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Physics · University

First law, ideal gases and entropy · revision notes

Use ΔU=Q−W, where W is work done by the system. A different sign convention is possible only when used consistently.

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Key ideas and conditions

Convention

Use ΔU=Q−W, where W is work done by the system. A different sign convention is possible only when used consistently.

Ideal gas

For a fixed amount of ideal monatomic gas, U=3nRT/2 in the classical translational model. Absolute temperature must be in kelvin.

Entropy

For a reversible path, dS=δQrev/T. A state-function change may be calculated along a reversible comparison path even when the actual path is irreversible.

Engines

Carnot efficiency is 1−Tc/Th for reversible operation between two reservoirs. Real engines need not reach that upper bound.

Explain without looking

  1. For a fixed amount of ideal gas in an isothermal process, what is ΔU?
  2. State how a consistent sign convention helps audit a first-law calculation.

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