The MS Solid State Physics Past Paper 2020 Punjab University is a document that contains the questions and answers of the Master of Science (MS) Solid State Physics examination held in 2020 by the University of the Punjab Lahore Pakistan.

## Suppose A = BC, where A has the dimension of \( \mathrm{\frac{L}{M}} \) and C has the dimension of \( \mathrm{\frac{L}{T}} \). Then B has the dimension of

- \( \mathrm{\frac{T}{M}} \)
- \( \mathrm{\frac{L^{2}}{TM}} \)
- \( \mathrm{\frac{TM}{L^{2}}} \)
- \( \mathrm{\frac{M}{L^{2}T}} \)

Correct Option: (A)

\[ \mathrm{A = BC} \]

\[ \mathrm{B = \frac{A}{C}} \]

\[ \mathrm{B = \frac{L/M}{L/T}} \]

\[ \mathrm{B = \frac{T}{M}} \]

## Why do raindrops fall with constant speed during the later stages of their descent?

- All the drops fall from the same height
- Air resistance just balances the force of gravity
- The gravitational force is the same for all drops
- The force of gravity is negligible for objects as small as raindrops

Correct Option: (B)

Raindrops fall with constant speed during the later stages of their descent because the force of air resistance balances the force of gravity.

## Which of the following bodies has the largest kinetic energy

- Mass 3M and speed V
- Mass M and speed 4V
- Mass 3M and speed 2V
- Mass 2M and speed 3V

Correct Option: (D)

The kinetic energy (KE) of an object is given by the formula

\[ \mathrm{KE = \frac{1}{2}mv^{2}} \]

\[ \mathrm{KE = \frac{1}{2}(3M)V^{2} = 1.5MV^{2}} \]

\[ \mathrm{KE = \frac{1}{2}(M)(4V)^{2} = 8MV^{2}} \]

\[ \mathrm{KE = \frac{1}{2}(3M)(2V)^{2} = 6MV^{2}} \]

\[ \mathrm{KE = \frac{1}{2}(2M)(3V)^{2} = 9MV^{2}} \]

Out of these options, mass 2M and speed 3V have the largest kinetic energy.

## When the temperature of a copper penny is increased by 100^{o}C and its diameter increases by 0.17%. The area of one of its faces increases by

- 0.17%
- 0.51%
- 0.34%
- 0.085%

Correct Option: (C)

\[ \mathrm{A = \pi r^{2}} \]

\[ \mathrm{A = \pi \left( \frac{d}{2} \right)^{2} = \frac{\pi d^{2}}{4}} \]

\[ \mathrm{\Delta A = \frac{\pi}{4} \ 2d \Delta d} \]

\[ \mathrm{\frac{\Delta A}{A} = \frac{\pi}{4} \ 2d \Delta d \ ÷ \ \frac{\pi d^{2}}{4}} \]

\[ \mathrm{\frac{\Delta A}{A} = 2\times \frac{\Delta d}{d}} \]

\[ \mathrm{\frac{\Delta A}{A} = 2\times 0.17 \%} \]

\[ \mathrm{\frac{\Delta A}{A} = 0.34 \%} \]

## Heat has the same units as

- Work
- Power
- Specific heat
- Temperature

Correct Option: (A)

Heat has the same units as work. Both heat and work are measured in joules (J).

## The torque exerted by an electric field on a dipole is

- Parallel to both the field and the dipole moment
- Perpendicular to both the field and the dipole moment
- Parallel to the dipole moment and perpendicular to the field
- Parallel to the field and perpendicular to the dipole moment

Correct Option: (B)

The torque exerted by an electric field on a dipole can be mathematically expressed as

\[ \mathrm{\overrightarrow{\tau} = \overrightarrow{p} \times \overrightarrow{E}} \]

The vector product of two vectors is perpendicular to both vectors. So the torque is perpendicular to both the electric field and the dipole moment.

## Lenz’s law is the consequence of the law of conservation of

- Energy
- Torque
- Current
- Momentum

Correct Option: (A)

Lenz’s law is the consequence of the law of conservation of energy.

## Each plate of a capacitor stores a charge of magnitude 1 mC when a 100 V potential difference is applied. The capacitance is

- 5 µF
- 10 µF
- 50 µF
- 100 µF

Correct Option: (B)

The capacitance of a capacitor is given as

\[ \mathrm{C = \frac{Q}{V}} \]

\[ \mathrm{C = \frac{1 \times 10^{-3}}{100}} \]

\[ \mathrm{C = 10^{-5} \ F} \]

\[ \mathrm{C = 10 \ \mu F} \]

## The units of magnetic dipole moment are

- ampere-meter
- ampere-meter
^{2} - ampere-meter
^{-1} - ampere-meter
^{-2}

Correct Option: (B)

A magnetic dipole moment is a vector quantity that describes the strength and orientation of a magnetic field.

It is defined as the product of the current flowing through a loop of wire and the area of the loop.

\[ \mathrm{\mu = I A} \]

Its SI unit is A-m^{2}.

## The most commonly used material in semiconductor devices is

- Silicon
- Graphene
- Germanium
- Gallium nitride

Correct Option: (A)

The most commonly used material in semiconductor devices is silicon because it is abundant, inexpensive, and has a bandgap that is ideal for many electronic applications.