CH 13 THERMAL PHYSICS PRACTICE TEST ANSWER KEY BY PHYSICS INN

ANSWER KEY & SOLUTIONS

VERSION 1 — ANSWER KEY

Q.1 — Multiple Choice Questions

1. (b) Random and irregular motion of particles/molecules in a fluid
2. (c) Mass of the particles
3. (b) High temperature and low pressure
4. (c) Perfectly elastic
5. (a)
6. (b)
7. (c) Molecules are tiny hard spheres undergoing inelastic collisions (In reality, collisions are perfectly elastic)
8. (c) Absolute temperature
9. (b)
10. (b) Statistical physics
11. (c) Nebulae or molecular clouds
12. (c) Hydrostatic equilibrium equation
13. (b) Main sequence

Q.2 — Short Question Solutions

1. Brownian motion: Random and irregular motion of small particles suspended in a fluid. Verified experimentally by Jean Perrin in 1908.
2. Assumptions: (i) Gas consists of a large number of tiny spherical particles far apart. (ii) Collisions are perfectly elastic.
3. Mean Free Path: The average distance a moving particle covers before undergoing a collision that significantly alters its direction or energy.
4. Yes: All ideal gases at the same temperature have the same average translational kinetic energy because , which depends only on temperature .
5. Relation: .
6. Pressure doubles: Since  at constant  (Boyle's Law), halving volume doubles the collision frequency on the container walls, doubling pressure.
7. : Energy gained by an electron when accelerated through . .
8. Formula: .
9. Pulsar: A rapidly rotating neutron star emitting beams of electromagnetic radiation.
10. Sun: Main sequence star, converting hydrogen into helium by nuclear fusion in its core.

VERSION 2 — ANSWER KEY

Q.1 — Multiple Choice Questions

1. (b) Lower viscosity leads to faster movement
2. (c)
3. (c) Be doubled
4. (a)
5. (b)
6. (c)
7. (c) The number of molecules increases, leading to more frequent wall collisions
8. (b) Decreases exponentially
9. (b) The star contracts under gravity
10. (c) Neutron star or black hole
11. (c) Degenerate gas
12. (c) Pulsars
13. (b) 1 : 4 (Explanation: )

Q.2 — Short Question Solutions

1. Force: Elastic collision produces momentum change . Rate of change of momentum gives force .
2. Yes: Average translational kinetic energy depends solely on temperature (). At thermal equilibrium,  is identical, so  is equal for both.
3. Pressure increase: Volume decrease reduces container surface area and increases molecular number density (), increasing wall collision rate per unit area.
4. No effect:  depends only on temperature  and mass , not on particle count .
5. Impossible: From , if  and  are constant,  must remain constant. Increasing  forces , , or both to increase.
6. Popcorn: Heat turns trapped moisture into high-pressure steam inside kernel; pressure ruptures outer shell.
7. Hydrostatic balance: The outward pressure gradient () balances the inward pull of gravity per unit volume ().
8. Red dwarfs: Consume hydrogen fuel extremely slowly due to low mass/luminosity; life cycle exceeds current age of universe ( billion years).
9. Magnetic contraction: Core collapse squeezes magnetic field lines close together, tremendously increasing magnetic field strength.
10. Factor 4: . If  and , . Temperature increases by factor of 4.

VERSION 3 — ANSWER KEY

Q.1 — Multiple Choice Questions

1. (c) Molecules bounce elastically with equal positive and negative directions, requiring velocity squaring before averaging
2. (b)  (Explanation: . New speed )
3. (c)  (Explanation: , . Ratio )
4. (c) Number of molecules per unit volume ()
5. (c)
6. (b) Molecular translational motion ceases as thermal energy
7. (c) 0 (Explanation: )
8. (b) Inert core contracts under gravity without fuel, triggering supernova collapse or white dwarf cooling
9. (b) Pressure decreases outward while gravity pulls inward
10. (c) 900 times
11. (b)  (Calculation: )
12. (b) 3.26 light years
13. (a) Increased by factor  (Explanation: )

Q.2 — Short Question Solutions

1. Derivation: Distance traveled between collisions with same wall . Time interval .
2. Derivation: . Total mass . Since density , substituting gives .
3. Reason: Sound waves rely on collective momentum transfer between adjacent molecules. Individual thermal molecule speeds in random directions () exceed macroscopically propagated pressure waves.
4. Zero K.E.: At , . Molecular translational motion ceases.
5. Ink dispersion: Water molecules undergo random thermal motion colliding with ink particles (Brownian motion), causing ink to diffuse throughout the liquid.
6. Boiling at room temp: By reducing ambient pressure using a vacuum pump until atmospheric pressure equals water's vapor pressure at room temperature, water boils without added heat.
7. Air Quality: Kinetic theory models collision rates and transport properties (diffusivity/viscosity) of pollutant molecules in atmospheric air.
8. Remnants: Low-mass stars  White Dwarf; massive stars ()  Neutron Star (if core collapse stops) or Black Hole (if collapse is total).
9. Calculation: . Since , internal pressure dominates and the star expands into a Red Giant.
10. Calculation: .