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

HEAT. MOLECULAR PHYSICS 283
343. The heat imparted to the gas is used to heat it and perform mecha..
nical work. According to the law of conservation of energy,
2-4. Properties of Liquids
344. It is more difficult to compress a litre of air, since more work has
to be done in this case.
Water has a small compressibility, and a small reduction in volume is
required to increase the pressure inside it to three atmospheres.
345. A maximum thermometer can be made as follows. A small unweUable
freely moving body is placed inside the tube of a horizontal thermometer
(Fig. 402). The position of the body will show the maximum temperature,
since the body will move along the tube when the liquid expands and will
remain in place when the liquid in the tube is compressed.
To make a minimum thermometer, a wettable body should be placed
inside the liquid in the tube.
346. When an elastic rubber film is stretched, the force of tension depends
on the amount of deformation of the film. The force of surface tension is
determined only by the properties of the liquid and does not change with
an increase of its surface.
347. The surface tension of pure petrol is less than that of petrol in
which grease is dissolved. For this reason the petrol applied to the edges
will contract the spot towards the centre. If the spot itself is wetted, it will
spread over the fabric.
348. Capillaries of the type shown in Fig. 403 form in a compact surface
layer of soil. They converge towards the top, and the water in them rises to
the surface, from which it is intensively evaporated. Harrowing destroys this
structure of the capillaries and the moisture is retained in the soil longer.
349. Leather contains a great number of capillaries. A drop of a wetting
liquid inside a capillary having a constant cross section will be in equllibrium.
When the liquid is heated, the surface tension diminishes and the
liquid is drawn towards the cold part of the capillary. The grease will be
drawn into the leather if it is heated outside.
350. The grease melts, and capillary forces carry it to the surface of the
cold fabric placed under the clothing (see Problem 349).
351. The end of the piece of wood in the shade is colder, and the caplltary
forces move the water in th is direction.
352. The hydrostatic pressure should be balanced by the capillary pres­
4~
sure: pgh={f' Hence, h=30 em.
353. The following forces act vertically on section abed of the film: weight,
surface tension F ab applied to line ab and surface tension Fcd applied to cd.
Fig. 402