NEB Class 12 Physics First Law of Thermodynamics Notes in PDF Complete Handwritten. Phyaics Notes 2082: All Chapters | New Curriculum | Class 12 Phyaics Notes download.
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NEB Class 12 Physics First Law of Thermodynamics Note Handwritten in PDF
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Thermodynamic Systems
A thermodynamic system is a defined quantity of matter separated from its surroundings.
Three types:
- Open system: Exchanges both matter and energy (boiling water in open pot)
- Closed system: Exchanges only energy (gas in sealed container)
- Isolated system: Exchanges neither (thermos flask, approximately)
Internal energy (U): Total microscopic energy of a system β sum of kinetic energies of all molecules plus their potential energies due to intermolecular forces. Internal energy is a state function β it depends only on the current state, not on how the system reached that state.
First Law β Statement and Equation
First Law: Energy cannot be created or destroyed. The heat supplied to a system equals the increase in internal energy plus the work done by the system.
dQ = dU + dW = dU + PdV
Or equivalently: dU = dQ β PdV
Sign conventions:
- Q positive β heat flows INTO system
- W positive β work done BY system (expansion)
- U positive β internal energy increases
Work done during volume change: W = β«P dV = area under PV diagram
Two Specific Heats of Gas: Cp and Cv
- Cv (at constant volume): All heat supplied increases internal energy only (no work done as no expansion): dQ = nCvdT
- Cp (at constant pressure): Heat goes to both internal energy AND work of expansion: dQ = nCpdT
Since Cp involves extra work of expansion: Cp > Cv always
Mayer’s relation: Cp β Cv = R (for 1 mole of ideal gas)
Ratio: Ξ³ = Cp/Cv
- Monatomic gas (He, Ar): Ξ³ = 5/3 β 1.67
- Diatomic gas (air, Oβ, Nβ): Ξ³ = 7/5 = 1.4
- Triatomic gas (COβ): Ξ³ β 1.33
The Four Thermodynamic Processes
1. Isothermal Process (T = constant)
- PV = constant (Boyle’s law)
- ΞU = 0 (temperature unchanged)
- Q = W (all heat converts to work)
- Work done = nRT ln(Vβ/Vβ)
- PV graph: hyperbola (isothermal curve)
2. Adiabatic Process (Q = 0)
- PVΞ³ = constant
- No heat exchange with surroundings
- ΞU = βW (work done at expense of internal energy)
- Temperature changes (gas cools on expansion, heats on compression)
- Work done = (PβVβ β PβVβ)/(Ξ³β1)
- Steeper curve on PV diagram than isothermal
3. Isochoric Process (V = constant)
- W = 0 (no expansion, no work)
- dU = dQ = nCvΞT
- All heat supplied changes internal energy
- Vertical line on PV diagram
4. Isobaric Process (P = constant)
- W = PΞV = nRΞT
- Q = nCpΞT
- Horizontal line on PV diagram
Reversible and Irreversible Processes
- Reversible process: Can be reversed along exact same path, restoring all properties. Only possible with infinitely slow (quasi-static) changes. Example: Carnot cycle (theoretical)
- Irreversible process: Cannot be reversed without changing surroundings. All real processes are irreversible. Examples: free expansion, mixing gases, heat flow across temperature difference, friction
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Frequently Asked Questions
What is the first law of thermodynamics Class 12?
First law: dQ = dU + PdV. Heat supplied either increases internal energy or does work. Energy is always conserved. Q positive means heat flows into system. W positive means work done by system. It is the thermodynamic version of conservation of energy applied to heat processes.
What is the difference between isothermal and adiabatic process?
Isothermal: temperature constant, ΞU=0, all heat converts to work, PV=constant, slow process. Adiabatic: no heat exchange Q=0, PVΞ³=constant, work changes internal energy, rapid process. On PV diagram adiabatic curve is steeper than isothermal. Both derivations are important for NEB 2082 board exam.
Why is Cp always greater than Cv for an ideal gas?
At constant pressure gas must expand β extra heat is needed for work done of expansion in addition to increasing internal energy. At constant volume no expansion occurs so all heat increases internal energy only. Therefore Cp is always greater than Cv. Their difference: Cp β Cv = R (Mayer’s relation).
What is Mayer’s relation in Class 12 thermodynamics?
Mayer’s relation: Cp β Cv = R for one mole of ideal gas. Where Cp is molar specific heat at constant pressure, Cv at constant volume, and R = 8.314 J/molΒ·K is universal gas constant. This is derived from first law of thermodynamics and is a standard NEB 2-mark question.
What is work done in isothermal process Class 12?
Work done in isothermal process: W = nRT ln(Vβ/Vβ). Since temperature is constant the gas follows Boyle’s law PV = constant. The work equals area under the PV curve β a hyperbola. If gas expands (Vβ > Vβ), work is positive. This derivation is frequently asked in NEB board exams.
What is internal energy in thermodynamics Class 12?
Internal energy (U) is total microscopic energy of a system β sum of kinetic energies of all molecules plus intermolecular potential energies. It is a state function depending only on current state. For ideal gas, internal energy depends only on temperature: ΞU = nCvΞT regardless of process.
What is an isochoric process in Class 12 Physics?
Isochoric process: volume stays constant, so no work is done (W = 0 since dV = 0). All heat supplied goes entirely to increasing internal energy: dQ = dU = nCvΞT. Represented by a vertical line on PV diagram. Example: heating gas in a rigid sealed container that cannot expand.
Which thermodynamics topics are most important for NEB 2083?
Most important: first law statement and equation, isothermal and adiabatic process differences, CpβCv=R derivation, work done in isothermal process, and PV diagram interpretation. Numerical problems comparing processes and calculating work done appear regularly in NEB Class 12 Physics board exams.
