 JEE Mains Previous Years Questions with Solutions

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1

AIEEE 2003

A strip of copper and another of germanium are cooled from room temperature to $80K.$ The resistance of
A
each of these decreases
B
copper strip increases and that of germanium decreases
C
copper strip decreases and that of germanium increases
D
each of these increases

Explanation

The resistance of metal (like $Cu$) decreases with decrease in temperature whereas the resistance of a semi-conductor (like $Ge$) increases with decrease in temperature.
2

AIEEE 2002

The part of a transistor which is most heavily doped to produce large number of majority carriers is
A
emmiter
B
base
C
collector
D
can be any of the above three

Explanation

Emitter sends the majority charge carries towards the collector. Therefore emitter is most heavily doped.
3

AIEEE 2002

The energy band gap is maximum in
A
metals
B
superconductors
C
insulator
D
semiconductor

Explanation

The energy band gap is maximum in insulators.
4

AIEEE 2002

By increasing the temperature, the specific resistance of a conductor and a semiconductor
A
increases for both
B
decreases for both
C
increases, decreases
D
decreases, increases

Explanation

Specific resistance is resistivity which is given by

$\rho = {m \over {m{e^2}\,\tau }}$

where $n=no.$ of free electrons per unit volume and $\tau$ $=$ average relaxation time

For a conductor with rise in temperature $n$ increases and $\tau$ decreases. But decrease in $\tau$ is more dominant than increase in $n$ resulting an increase in the value of $\rho$

For a semiconductor with rise in temperature, $n$ increases and $\tau$ decreases. But the increase in $n$ is more dominant than decrease in $\tau$ resulting in decrease in the value of $\rho .$