phy soham-1.pptx

Physics

1. Special Cases of Capillary Rise The following are some special cases of capillary rise

2. Rising of a liquid in a capillary tube if insufficient length Suppose a liquid of density p and surface tension T rises in a capillary tube to a height h. Then, hpg = 2T/R where R is the radius of curvature of the liquid meniscus in the tube. From this we may write 2T hR / pg = = constant (for a given liquid ). When the length of the tube is greater than h, the liquid rises in the tube to a height so as to satisfy the above relation. But if the length of the tube is less than h, say h', then the liquid rises up to the top of the tube and then spreads out until its radius of curvature R increases to R', such that h'R ' = hR =2T/ pg It is clear that liquid cannot emerge in the form of a fountain from the upper end of a short capillary tube.

3. Rise of Liquid in an Inclined Capillary Tube The height of the liquid column in a glass capillary tube of radius r is h=2T cosA / rpg where T is surface tension of the liquid and A is the angle of contact f liquid glass. If we incline the capillary tube at an angle B with the vertical, then for balancing the weight of the liquid column raised in the tube by the force of surface tension, the 'vertical' height of the liquid in the capillary tube will still be h. For this, the 'length' of the liquid in the tube will increase to h', where h’=h/ cosB Substituting the value of h from eq. ( i ), we have h '= 2 T cos A/ rpg cosB .

4. Some Examples of Capillarity The Water given to the fields rises in the innumerable capillaries formed in the stems of plants and trees and reaches the branches and the leaves

5. Some Examples of Capillarity The kerosene oil in lanterns and the melted wax in a candle , rise in capillaries formed in the cotton wick and burns

6. Some Examples of Capillarity If one end of a towel is dipped in a vessel filled with water then the water began to rise in the innumerable holes (which act like capillaries ) formed in between the threads of the towel and ultimately the entire towel is wetted

7. Some Examples of Capillarity Coffee powder is easily soluble in water because water immediately wets the fine granules of coffee by the actin of capillary.

8. Some Examples of Capillarity Writing nib is split in the middle so that a fine capillary is formed in it, when it is dipped in ink the ink rises in the capillary

9. Some Examples of Capillarity The farmers plough their fields after rains so that the capillaries formed in the soil are broken a the water remains in the lower layers of the soil. This water is taken up by plants. If ploughing is done, the water of the lower layers will rise through the capillaries in the soil and evaporate.

10. Pressure Difference between the two sides of a curved surface The Laplace pressure is the pressure difference between the inside and the outside of a curved surface that forms the boundary between two fluid regions.The pressure difference is caused by the surface tension of the interface between liquid and gas, or between two immiscible liquids . The Laplace pressure is determined from the Young–Laplace equation given as