Q.Explain in detail an adiabatic process.

Step 1. An adiabatic process is one in which no heat flows into or out of the system at all, $Q=0$ -- either because the system is thermally insulated, or because the process happens too fast for any heat exchange even without insulation.

Step 2. Applying the first law with $Q=0$: $\Delta U=-W$. This means an adiabatically expanding gas can only expand at the expense of its own internal energy (cooling as it does so), while an adiabatically compressed gas is heated by the external work done on it.

Step 3. The equation of state for a quasi-static adiabatic process is $PV^\gamma=\text{constant}$, where $\gamma=C_p/C_v$ is the adiabatic exponent (equivalently $TV^{\gamma-1}=\text{constant}$ or $T^\gamma P^{1-\gamma}=\text{constant}$); on a P-V diagram this curve (an "adiabat") is always steeper than the isotherm passing through the same point, since $\gamma>1$. …

Define via Q=0, derive $\Delta U=-W$ from the first law, and state (without full derivation) …

• Confusing 'adiabatic' with 'isothermal' -- adiabatic processes generally DO change temperature (ofte …