CH 13 THERMAL PHYSICS MCQS SOLUTION AND EXPLANATION CLASS 12 PHYSICS NEW BOOK PHYSICS INN

 

Topic: Brownian Motion 

Answer Key with Hints/Solutions

1. (b) Sol: Jean Perrin verified Einstein's theoretical explanation in 1908.
2. (b) Sol: Einstein published the theoretical explanation of Brownian motion in 1905.
3. (b) Sol: Irregular collisions by surrounding fluid molecules from random directions exert unequal instantaneous forces on the particle.
4. (a) Sol: Speed gained from momentum transfer during collision is inversely proportional to particle mass ().
5. (b) Sol: Collisions between molecules result in the exchange/transfer of momentum and kinetic energy.
6. (b) Sol: Speed of Brownian motion is inversely proportional to fluid viscosity; glycerine has higher viscosity.
7. (b) Sol: Unseen, fast-moving air molecules continuously collide with suspended dust particles from all sides.
8. (b) Sol: Viscosity describes internal friction and resistance to fluid flow.
9. (c) Sol: In the absence of external forces during free flight, a molecule travels in a straight line between successive collisions.
10. (a) Sol: Lighter/smaller particles acquire higher speeds upon collision with gas molecules.
11. (c) Sol: Jean Perrin received the Nobel Prize in Physics in 1926.
12. (b) Sol: Brownian speed is inversely proportional to fluid viscosity, so lower viscosity allows faster motion.
13. (a) Sol: The visible chaotic movement of larger suspended particles reflects the unseen thermal motion of fluid molecules.
14. (b) Sol: Brownian motion illustrates random, chaotic molecular dynamics.
15. (b) Sol: Since particle displacement vectors are randomly oriented in all 3D directions, their vector sum averages to zero over time.

Topic: Kinetic Theory of Gases

Answer Key with Hints/Solutions

1.      (b) Sol: Kinetic theory assumes collisions between gas particles and container walls are perfectly elastic.

2.    (c) Sol: Assumption 5 states that average kinetic energy .

3.      (b) Sol: Assumption 1 states that empty space is vast relative to particle sizes, making individual particle volume negligible.

4.      (c) Sol: Gas pressure arises from particles continuously colliding with container walls and exerting force per unit area.

5.      (c) Sol: Assumption 4 specifies no attractive or repulsive forces exist between particles except during collisions.

6.      (b) Sol: By definition, the mean free path is the average distance covered between significant collisions.

7.    (c) Sol: Since , doubling the absolute temperature directly doubles the average kinetic energy.

8.      (b) Sol: Heating raises temperature, increasing molecular speeds and wall collisions, driving expansion.

9.      (b) Sol: The margin note states a small cube of air can contain up to  molecules.

10.   (b) Sol: Kinetic theory defines macroscopic transport properties including viscosity, thermal conductivity, and diffusivity.

11.  (b) Sol: Since , as , average molecular kinetic energy drops to zero.

12.   (a) Sol: Increased temperature leads to faster molecular speeds, increasing force and rate of impacts on tire walls.

13.   (b) Sol: Kinetic molecular theory applies fundamental motion assumptions to model ideal gases.

14.   (a) Sol: Continuous, rapid, and random collisions constantly alter the directional paths of molecules.

15.   (c) Sol: Average translational kinetic energy per molecule depends solely on absolute temperature () regardless of gas species.

 

Topic: Pressure in a Gas and Derivation of    

Answer Key with Hints/Solutions

1.      (c) Sol:  along the x-axis.

2.    (b) Sol: Distance traveled between consecutive collisions with the same wall is , so .

3.      (b) Sol: Random motion means molecules move equally in opposite directions, making .

4.      (c) Sol: Since , .

5.      (a) Sol: Substituting density  into  gives .

6.      (b) Sol: .

7.      (b) Sol: .

8.      (c) Sol: From , pressure ; halving volume doubles pressure.

9.      (b) Sol: .

10.   (b) Sol: ; multiplying  by  divides  by .

11.    (b) Sol: Gas constant per molecule is .

12.  (b) Sol: ; multiplying  by  doubles speed ().

13.   (b) Sol: .

14.  (a) Sol: .

15.   (c) Sol: Standard physical constant .

 

Topic: Average Translational Kinetic Energy of a Gas

Answer Key with Hints/Solutions

1.      (b) Sol: Temperature is a direct measure of average translational kinetic energy ().

2.    (b) Sol: Average translational kinetic energy per molecule is given by .

3.      (b) Sol: Boltzmann constant in electron-volts is .

4.    (a) Sol:  is energy gained across a potential difference of .

5.      (b) Sol: The linear proportionality constant  links temperature directly to molecular kinetic energy.

6.    (c) Sol: Kinetic energy is directly proportional to absolute temperature (), so tripling  triples kinetic energy.

7.    (b) Sol: Quantities like  are extremely small in Joules (), making electron-volts more convenient.

8.    (a) Sol: .

9.      (a) Sol: Trapped moisture turns to steam, building pressure that ruptures the kernel cover.

10.   (b) Sol: Pumping air increases molecule count, raising collision frequency and pressure against walls.

11.  (c) Sol: .

12.  (b) Sol: .

13.  (c) Sol: .

14.   (b) Sol: Pressure relation is .

15.  (c) Sol: By definition, . 

 

Topic: Kinetic Theory and Statistical Physics

Answer Key with Hints/Solutions

1.      (b) Sol: Statistical physics uses probability theory to analyze systems containing large numbers of particles.

2.      (b) Sol: James Clerk Maxwell derived the speed distribution equation for molecular speeds in 1860.

3.      (b) Sol: The Boltzmann distribution law states .

4.      (b) Sol:  is explicitly defined as the Boltzmann factor.

5.      (b) Sol: Because of the negative exponential dependence on energy, lower energy states retain larger populations.

6.      (b) Sol: Huge particle counts undergoing continuous collisions require probabilistic/statistical mechanics rather than tracking individual particles.

7.    (a) Sol: As , , making , so higher levels become populated up to equality.

8.    (c) Sol: Statistical kinetic theory gives average thermal energy .

9.    (b) Sol: As , , meaning higher energy levels become unpopulated.

10.   (c) Sol: Margin text notes that experimental verification occurred about 60 years after Maxwell's 1860 prediction.

11.   (b) Sol: Text states that number density in a state is directly proportional to pressure.

12.   (b) Sol: Statistical equilibrium ensures that individual kinetic energy variations cancel out over  particles.

13.   (b) Sol: Section 13.5 states  is effectively the gas constant per molecule ().

14.  (b) Sol: Large positive exponent ratio makes , yielding negligible .

15.   (b) Sol: The Boltzmann kinetic equation models dynamic transport processes in multi-particle gas systems. 

 

Topic: Stellar Evolution

Answer Key with Hints/Solutions

1. (c) Sol: Stars originate from interstellar gas and dust clouds named nebulae or molecular clouds.
2. (b) Sol: Hydrostatic balance between pressure and gravity is given by .
3. (b) Sol: Gravitational contraction converts potential energy to kinetic energy (), increasing density, pressure, and temperature.
4. (b) Sol: The negative sign indicates that pressure decreases moving outward from the core while gravity acts inward.
5. (c) Sol: Stars with masses  end their lives in massive supernova explosions.
6. (b) Sol: Red dwarfs burn hydrogen extremely slowly due to low mass; their lifespan exceeds the current age of the universe.
7. (c) Sol: Highly compressed degenerate neutron matter provides outward neutron degeneracy pressure to halt gravitational collapse.
8. (c) Sol: Rapidly spinning, highly magnetized young neutron stars that emit directional radiation beams are called pulsars.
9. (a) Sol: One parsec is an astronomical distance unit equal to approximately 3.26 light-years.
10. (b) Sol: The Sun formed approximately 4.6 billion years ago from a solar nebula.
11. (a) Sol: Fuel depletion breaks equilibrium; core contraction heats surrounding shells, expanding outer layers into a red giant.
12. (b) Sol: Compressing the stellar core squeezes magnetic field lines into a smaller area, greatly strengthening the field.
13. (c) Sol: Low-to-intermediate mass stars shed outer layers as planetary nebulae, leaving behind dense white dwarf cores.
14. (b) Sol: Hydrostatic balance requires inward self-gravity to match outward thermal gas pressure from core fusion.
15. (b) Sol: Massive star core nucleosynthesis fuses progressively heavier nuclei up to iron.

Topic: Our Sun

Answer Key with Hints/Solutions

1.      (c) Sol: The Sun was formed about 4.6 billion years ago.

2.      (b) Sol: The primordial cloud that collapsed under gravity to form our Sun is called the solar nebula.

3.      (b) Sol: The Sun is currently in its main sequence phase, converting hydrogen to helium.

4.      (c) Sol: Radiation is the primary mode of heat transfer from the Sun through vacuum.

5.      (b) Sol: High density and temperature at the central core initiate nuclear fusion.

6.      (b) Sol: Gravitational contraction converts potential energy into translational kinetic energy of gas particles, raising temperature.

7.      (b) Sol: Main sequence stars convert hydrogen into helium via nuclear fusion.

8.      (a) Sol: The Sun serves as Earth's natural primary source of thermal energy transferred via radiation.

9.    (a) Sol: The life stages are nebula  protostar  main sequence  red giant  planetary nebula  white dwarf.

10.   (b) Sol: Hydrostatic balance exists between inward gravitational pull and outward internal pressure generated by energy release.

11.   (b) Sol: Depletion of core hydrogen breaks equilibrium, causing outer layers to expand into a red giant.

12.   (a) Sol: Solar radiation directly provides the incoming radiative heat flux for Earth's surface and atmosphere.

13.  (a) Sol: .

14.   (c) Sol: .

15.   (c) Sol: Intermediate-mass stars like our Sun end their stellar evolution as white dwarfs.