## Chapter 27: SURFACE TENSION

### Introduction

Surface tension explains why Bubbles are spherical.

Water doesn’t spread on some surfaces but forms droplets.

Oil and water don’t mix; water wets us but not ducks.

Hairs of brush stick together when removed out of water.

Raindrops are spherical. oil rises in wick; sap and water rise in plants.

Particles of soil remain separated under water but stick together when taken out.

When glass pieces are melted the edges take rounded shape and become smooth, this is called fire polishing.

### Properties of Surface Tension

Surface tension decreases with temperature.

Angle of contact determines if liquid shall spread or form droplets. It is reduced by wetting agents so they can penetrate well; whereas it is increased by waterproofing agents so water doesn’t stick. Detergents molecules attract water on one side and grease-dirt on other so they help in washing by reducing surface tension of water-oil.

Fig 1: Angle of Contact

Viscosity is due to the friction between different liquid layers as they slip past each other when the liquid flows.

### Properties of Viscosity

Higher viscosity means slower is the flow. Increasing the temperature decreases the viscosity.

E.g.: Glass is an extremely viscous liquid; it’s so viscous that it resembles solids. The windowpanes of old buildings are thicker at the bottom as glass flows down due to gravity.

### Friction

Rolling friction [rolling a body on ground] is less than sliding friction [pushing a body already in motion] is less than static friction [pushing a static object].

e.g.: Use of ball bearings in hubs and axels of ceiling fans and bikes to reduce contact surface between two solid parts. Thin cushion of air is maintained between solids in relative motion for same purpose. Lubricants too reduce friction.

Earthquakes occur when continental plates brush past each other or collide. Earthquakes are measured in Richter’s scale. An earthquake of intensity 6 is 1000 times more destructible than earthquake of intensity 2, so Richter scale is not linear.

### Satellites

The escape velocity on moon is lower so gas molecules if formed escape as they can easily overcome the gravitational pull of the moon. Thus the moon has no atmosphere.

Geostationary satellites appear to be appearing fixed from any point on the earth. To do this the satellite must be at a height of 35800km from the earth’s surface and in the equatorial plane. Since EM waves beyond a certain frequency can’t be reflected by the ionosphere [TV], for reflecting such waves we use geostationary satellites.

Polar satellites go around earth in north south direction, their time period is 100 mins approx and their altitude is 500-800km. The satellites at a higher height have a lower speed of revolution to stay in orbit as gravitational pull is lesser but this is independent of their weights. They don’t fall as earth’s gravitation provides necessary acceleration to it.

Fig 2: Types of Satellite

Satellites are launched preferably from equator and eastwards as it aids the speed due to lower acceleration due to gravity ‘g’. Speed of rotation of satellites is independent of its size and mass.

Note: centrifugal force is more at equator than poles. Gravity is higher at poles than equator.

### Electromagnetic Waves

EM waves in decreasing order of wavelength

Long Radio wave AM radio > TV and FM waves > short radio > microwave > infrared > visible > UV > X-ray > gamma

In satellites, the acceleration towards the center of the earth is equal to earth’s acceleration due to gravity at that point. Thus the objects inside the satellite experience weightlessness. A body in free fall experiences no upward force on him and so feels weightless.

### Quiz

Score more than 80% marks and move ahead else stay back and read again!