Chapter 1: Thermal Physics — Important Short Questions
Overview
Ye 30 short questions poore chapter ka quick revision hain — Brownian motion, kinetic theory ki assumptions, gas pressure, rms speed, Boltzmann distribution, aur stellar evolution (stars ka irtiqa) sab topics cover ho rahe hain. Ye chhote answers hain isliye exam mein 1-2 line ka concept clear jawab dena kaafi hota hai, lambi calculation ki zaroorat nahi.
Brownian Motion Se Related Sawalat (Q1–5)
Q1 (Definition): Brownian motion ka matlab hai ke chhote (microscopic) particles jab kisi fluid (liquid ya gas) mein suspended hote hain, to wo musalsal, tez, random zigzag harkat karte hain.
Q2 (Kis ne discover kiya): Botanist Robert Brown ne 1827 mein sabse pehle observe kiya (pollen/dust particles paani mein random move kar rahe the). Baad mein Einstein ne 1905 mein mathematically sabit kiya ke ye motion fluid molecules ke takrane (bombardment) ki wajah se hoti hai.
Q3 (Speed ko affect karne wale factors): Do factors:
- Size/Mass: Chhote/halke particles zyada tez move karte hain, kyunke collisions ka unpar zyada asar hota hai
- Viscosity: Kam viscosity (kam internal friction) hone se particles azaad aur tez move kar sakte hain
Q4 (Physics mein ahmiyat): Brownian motion ne pehli dafa direct visual proof diya ke atoms aur molecules waqai mojood hain, aur matter ki kinetic nature ko confirm kiya.
Q5 (Bare particles Brownian motion kyun nahi dikhate): Bare particles ka mass aur inertia zyada hota hai. Unke bare surface area par jo random collisions hoti hain, wo cancel ho jaati hain (ek taraf se force doosri taraf se balance ho jaati hai), isliye net force itni kam hoti hai ke visible movement nahi hoti.
Kinetic Theory Ki Bunyadi Baatein (Q6–11)
Q6 (Core assumptions):
- Gas mein bohot saare chhote, identical, gol particles hote hain
- Molecules ka apna volume negligible hota hai container ke volume ke muqablay mein
- Molecules musalsal, tez aur random motion mein hote hain
Q7 (Collisions ke baare mein assumption): Kinetic theory maanti hai ke tamam collisions (molecules ke aapas mein, aur walls ke saath) perfectly elastic hoti hain — matlab total kinetic energy aur momentum dono conserve hote hain.
Q8 (Mean Free Path): Ye wo average distance hai jo ek gas molecule do successive collisions ke darmiyan tay karta hai.
Q9 (Real gas vs Ideal gas — intermolecular forces): Ideal gas mein molecules ke darmiyan koi bhi force nahi hoti (sirf collision ke waqt chhoti si force). Lekin real gases mein high pressure aur low temperature par halki intermolecular forces mojood hoti hain.
Q10 (Gas container ko kyun fill karti hai): Gas molecules har direction mein tezi se move karte hain aur unke darmiyan attraction forces negligible hoti hain, isliye wo azaad tareeqe se phail kar container ki boundaries tak pohanch jate hain.
Q11 (Pressure kaise paida hoti hai): Gas molecules jab container ki walls se musalsal takrate hain, to har collision mein unki momentum change hoti hai, jisse force paida hoti hai. Yehi force per unit area pressure kehlati hai.
rms Speed Aur Pressure Equation (Q12–16)
Q12 (rms speed ki definition aur formula): Sab molecules ki speeds ke squares ka average lekar, uska square root nikalne se rms speed milti hai:
vrms=3Pρv_{rms} = \sqrt{\frac{3P}{\rho}}
Q13 (Density double hone ka asar, pressure constant rehte): Formula se pata chalta hai ke vrms, density ke square root ke ulta proportional hai. Isliye density double hone se vrms √2 (≈1.41) guna kam ho jayegi.
Q14 (Fundamental pressure equation):
PV=13Nm⟨v2⟩PV = \frac{1}{3}Nm\langle v^2\rangle
Jahan P=pressure, V=volume, N=molecules ki tadaad, m=ek molecule ka mass, ⟨v²⟩=mean square velocity.
Q15 (Mean square velocity kyun use karte hain, simple average kyun nahi): Chunke molecules har direction mein random move karte hain, unki velocities ke +ve aur -ve signs hote hain, isliye simple average zero ho jata. Squaring karne se sab negative values positive ban jaati hain, aur ek meaningful average milta hai.
Temperature Aur Kinetic Energy Ka Taluq (Q16–18)
Q16 (Temperature aur average KE ka relation): Absolute temperature (T), molecules ki average translational kinetic energy ke seedha proportional hoti hai:
12m⟨v2⟩=32kBT\frac{1}{2}m\langle v^2\rangle = \frac{3}{2}k_BT
Q17 (Boltzmann constant): Ye ek constant hai jo gas constant per molecule ko represent karta hai — matlab macroscopic temperature ko molecule ki microscopic KE se jorta hai. Value: 1.38×10⁻²³ J/K
Q18 (Absolute zero ki physical significance): Jab T=0K, to average KE bhi zero ho jaati hai. Nazariati tor par, is point par poori molecular motion ruk jaati hai.
Real-Life Applications (Q19–20)
Q19 (Popcorn kaise pop karta hai): Kernel ke andar thora moisture hota hai. Heat lagne se ye paani steam mein badalta hai, jiski KE aur speed barh jaati hai. Isse internal pressure itna barh jata hai ke hard hull phat jaata hai aur starch violently phailta hai.
Q20 (Lambi drive ke baad tyre ka pressure kyun barhta hai): Tyre aur road ke darmiyan friction se heat paida hoti hai, jisse tyre ke andar air ka temperature barhta hai. Isse molecules ki KE barhti hai, wo walls se zyada bar aur zyada force se takrate hain, jisse pressure barh jata hai.
Statistical Physics Aur Boltzmann Distribution (Q21–25)
Q21 (Statistical physics ka maqsad): Ye microscopic properties (molecules ki position, velocity, energy) ko macroscopic properties (pressure, temperature, volume) ke saath probability laws ke zariye jorta hai, jab particles ki tadaad bohot zyada ho.
Q22 (Boltzmann Distribution Law — statement): Kisi khaas energy state mein particles milne ka probability, us energy ke barhne ke saath exponentially kam hoti jaati hai. Matlab lower energy states hamesha zyada populated hote hain higher states ke muqablay mein.
Q23 (Equation aur parameters):
N2N1=e−ΔE/kBT\frac{N_2}{N_1} = e^{-\Delta E/k_BT}
N₁, N₂ = states 1 aur 2 mein particles ki tadaad, ΔE = energy ka farq, kB = Boltzmann constant, T = absolute temperature.
Q24 (Temperature barhne se higher energy states ka kya hota hai): Jaise jaise T barhta hai, exponent ki negative value kam hoti jaati hai, isliye zyada molecules ke paas higher energy states occupy karne ke liye kaafi thermal energy hoti hai (N₂ barh jata hai).
Q25 (Pahar ki choti par kam molecules kyun hote hain): Ye Boltzmann Distribution ka ek application hai jahan potential energy ko energy state maana jata hai. Earth ki gravity ki wajah se molecules kam gravitational energy wali states (sea level) ko prefer karte hain, isliye altitude barhne ke saath air ki density aur pressure exponentially kam hoti jaati hai.
Stars Aur Stellar Evolution (Q26–30)
Q26 (Hydrostatic Equilibrium ki definition): Ye star ki wo stable state hai jahan gravity ka andar ki taraf khenchna, aur core mein nuclear reactions se paida hone wali gas/radiation pressure ka bahar ki taraf dhakka, dono barabar hote hain.
Q27 (Governing equation):
ΔPΔr=−GMrρrr2\frac{\Delta P}{\Delta r} = -\frac{GM_r\rho_r}{r^2}
Negative sign dikhata hai ke jaise jaise radius (r) core se surface ki taraf barhta hai, pressure kam hoti jaati hai.
Q28 (Sun jaise star ka final anjaam): Fuel khatam hone ke baad, Sun apni bahar ki layers ko planetary nebula ki soorat mein shed kar dega, aur bacha hua core sikur kar ek chhota, dense, non-fusing remnant banega jise White Dwarf kehte hain.
Q29 (Supernova): Ye ek bara, tabahkun explosion hai jo high-mass stars (Sun se taqreeban 10 guna ya zyada massive) ki zindagi ke aakhir mein hota hai, jab unka core achanak collapse ho jata hai.
Q30 (Pulsars): Ye strongly magnetized, tezi se ghumte hue neutron stars hote hain. Unke mazboot magnetic fields charged particles ko accelerate karte hain, jisse magnetic poles se tez, directional radiation ke beams nikalte hain — jo star ke ghumne ki wajah se pulse karte hue mehsoos hote hain.
Sabse Zaroori Tip (Exam Ke Liye):
Short questions mein examiner precise definitions aur key terms dekhta hai — jaise “elastic collisions”, “mean free path”, “degeneracy pressure”, “hydrostatic equilibrium”. Har answer ko 2-3 lines mein, seedha point par likhna chahiye, faltu tafseel (detail) ki zaroorat nahi hoti.