Thermodynamic Processes — Isothermal, adiabatic, isochoric, isobaric
Thermodynamics & KTG
21
JEE Qs
8%
Hard
75
min
Master the P-V diagrams for each process, understand the sign conventions for work and heat, and practice applying the First Law for various combinations of processes.
🧮 Key Formulas
✅ Key Points for JEE
- 1The sign convention for Work (W) is crucial: in JEE, W usually denotes work done *by* the system. If W is work *on* the system, the First Law becomes ΔU = Q + W. Be consistent.
- 2For an ideal gas, internal energy (ΔU) depends ONLY on temperature (T). Thus, ΔU = 0 for any isothermal process and ΔU = nCvΔT for any process.
- 3Work done by the gas is the area under the P-V curve. On a P-V diagram, an adiabatic curve is steeper than an isothermal curve starting from the same point.
- 4Heat exchange (Q) and Work (W) are path-dependent, but Internal Energy (ΔU) is a state function (path-independent).
- 5Remember that γ (gamma) = Cp/Cv is always > 1 and depends on the atomicity of the gas (e.g., monatomic: γ=5/3, diatomic: γ=7/5).
⚠️ Common Mistakes
- ✕Incorrectly applying the sign convention for work (W) in the First Law of Thermodynamics.
- ✕Confusing the characteristic equations or work done formulas for adiabatic and isothermal processes.
- ✕Using incorrect values for specific heats (Cp, Cv) or gamma (γ) for different types of gases.
- ✕Misinterpreting P-V diagrams, especially the area under the curve for work done and the relative slopes of isothermal vs. adiabatic processes.
📝 Practice Questions
See allQ29.Given below are two statements. One is labelled as Assertion (A) and the other is labelled as Reason (R). Assertion (A) : With the increase in the pressure of an ideal gas, the volume falls off more rapidly in an isothermal process in comparison to the adiabatic process. Reason (R) : In isothermal process, PV = constant, while in adiabatic process PVγ = constant. Here γ is the ratio of specific heats, P is the pressure and V is the volume of the ideal gas. In the light of the above statements, choose the correct answer from the options given below : (1) Both (A) and (R) are true and (R) is the correct (2) Both (A) and (R) are true but (R) is NOT the explanation of (A) correct explanation of (A) (3) (A) is true but (R) is false (4) (A) is false but (R) is true 2025 (29 Jan Shift 2) JEE Main Previous Year Paper
Q40.A cup of coffee cools from 90∘C to 80∘C in t minutes when the room temperature is 20∘C. The time taken by the similar cup of coffee to cool from 80∘C to 60∘C at the same room temperature is : (1) 13 10 t (2) 1013 t (3) 13 5 t (4) 135 t
Q42. A poly-atomic molecule ( CV = 3R, CP = 4R, where R is gas constant) goes from phase space point A (PA = 105 Pa, VA = 4 × 10−6 m3) to point B (PB = 5 × 104 Pa, VB = 6 × 10−6 m3) to point 2025 (29 Jan Shift 2) JEE Main Previous Year Paper C (PC = 104 Pa, VC = 8 × 10−6 m3). A to B is an adiabatic path and B to C is an isothermal path. The net heat absorbed per unit mole by the system is : (1) 500R(ln 3 + ln 4) (2) 450R(ln 4 −ln 3) (3) 500R ln 2 (4) 400R ln 4
Q29.An amount of ice of mass 10−3 kg and temperature −10∘C is transformed to vapour of temperature 110∘C by applying heat. The total amount of work required for this conversion is, (Take, specific heat of ice = 2100Jkg−1 K−1 , specific heat of water = 4180Jkg−1 K−1 , specific heat of steam = 1920Jkg−1 K−1 , Latent heat of ice = 3.35 × 105Jkg−1 and Latent heat of steam = 2.25 × 106 Jkg−1 ) (1) 3043 J (2) 3024 J (3) 3003 J (4) 3022 J
Q41.Two spherical bodies of same materials having radii 0.2 m and 0.8 m are placed in same atmosphere. The temperature of the smaller body is 800 K and temperature of the bigger body is 400 K . If the energy radiated from the smaller body is E, the energy radiated from the bigger body is (assume, effect of the surrounding temperature to be negligible), (1) 16 E (2) E (3) 64 E (4) 256 E
Q49.Three conductors of same length having thermal conductivity k1, k2 and k3 are connected as shown in figure. Area of cross sections of 1st and 2nd conductor are same and for 3rd conductor it is double of the 1st conductor. The temperatures are given in the figure. In steady state condition, the value of θ is _______ ∘C. (Given : k1 = 60Js−1 m−1 K−1, k2 = 120Js−1 m−1 K−1, k3 = 135Js−1 m−1 K−1 ) ∣ ∣∣ 2025 (22 Jan Shift 1) JEE Main Previous Year Paper
NCERT Chapters
- Class 11 Physics Ch 12: Thermodynamics