Bukhovtsev-et-al-Problems-in-Elementary-Physics

HEAT. MOLECULAR PHYSICS 77
353. A soapy film is stretched over a rectangular vertical wire
frame (Fig. 133). What forces hold section abed in equilibrium?
354. A cube with a mass m = 20 g wettable by water floats
on the surface of water. Each face of the cube a is 3 em long.
What is the distance between the lower face of the cube and
the surface of the water?
355. The end of a capillary tube with a radius r is immersed
into water. What amount of heat will be evolved when the
water rises in the tube?
356. A capillary tube is lowered into a vessel with a liquid
whose vapour pressure may be neglected. The density of the
liquid is p. The vessel and the tube are in a. vacuum under the
bell of an air pump (Fig. 134). Find the pressure inside the
liquid in the capillary tube at a height h above the level of
the liquid in the vessel.
357. The course of reasoning- given below is usually followed
to prove that the molecules of the surface layer of a liquid have
surplus potential energy. A molecule inside the liquid is acted
upon by the forces of attraction from the other molecules which
compensate each other on the average. If a molecule is singled
out on the surf ace, the resulting force of attraction from the
other molecule is directed into the liquid. For this reason the
molecule tends to move into the liquid. and definite work should
a ~--------------- 6
C 1-0----____________ tl
.: h
Fig. /33
Fig. 134