 kV/mm

(d) 200 kV/mm

Ans: c

16. Low tension cables are generally used up to

(a) 200 V

(b) 500 V

(d) 1000 V

Ans: d

17. In a cable, the maximum stress under operating conditions is at

(a) insulation layer

(b) sheath

(d) conductor surface

Ans: d

18. High tension cables are generally used up to

(a) 11kV

(b) 33kV

(d) 132 kV

Ans: a

19. The surge resistance of cable is

(a) 5 ohms

(b) 20 ohms

(d) 100 ohms

Ans: c

20. PVC stands for

(a) polyvinyl chloride

(b) post varnish conductor

(d) positive voltage conductor

(e) none of the above

Ans: a

LOGIC CIRCUITS Questions :-

1. What is a combinational circuit?

In a combinational circuit, the output depends upon present input(s) only i.e, not dependant on the previous input(s). The combinational circuit has no memory element. It consists of logic gates only.

2. Write two characteristics of combinational circuits.

The two characteristics of combinational circuits are:

In combinational circuits, the output exists as long as the input exists.

A combinational circuit will always respond in the same fashion to the input function, when we apply signal to the input terminal of the combinational logic circuit.

3. What is a half-adder?

A logic circuit, that can add two 1-bit numbers and produce outputs for sum and carry, is called a half-adder.

4. What is a full-adder?

A binary adder, which can add two 1-bit binary numbers along with a carry bit and produces outputs for sum and carry is called a full-adder.

LOGIC CIRCUITS Questions with Answers ::

5. What is a flip-flop?

A flip-flop is a basic memory element that is made of an assembly of logic gates and is used to store 1-bit of information.

6. Explain what is a latch?

It is a D-type of flip-flop and stores one bit of data.

7. Explain what is an excitation table?

Excitation table gives an information about Explain what should be the flip-flop inputs if the outputs are specified before and after the clock pulses.

8. Explain what is a state table?

State table consists of complete information about present state, next state, and outputs of a sequential circuit.

DIGITAL ELECTRONICS Questions :-

1.In which of the following base systems is 123 not a valid number?

(a) Base 10

(b) Base 16

(c)Base8

(d) Base 3

Ans:d

2. Storage of 1 KB means the following number of bytes

(a) 1000

(b)964

(d) 1064

Ans:c

3. What is the octal equivalent of the binary number:

10111101

(a)675

(b)275

(d) 573.

Ans:b

4. Pick out the CORRECT statement:

(a) In a positional number system, each symbol represents the same value irrespective of its position

(b) The highest symbol in a position number system as a value equal to the number of symbols in the system

(d) Each hexadecimal digit can be represented as a sequence of three binary symbols.

Ans:c

5.The binary code of (21.125)10 is

(a) 10101.001

(b) 10100.001

(d) 10100.111.

Ans:a

6.A NAND gate is called a universal logic element because

(a) it is used by everybody

(b) any logic function can be realized by NAND gates alone

(d) many digital computers use NAND gates.

Ans:b

7. Digital computers are more widely used as compared to analog computers, because they are

(a) less expensive

(b) always more accurate and faster

(d) easier to maintain.

Ans:c

8. Most of the digital computers do not have floating point hardware because

(a) floating point hardware is costly

(b) it is slower than software

(d) of no specific reason.

Ans:a

9. The number 1000 would appear just immediately after

(a) FFFF (hex)

(b) 1111 (binary)

(d) All of the above.

Ans:d

10. (1(10101)2 is

(a) (37)10

(b) ( 69)10

(d) — (5)10

Ans:a

11. The number of Boolean functions that can be generated by n variables is equal to

(a) 2n

(b) 22 n

(d) — 2n

Ans:b

12. Consider the representation of six-bit numbers by two’s complement, one’s complement, or by sign and magnitude: In which representation is there overflow from the addition of the integers 011000 and 011000?

(a) Two’s complement only

(b) Sign and magnitude and one’s complement only

(d) All three representations.

Ans:d

13. A hexadecimal odometer displays F 52 F. The next reading will be

(a)F52E

(b)G52F

(c)F53F

(d)F53O.

Ans:d

14. Positive logic in a logic circuit is one in which

(a) logic 0 and 1 are represented by 0 and positive voltage respectively

(b) logic 0 and, -1 are represented by negative and positive voltages respectively

(d) logic 0 voltage level is lower than logic 1 voltage level.

Ans:d

15. Which of the following gate is a two-level logic gate

(a) OR gate

(b) NAND gate

(d) NOT gate.

Ans:c

DIGITAL ELECTRONICS Interview Questions ::

16. Among the logic families, the family which can be used at very high frequency greater than 100 MHz in a 4 bit synchronous counter is

(a) TTLAS

(b) CMOS

(c)ECL

(d)TTLLS

Ans:c

17. An AND gate will function as OR if

(a) all the inputs to the gates are “1”

(b) all the inputs are ‘0’

(d) all the inputs and outputs are complemented.

Ans:d

18. An OR gate has 6 inputs. The number of input words in its truth table are

(a)6

(b)32

(d) 128

Ans:c

19. A debouncing circuit is

(a) an astable MV

(b) a bistable MV

(d) a monostable MV.

Ans:c

20. NAND. gates are preferred over others because these

(a) have lower fabrication area

(b) can be used to make any gate

(d) provide maximum density in a chip.

Ans:b

MAGNETISM and ELECTROMAGNETISM Questions :-

1. Tesla is a unit of

(a) field strength

(b) inductance

(d) flux

Ans: c

2. A permeable substance is one

(a) which is a good conductor

(6) which is a bad conductor

(d) through which the magnetic lines of force can pass very easily

Ans: d

3. The materials having low retentivity are suitable for making

(a) weak magnets

(b) temporary magnets

(d) none of the above

Ans: b

4. A magnetic field exists around

(a) iron

(b) copper

(d) moving charges

Ans: d

5. Ferrites are materials.

(a) paramagnetic

(b) diamagnetic

(d) none of the above

Ans: c

6. Air gap has_______eluctance as compared to iron or steel path

(a) little

(b) lower

(d) zero

Ans: b

7. The direction of magnetic lines of force is

(a) from south pole to north pole

(b) from north pole to south pole

(d) none of the above

Ans: b

8. Which of the following is a vector quantity ?

(a) Relative permeability

(b) Magnetic field intensity

(d) Magnetic potential

Ans: b

9. The two conductors of a transmission line carry equal current I in opposite directions. The force on each conductor is

(a) proportional to 7

(b) proportional to X

(d) inversely proportional to I

Ans: b

10. A material which is slightly repelled by a magnetic field is known as

(a) ferromagnetic material

(b) diamagnetic material

(d) conducting material

Ans: b

11. When an iron piece is placed in a magnetic field

(a) the magnetic lines of force will bend away from their usual paths in order to go away from the piece

(b) the magnetic lines of force will bend away from their usual paths in order to pass through the piece

(d) the iron piece will break

Ans: b

12. Fleming’s left hand rule is used to find

(a) direction of magnetic field due to current carrying conductor

(b) direction of flux in a solenoid

(d) polarity of a magnetic pole

Ans: c

13. The ratio of intensity of magnetisation to the magnetisation force is known as

(a) flux density

(b) susceptibility

(d) none of the above

Ans: b

14. Magnetising steel is normals difficult because

(a) it corrodes easily

(6) it has high permeability

(d) it has low permeability

Ans: d

15. The left hand rule correlates to

(a) current, induced e.m.f. and direction of force on a conductor

(b) magnetic field, electric field and direction of force on a conductor

(d) current, magnetic field and direction of force on a conductor

Ans: d

16. The unit of relative permeability is

(a) henry/metre

(b) henry

(d) it is dimensionless

Ans: d

17. A conductor of length L has current I passing through it, when it is placed parallel to a magnetic field. The force experienced by the conductor will be

(a) zero

(b) BLI

(d) BLI2

Ans: a

18. The force between two long parallel conductors is inversely proportional to

(a) radius of conductors

(b) current in one conductor

(d) distance between the conductors

Ans: d

19. Materials subjected to rapid reversal of magnetism should have

(a) large area oiB-H loop

(b) high permeability and low hysteresis loss

(d) high co-ercivity and low density

Ans: b

20. Indicate which of the following material does not retain magnetism permanently.

(a) Soft iron

(b) Stainless steel

(e) Hardened steel

(d) None of the above

Ans: a

100 M0ST IMPORTANT QUESTION ON JKSSB JE AND JKPSC AE EXAM 2019