NEET · Physics
Teach official Unit 8: thermal equilibrium, zeroth law, temperature concept, heat/work/internal energy, first law, isothermal and adiabatic processes, and second-law reversible/irreversible process distinction.
In short
Thermodynamics tracks energy transfer and state changes of a macroscopic system. Rank Sarthi uses the sign convention Q = ΔU + W, where W is work done by the system. Every worked item must state that convention. Isothermal and adiabatic labels describe different process constraints and should never be used interchangeably.
Subject faculty
Also in Physics: Kumar Gandharva Saxena
Internal energy is a state function. Heat and work describe energy transfer across the system boundary during a process and are not state functions. A page should state the system boundary before applying the first law.
This package uses the NCERT-style form Q = ΔU + W, with Q>0 for heat supplied to the system and W>0 for work done by the system on the surroundings. Equivalent conventions are possible, but mixing them inside one solution produces sign errors.
An isothermal process occurs at constant temperature. For an ideal gas, internal energy depends only on temperature, so ΔU=0 in an isothermal process. An adiabatic process has no heat transfer, Q=0; temperature can change. Ideal-gas adiabatic relations require the additional quasistatic/reversible idealization when using PV^γ=constant.
The second law introduces directionality and distinguishes reversible idealizations from irreversible real processes. Keep explanations within current syllabus scope and avoid adding unnecessary heat-engine depth not required by the official unit wording.
First law, chosen convention: Q = ΔU + W. J. Q>0 heat added to system; W>0 work done by system. State convention in every calculation.
First law, chosen convention
Use when — J. Q>0 heat added to system; W>0 work done by system. State convention in every calculation.
Constant-pressure boundary work: W = P(V_2-V_1). J. Quasistatic process at constant external/system pressure as modeled; expansion gives positive W under chosen convention.
Constant-pressure boundary work
Use when — J. Quasistatic process at constant external/system pressure as modeled; expansion gives positive W under chosen convention.
Ideal-gas isothermal work: W = nRT ln(V_2/V_1). J. Ideal gas, isothermal, quasistatic/reversible path; T K. For ideal gas ΔU=0 in isothermal process.
Ideal-gas isothermal work
Use when — J. Ideal gas, isothermal, quasistatic/reversible path; T K. For ideal gas ΔU=0 in isothermal process.
Adiabatic energy balance: Q=0, so ΔU = -W under chosen convention. Adiabatic means no heat transfer. Sign follows chosen convention.
Adiabatic energy balance
Use when — Adiabatic means no heat transfer. Sign follows chosen convention.
Ideal-gas reversible adiabatic: PV^γ = constant. Ideal gas with fixed composition, quasistatic/reversible adiabatic process and approximately constant γ over the range.
Ideal-gas reversible adiabatic
Use when — Ideal gas with fixed composition, quasistatic/reversible adiabatic process and approximately constant γ over the range.
Concept being tested: First law
Prompt: A gas absorbs 500 J of heat and does 200 J of work on the surroundings. Using Q=ΔU+W, find ΔU.
Decisive condition: The sign convention is stated; heat added and work done by system are both positive.
Method choice: Rearrange ΔU = Q - W.
Reasoning/calculation: ΔU = 500 - 200 = 300 J.
Unit/sign/condition check: Energy unit is joule; positive ΔU means internal energy increased under this process description.
PI if answered incorrectly: Execution Error if work sign is reversed without changing the first-law convention.
Mixing sign conventions for work
Execution errorWhy it happens
Preparation Intelligence classification: Execution Error.
How it is corrected
State the convention before writing the first law.
Assuming adiabatic means constant temperature
Knowledge gapWhy it happens
Preparation Intelligence classification: Knowledge Gap.
How it is corrected
Adiabatic means Q=0; temperature may change.
Using PV^γ=constant for an arbitrary irreversible adiabatic process
Decision / selection errorWhy it happens
Preparation Intelligence classification: Decision / Selection Error.
How it is corrected
Require the reversible/quasistatic ideal-gas condition.
Treating heat as a state function
Knowledge gapWhy it happens
Preparation Intelligence classification: Knowledge Gap.
How it is corrected
Heat and work are process/path transfers.
FAQ
Straight answers about how Rank Sarthi fits into serious exam preparation.
Q = ΔU + W, with W positive for work done by the system.
No. It means no heat transfer.
For the ideal-gas quasistatic/reversible adiabatic model with appropriate constant γ.
Official scope is taken from the NEET (UG) 2026 syllabus and official NTA documents; concept structure follows current NCERT contents. No performance, weightage or question-frequency data is asserted.
