## Saturday, 6 September 2014

### Surface tension PROBLEMS

Surface tension

1. Explain the following observations in terms of surface tension:
(a)  A pond-skater can walk on water but a person cannot.
(b)  A wet tent will let in water if the inside is touched.
(c)  Water will rise up the capillaries inside a plant stem (surface tension only partly explains this effect, however).
(d)  A needle may be made to float on water.
(e)  A small piece of soap fixed to the back of a piece of cardboard that is floating on water will cause the cardboard to move over the water surface.
(f)  Lead shot is made by pouring a molten stream of lead from a tall tower.

2. A circular ring of thin wire of mean radius 2 cm is suspended horizontally by a thread passing through the 5 cm mark on a metre ruler pivoted at its centre, and the ring is balanced by a 5 g mass suspended from the 70 cm mark. A beaker of liquid is then placed so that the ring just touches the liquid surface when the ring is horizontal. If the 5 g mass is moved to the 80 cm mark the ring just parts from the surface.
Find the surface tension of the liquid.

3.  A soap bubble of radius 8 cm is blown on the end of a tube which is connected to a U-tube containing water.
(a)  What difference in water levels would be produced?
(b)  If another soap bubble is now allowed to make contact with the first so that the radius of curvature of the common surface is 2 cm, calculate the radius of the second bubble.
(Surface tension of soap solution = 3.5 x 10-2 Nm-1, density of water = 1000 kg m~3.)

4. (a) What is the excess pressure inside a spherical soap bubble of radius 5 cm if the surface tension c the soap film is 3.5 x 10-2 Nm-1?
(b) What is the work done in blowing the bubble?

5. A glass capillary tube with a uniform internal diameter is placed vertically with one end dipping into paraffin, for which the surface tension is 2.7 x 10-2 Nm-1, the angle of contact 26° and the density 865 kg m-3.If the paraffin rises 4.5 cm up the tube, what is the diameter of the tube?

6. A U-tube which has its ends open and its limbs vertical contains a liquid of surface tension 2.4 x 10-2 Nm-1and density 800 kgm-3, the angle of contact between the tube and the liquid being 20°. The internal diam­eter of one limb is 0.4 mm and the other 0.2 mm.
Calculate the difference in the liquid levels in the two limbs.

7. A small drop of water is placed between two glass plates and the plates are then squeezed together until the drop forms a thin circular film. If the radius of the film is 0.03 m and its thickness 0.10 mm, cal­culate the force required at right angles to the glass plates to pull them apart. (Surface tension water = 7.2 x 10-2 Nm-1)