Sol. The pressure of the water changes linearly with the increase in height. At the bottom of the meniscus it is equal to the external atmospheric pressure p 0 , and at the top to p 0 – ρ gh. The average pressure exerted on the wall is p average = p 0 – ρ gh/2. The force corresponding to this value, for an aquarium with side walls of length λ , is F 1 = λ p average h.

Consider the horizontal forces acting on the volume of water enclosed by the dashed lines in the figure. The wall pushes it to the right with force F 1 , the external air pushes it to the left with force F 2 = λ p 0 h, and the surface tension of the rest of the water pulls it to the right with a force F 3 = λ γ . The resultant of these forces has to be zero, since the volume itself is at rest. This means that

λ h – p 0 λ h + λ γ = 0,

which we can write as

h = = = 0.0038 m.

Water rises by approximately 4 mm up the wall of the aquarium.