Question

Air in a cylinder is suddenly compressed by a piston, which is then maintained at the same position. With the passage of time

• A. the pressure decreases
• B. the pressure increases
• C. the pressure remains the same
• D. the pressure may increase or decrease depending upon the nature of the gas

Answer 1

Answer: (A)

From the First Law of Thermodynamics,
$dQ=dU+dW$
Sudden processes are adiabatic, so $dQ=0$
$dU=-dW$
In compression, $dW<0$
So, $dU>0$ which means $dT>0$
With passage of time (i.e. slow process), piston is maintained at the same place. Hence, Volume is constant.
So, P $\alpha$ T.
As temperature decreases with time, pressure decreases too.