# Electronic Science (Test 2)

## Ugc Net Entrance Exams : Science

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Q.1
In the case of metals the valence and conduction bands have
A. no overlap, energy gap =0
B. no overlap, energy gap is small
C. overlap, energy gap =0
D. no overlap, energy gap is large
Explaination / Solution:

The materials can be classified by the energy gap between their valence band and the conduction band. The valence band is the band consisting of the valence electron, and the conduction band remains empty. Conduction takes place when an electron jumps from valence band to conduction band and the gap between these two bands is forbidden energy gap. Wider the gap between the valence and conduction bands, higher the energy it requires for shifting an electron from valence band to the conduction band.

• In the case of conductors, this energy gap is absent or in other words conduction band, and valence band overlaps each other. Thus, electron requires minimum energy to jump from valence band. The typical examples of conductors are Silver, Copper, and Aluminium.
• In insulators, this gap is vast. Therefore, it requires a significant amount of energy to shift an electron from valence to conduction band. Thus, insulators are poor conductors of electricity. Mica and Ceramic are the well-known examples of insulation material.
• Semiconductors, on the other hand, have an energy gap which is in between that of conductors and insulators. This gap is typically more or less 1 eV, and thus, one electron requires energy more than conductors but less than insulators for shifting valence band to conduction band.

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Q.2
in p-type semiconductor the dopant is
A. always germanium
B. always silicon
C. donor
D. acceptor
Explaination / Solution:

The term p-type refers to the positive charge of the hole. In p-type semiconductors, holes are the majority carriers and electrons are the minority carriers. P-type semiconductors are created by doping an intrinsic semiconductor with acceptor impurities (or doping an n-type semiconductor).

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Q.3
When the conductivity of a semiconductor is only due to breaking of the covalent bonds, the semiconductor is called
A. extrinsic
B. intrinsic
C. acceptor
D. donor
Explaination / Solution:

A pure semiconductor is called intrinsic semiconductor, e.g., silicon, germanium. The presence of the mobile charge carriers is the intrinsic property of the material. At room temperature, some covalent bonds are broken and electrons are made free. The absence of electron in the covalent bond form hole. The electrical conduction is by means of mobile electrons and holes. Hole act as positive charge, because it can attract an electron. If some other bond is broken and the electron thus freed fills this hole(vacancy), it seems as though the hole is moving. Actually an electron is travelling in opposite direction. In a pure(intrinsic) semiconductor, the number of holes is equal to the number of free electrons.

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Q.4
An N-type Ge is obtained on doping the Ge- crystal with
A. gold
B. aluminum
C. phosphorus
D. boron
Explaination / Solution:

The addition of pentavalent impurities such as antimony, arsenic or phosphorous contributes free electrons, greatly increasing the conductivity of the intrinsic semiconductor.

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Q.5
The number of junctions in a transistor is/are
A. three
B. four
C. two
D. one
Explaination / Solution: A transistor consisting of two p-n junctions combined to form either an n-p-n or a p-n-p transistor, having the three electrodes, the emitter, base, and collector

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Q.6
Which of the following statements about bar magnets is correct ?
A. North Pole atrracts North Pole.
B. We can isolate the north and south pole of a bar magnet.
C. South Pole atrracts South Pole.
D. We cannot isolate the north and south pole of a bar magnet.
Explaination / Solution:

As magnetic monopole does not exist. If we split the bar magnet into two pieces each part will have its own north and south pole.

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Q.7
magnetic declination and dip refer to
A. angle between longitudinal magnetic axis and latitdinal geographic axis,angle up or down with respect to horizontal of a magnetic needle
B. angle between magnetic axis and geographic axis,angle up or down with respect to horizontal of a magnetic needle
C. angle between magnetic axis and geographic axis,angle up or down with respect to longitude of a magnetic needle
D. angle between latitude of magnetic axis and geographic axis,angle up or down with respect to horizontal of a magnetic needle
Explaination / Solution:
No Explaination.

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Q.8
For strong electromagnets use is made of
A. air core
B. ferromagnetic core
C. paramagnetic core
D. diamagnetic core
Explaination / Solution:
No Explaination.

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Q.9
A cube-shaped permanent magnet is made of a ferromagnetic material with a magnetization M of about The side length is 2 cm. Magnetic dipole moment of the magnet is.
A. 4Am2
B. 6Am2
C. 3Am2
D. 5Am2
Explaination / Solution:
No Explaination.

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Q.10
A closely wound solenoid of 800 turns and area of cross section  carries a current of 3.0 A. What is its associated magneticm oment?

A. 0.8 J/T
B. 0.5 J/T
C. 0.4 J/T
D. 0.6 J/T