MHT-CET : Physics Entrance Exam

### MHT - CET : Physics - Properties of Fluids Page 2

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4.

Angle of Contact

Definition: For a liquid in contact with a solid, the angle between the surface of the solid and the tangent drawn to the free surface of the liquid at the point of contact measured from the side of the liquid is called the angle of contact.

Characteristics of the Angle of Contact:

• For a liquid that wets the solid, the angle of contact is acute. The angle of contact between clean water and glass is nearly zero. Water almost completely wets glass.
• Liquids that partially wet the solid have an acute angle, e.g. kerosene and glass.
• For liquids that do not wet the solid, the angle of contact is obtuse, e.g. mercury and glass.
• The angle of contact for a given liquid-solid pair is always constant.

5.

Capillary Action

Capillary Tube: A tube with a very fine bore is called a capillary tube.

• The rise or fall of a liquid in a capillary tube is called capillarity
• If a capillary tube is dipped in a liquid that wets the tube, the liquid level rises in the tube.
• If a capillary tube is dipped in a liquid that does not wet the tube, the liquid level falls in the tube

Applications of Capillarity:

• Blotting paper absorbs ink due to capillarity.
• Oil rises in the wick of a lamp due to capillarity.
• Sap rises in trees due to capillary action.
• Water in soil rises to the surface through tiny capillaries formed in the soil.

6.

Experiment to Find Surface Tension of a Liquid by Capillary Rise Method

Principle: The height, h, to which a liquid rises in the tube and the radius of the tube are carefully measured. Surface tension is calculated using the appropriate formula.

Apparatus: A clear capillary tube is dipped in a liquid and clamped vertically. A bent index pin is attached to the capillary tube and adjusted so that its tip just touches the water surface outside the capillary.

Measurement of
h:
A travelling microscope with known least count is focused on the liquid meniscus in the capillary. The horizontal cross-wire is made tangential to the liquid meniscus and the microscope reading is noted.

The beaker of water is removed. The microscope is focused on the tip of the index pin and the reading is noted. The difference between these two readings gives the capillary rise.

Measurement of r:

• The capillary tube is clamped horizontally and the microscope is focused at its end. The horizontal cross-wire is made tangential to one edge of the bore. The reading is noted.
• The microscope is moved with the help of the fine adjustment screw to make the horizontal cross-wire tangential to the opposite edge of the bore. The reading is noted. This reading is the difference between those readings gives the diameter (vertical) of the bore.
• The above procedure is repeated by making the vertical cross-wire tangential first with one edge and then the other of the bore. The difference between these two readings gives the diameter (horizontal) of the bore.
• The mean diameter and then the mean radius of the bore in determined.

Formula:

 The surface tension is calculated using the formula T = rhdg 2 cos q

where r is the radius of the capillary tube, h is the capillary rise, d is the density of the liquid, g is the acceleration due to gravity and q is the angle of contact.

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