materials and components section 3

8/18/2019 Materials and Components Section 3

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ELECTRONICS AND COMMUNICATIONS ENGINEERING:MATERIALS AND COMPONENTS sec. 3

1. At a very low temperature, a semiconductor

A. becomes a conductor

B.becomes an insulator

C. may become a conductor or an insulator

D. will remain a semiconductor becomes a conductor or an insulator

Semi conductor becomes insulator at very low temperature.

2. The colour code on a carbon resistor is red-red-black-silver. The value of resistor is

A. 22000

B. 2200

C. 22 ! "#

D.22 ! 10#

$. Some typical e%amples of macroscopic &uantities are

A. conductivity

B. 'r

C. dielectric constant

D.all of the above

(. The temperature coefficient of a thermistor

A. is positive

B.is ne)ative


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C.7

D. 8

9. :ermeability is analo)ous to

A. resistivity B. retentivity

C.conductivity

D. coercivity

10. The colour code on a carbon resistor is )reen-blue-yellow. The tolerance is

A. ! "#

B. ! 10#

C. ! 20#

D.! $0#

11. The resistivity of intrinsic semiconductors, at room temperature, is of the order of

A.1 -m

B. 1000 -m

C. 10/ -m

D. 10-$ -m

12. Assertion (A): 8erroma)netic materials possess ferroma)netic properties only when temperature

is less than parama)netic curie temperature.

Reason (R): 8or a ferroma)netic material for T ;


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D. A is false but is true

1$. 3n terms of classical representation of electrons

A.an atom is essentially empty

B. an atom is essentially full

C. an atom may be essentially empty or full dependin) on the material

D. an atom may be essentially empty or full dependin) on temperature

1(. To produce ma)netic flu%, mmf is necessary.

A.True

B. 8alse

1". 8or a polyatomic )as, the e&uation relatin) dielectric constant ∈r with molecular properties is

A.

B.

C.

D.

1/. The rela%ation time of electrons in a metal is of the order of

A. 10-/ secs

B. 10-10 secs

C.10-1( secs


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D. 10-20 secs

1. Assertion (A): 8or elemental dielectrics, : αe6i where N is number of atoms=m$, : is

polari*ation, αe is electronic polari*ation and 6 i is internal electric field.

Reason (R): 3n elemental dielectrics there are no permanent dipoles or ions.

A.oth A and are true and is correct e%planation of A

B. oth A and are true but is not correct e%planation of A

C. A is true but is false


3n elemental dielectrics :i :o 0.

1. The circumference of an electron orbit is e&ual to inte)er times the wavelen)th of electron.

A.True

B. 8alse

19. 3n a specimen of silicon steel, a field stren)th of 200 A=m causes a flu% density of 0." T. 3n a

specimen of cast iron, the same + would cause to be

A. 0." T

B.less than 0." T

C. more than 0." T

D. 0." T or more or less

ecause cast iron has lower permeability than silicon steel.

20. 3n a rare )as, the molecules of some atoms have an e%cess positive or ne)ative char)e. The

electric field tends to shift positive ions relative to ne)ative ions. This is known as ionic

polari*ation.

A.True

B. 8alse


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21.

The dielectric constant of ferroelectric materials can be e%pressed by the e&uation

22. A li&uid has permanent dipoles > p per unit volume and has been sub?ected to dc field for a lon)

time such that orientation polari*ation is P 0. At t 0, the field is suddenly removed, then

polari*ation

A. will remain constant at P 0

B. will become *ero at t 0

C.will delay to *ero with a time constant

D. either b or c

2$. Assertion (A): @rbital ma)netic moment of an electron in an atom is of the order of 1 ohr

ma)neton.

Reason (R): 1 ohr ma)neton 9.2 % 10 -2( A=m2.

A. oth A and are true and is correct e%planation of A

B.oth A and are true but is not correct e%planation of A




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2(. As per ohrs postulate, only those circular orbits are stable for which the an)ular momentum ise&ual to an inte)er times .

A.

True

B. 8alse

3t is a &uantum condition postulated by ohr.

2". 3nductors used at B.+.8. are made usin)

A. air core

B. cast iron cored

C. sheet steel cored

D.thin film techni&ue

Thin film techni&ue ensure low hysteresis and eddy current losses.

2/.

3f a capacitor 7 havin) selectivity factor C is scaled down in all its dimensions by a factor D

without any alterin) the dielectric material, the new capacitor will have capacitance and selectivity

as

Since the dimension of plates is scaled down by D, the area decreases by a factor of D2, the

distance between plates decreases by a factor of D.

Therefore, capacitance chan)es to .

The selectivity will not be affected.

2. The hysteresis loss is


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A.proportional to fre&uency

B. independent of fre&uency

C. proportional to Efre&uencyF2

D.

proportional to 1= f

2.

3f m, v , r are the mass, velocity and radius of orbit of electron and h is :lancks

constant, &uantum condition for electrons orbit is

29. Assertion (A): 8or a ma)netic material G E>r - 1F +, where G is ma)netic dipole moment=m$, +

is field intensity A=m and >r is relative permeability.

Reason (R): >r can have any value from 0 to hundreds of thousands.



C.A is true but is false


>r is always more than 1. 3t cannot be 0 or less than 1.

$0. As re)ards ∈0 and ∈r Eabsolute and relative permittivityF

A. both ∈0 and ∈r can be interpreted in terms of atomic properties of medium

B.only ∈r can be interpreted in terms of atomic properties of medium


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C. only ∈0 can be interpreted in terms of atomic properties

D. ∈r has no physical si)nificance

$1. Thermistor is used in

A. temperature measurement

B. temperature control

C. hi)h temperature alarm circuits of devices

D.all of the above

$2. ichrome is used for

A. overhead line wires

B.heater coils

C. lamp filaments

D. all of the above

The system is neutral and hence sum is *ero.

$(. The class of insulation of impre)nated paper is

A.A

B.


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C. 7 D. 8

$". Assertion (A): The resistance of a copper specimen chan)es by about 22# when temperature is

increased from room temperature to 00H7.

Reason (R): The physical properties of a material are determined mostly by the e%tent of

impurities.





The increase of resistance with temperature is )overned by temperature coefficient of

resistance.

$/. Assertion (A): 8or most metals, 8ermi ener)y 68 is less than 10 eB.

Reason (R): 8ermi level of a metal is )iven by 68 $./( % 10-19 EnF2=$ where n is number of free

electrons=m$ of metal.

A.oth A and are true and is correct e%planation of A




$. The capacitance of an insulated conductin) sphere of radius in vacuum is

A. 2π∈0

B.(π∈0

C. (π∈02


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D.

(π

(0. 6ven at room temperature, an intrinsic semiconductor has some holes. These holes are due to

A. electron

B. dopin)

C.thermal ener)y

D. free electrons

(1. 3n carbon resistor colour code, if the third band is of silver, it indicates

A. that three *eros are to be added to indicate the resistance value

B. multiplier is 0.01

C. multiplier is 0.1

D. tolerance is ! "#

(2. 3n a metal, the drift velocity of electrons in the presence of a electric field 6 is


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A.proportional to 6

B. proportional to 6

C. inversely proportional to 6

D. proportional to 62

Irift velocity is proportional to field stren)th.

($. At a temperature of about 1 D, aluminium is a

A. conductor B. insulator

C. photoconductor D.superconductor

(". 3f v is the velocity of electron, τc is the avera)e time between collisions, the mean free path J

v τc .

A.True

B. 8alse


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(/. Assertion (A): An electron havin) char)e - e and mass m and lyin) in an electric field 6 is

sub?ected to an acceleration - Ee =mF 6.

Reason (R): 3f there are n electrons=m$ movin) with velocity v x and each electron has a char)e

- e, the current density K - n ev x .





(. 4hen a pentavalent impurity is added, the semiconductor becomes

A. p type

B. n type

C. intrinsic

D. neutral

(. 3n body centred cubic structure, comers and centre of the cube are occupied by identical atoms.

A.True

B. 8alse

(9. 8or a multiplate capacitor havin) n plates, A as area of each plate, the capacitance is proportional

to

A. n A

B.

En - 1F A

C. En L 1F A

D. nA=2

"0. The orientation polari*ation is


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1. directly proportional to temperature

2. inversely proportional to temperature

$. inversely proportional to s&uare of permanent dipole moment

(. directly proportional to s&uare of permanent dipole moment.

4hich of the above are correct5

A. 1 and $ only

B. 2 and $ only

C. 1 and ( only

D.2 and ( only

materials and components section 3

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