Question

A liquid is kept in a bowl opened to atmosphere.

Consider two consecutive layers $A$ and $B$ as shown above, which of the following statement(s) is/are correct?

• A. Total energy of surface $A=$ Total energy of surface $B$.
• B. Number of molecules in surface $A>$ Number of molecules in surface $B$.
• C. Energy of a molecule of layer $B$ is nearly half of energy of layer $A$.
• D. Net force on a molecule of surface $B$ is zero.

Answer 1

Answer: (D)

From the property of surface tension, there are least number of molecules in topmost layer of any liquid and for topmost layers of a fluid, energy is maximum.
So, total energy of surface $A>$ total energy of surface $B$.
Number of molecules on surface $A<$ Number of molecules in surface $B$.
The molecule on the surface of the liquid, i.e. layer $A$ is surrounded from half side by liquid molecules. Thus, its potential energy is half that of a molecule inside the liquid, i.e. layer $B$.
However, as the liquid molecule inside the liquid is surrounded equally from all sides, so net force on a molecule of surface $B$ will be zero.
Thus, the statement given in option is correct, rest are incorrect..