Question

A liquid when kept inside a thermally insulated closed vessel at $25^{\circ}C$ was mechanically stirred from outside. What will be the correct option for the following thermodynamic parameters?

• A. $\Delta U>0, q=0, w>0$
• B. $\Delta U=0, q=0, w=0$
• C. $\Delta U<0, q=0, w>0$
• D. $\Delta U=0, q<0, w>0$

Hint:

In a thermally insulated system, no heat exchange occurs (q=0). Work done on the system is positive (w>0), and work done by the system is negative (w\>0).

Answer 1

The Correct Option is A

Thermally insulated $\Rightarrow q=0$
No heat exchange From $I^{st}$ law, $\Delta U=q+w$ 
First law of thermodynamics $\Delta U=w$ 
Since q=0
When the liquid is mechanically stirred, work is done on the system, so $w>0$.
Therefore, $\Delta U>0$.