Class 11 Physics: Thermodynamics Notes

1. Introduction

• Thermodynamics studies heat, work, energy, and their interconversion.
• Focuses on macroscopic properties: pressure, volume, temperature, internal energy.

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2. System and Surroundings

• System: The part of the universe under study.
  • Open: exchanges energy & matter
  • Closed: exchanges energy only
  • Isolated: no exchange of energy or matter
• Surroundings: Everything outside the system
• Boundary: real or imaginary surface separating system & surroundings

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3. Work in Thermodynamics

• Work done by a system during expansion/compression:

$$W = \int P \, dV$$W=∫PdV

• Graphical interpretation: Area under P-V curve

Special Cases:

• Isobaric: $W = P \Delta V$W=PΔV
• Isochoric: $W = 0$W=0

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4. Internal Energy (U)

• Total microscopic energy of system (kinetic + potential of particles)
• Depends on temperature for an ideal gas
• Change in internal energy:

$$\Delta U = U_f – U_i$$ΔU=Uf​−Ui​

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5. Heat Capacity

• Definition: Heat required to raise the temperature by 1 K

$$C = \frac{Q}{\Delta T}$$C=ΔTQ​

• Specific Heats for Ideal Gas:
  • $C_v$Cv​ – at constant volume
  • $C_p$Cp​ – at constant pressure
  • Relation: $C_p – C_v = R$Cp​−Cv​=R

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6. First Law of Thermodynamics

• Statement:

$$\Delta U = Q – W$$ΔU=Q−W

Where:

• $Q$Q = heat absorbed by system
• $W$W = work done by system

Special Cases:

• Isothermal (ΔT = 0): ΔU = 0 → Q = W
• Adiabatic (Q = 0): ΔU = −W
• Isochoric (ΔV = 0): W = 0 → ΔU = Q
• Isobaric (P constant): ΔU = Q − PΔV

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7. Thermodynamic Processes

Process	Condition	Work Done	Heat Exchange
Isothermal	ΔT = 0	W = nRT ln(Vf/Vi)	Q = W
Adiabatic	Q = 0	W = ΔU	Q = 0
Isochoric	ΔV = 0	W = 0	Q = ΔU
Isobaric	P = constant	W = PΔV	Q = ΔU + PΔV

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8. Enthalpy (H)

• Definition:

$$H = U + PV$$H=U+PV

• At constant pressure: ΔH = Q (heat absorbed/released)

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9. Second Law of Thermodynamics

• Statement: Heat cannot spontaneously flow from a cold body to a hot body
• Introduces entropy (S): measure of disorder
• Carnot engine: Maximum efficiency = $1 – \frac{T_C}{T_H}$1−TH​TC​​

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10. Important Equations (NCERT Focused)

1. Ideal gas equation: $PV = nRT$PV=nRT
2. Work: $W = \int P dV$W=∫PdV
3. Internal energy (ideal gas): $\Delta U = n C_v \Delta T$ΔU=nCv​ΔT
4. Heat at constant pressure: $Q = n C_p \Delta T$Q=nCp​ΔT
5. Relation: $C_p – C_v = R$Cp​−Cv​=R
6. Adiabatic condition: $PV^\gamma = \text{constant}$PVγ=constant

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11. Key Points to Remember (NCERT Style)

• Energy conservation → first law
• Adiabatic process → no heat exchange
• Isothermal → ΔU = 0
• C_p > C_v always
• Work = area under P-V curve