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# What is convection?

“Transfer of heat by actual movement of molecules from hot place to a cold place is known as convection.”

Liquids and gases are poor conductors of heat.However,heat is transfer through  fluids(liquids or gases)easily by another method called convection.

Why a balloon inflated with hot shown in figure .rise up?a liquid or a gas becomes lighter (less dense) as it expands on heating. Hot liquid or gas rises up above the heated area.The cooler liquid or gas from the surrounding fills the places which in turns is heated up.in this way,all the fluid is heated up.Therefore,transfer of heat through fluids takes place by the actual movement of heated molecules from hot to cold parts of the fluid.

Unlike particles of solids,particles in liquids and gases move from one place to another.Take a beaker and put small pieces of paper in it.

Fill half of the beaker with water.Heat the beaker by a spirit lamp.We shall see that pieces of paper rise to the top of water,move sideways and sink to the bottom.The water in the beaker also gets warm.The molecules of water absorb heat energy from the bottom of the beaker and rise to the top.Other surroundings molecules of water come to the bottom to absorb heat energy.From above experiment we can also defined convection as:”The transfer of heat in which molecules of a medium actually move to the source of heat energy to absorb heat and then move away from it,is called convection.”

### Is Convection occurs in solids?

Convection occurs in liquids and gases only because their molecules can move freely.The molecules of a solid are held closely together.They cannot move freely.The molecules of a solid are held closely together.They cannot move freely,therefore,convection is not possible in solids.

## Convection current:

The upward and downward movement of molecules of water or air is called a convection current.

### Atmospheric Convection:

Atmospheric convection plays a fundamental role in determining the global climate patterns and in our daily weather variations.Gliders pilots and condors alike seek the convection thermals that ,rising from the warmer Earth beneath,keep them aloft.Huge energy transfers take place within the oceans by the same process.The outer region of the sun ,called the stratosphere ,contains a vast array of convection cells that transport energy to the solar surface and give the surface a granulated appearance.

### Winds and Ocean currents:

We know that convection is the transfer of heat by the actual movement of the particles in materials.Winds and ocean currents are examples of effects of convection.

#### Wind current:

The heat of the sun heats up the surface of the Earth and the air near it also gets hot.The air expands and gets lighter.So,it rises up and cool air from the neighbouring regions moves in to fill its space.The rising warm air reaches upper colder layers of the air and cools down.Cool heavy air sinks to the Earth in cold regions to blow again to take the place of the rising air.Thus,convection currents are setup and wind system goes on.

#### Ocean Currents:

ocean currents are also set up due to convection of heat.Water of the hot regions of an ocean gets hot,it expands and gets lighter,but water in the colder regions remains cold and heavy.hot water moves along the surface of the ocean towards the colder regions.The cold water flows below the surface of the ocean towards the hot regions.in this way,ocean currents are set up.

#### What is Gliding Flights of Birds?

Convection currents also take place in atmosphere.The heat form the Sun warms the air near the ground.The warm air expands and becomes lighter in weight.As warm air rises,colder air rushes in to fill its place near the ground.This process continuous.Birds like eagles,hawks,vultures and gulls advantage of this phenomenon.They enjoy gliding.During gliding flight a bird does not move its wings,but glides on air currents.A lot of energy of birds of saved during gliding.

### Application of Convection Currents:

We can observe the use of convection currents in our surrounding.

• Household ventilation can make our house cool.The air which we breathe out is warmer and lighter.It moves up in the room to go out of the ventilators near the top side of the walls.Fresh and cool air enters the room through windows and doors.
• In a domestic water heater,water is heated in the boiler by gas burner or heating coil.the hot water expands and becomes lighter in weight.this water rises and flows into the upper part of the water heater.To take the place of hot water,cold water from storage tank (cistern)  falls to the lower part of the water heater to be hot.We take the hot water from the tap attached to the water heater,convection currents help in the continuous supply of the water.
• An air conditioner also uses convection currents to cool a room.Air conditioners are installed near to be ceiling.The rotary fan of an air conditioner releases cool dry air.The cool air is heavier in weight,so its sinks.the warm air of the room rises because it becomes lighter in weight.The air conditioner draws this warm air to make it cool.In this way,the air circulates again and again and desired temperature is gained.
• convection currents occur on a large scale in nature.The day _to_day temperature changes in the atmosphere result from the circulation of warm or cold air that travels across the regions.land and see breezes are also the examples of convection currents.

#### What is difference between land breeze and sea breeze?

Land Breeze:

Land and sea breezes are the result of convection.On a hot day,the temperature of the land increases more quickly than the sea.It is because the specific heat of land is much smaller as compared to water.The air above land gets hot and rises up.cold air from the sea begins to move towards the land.It is called see breeze.

Sea Breeze:

At night,the land cools faster than the sea.Therefore,air above the sea is warmer,rises up and the cold air from the land begins to move towards the sea as illustrated .it is called sea breeze.

##### GLIDING:

What cause a glider to remain in air?

A glider such as look like a small aero plane without engine.Glider pilots use upward movement of hot air currents due to convection of heat.These rising currents of hot air are called thermals.glider ride over these thermal.The upward movement of air currents in thermal help them to stay in air for a long period.

How do thermals help birds to fly for hours without flapping their wings?

The birds stretch out their wings and circle in these thermals.The upward movement of air helps birds to climb up with it.Eagles ,hawks and vultures are expert thermal climbers.After getting the free lift,birds are able to fly for hours without flapping their wings.They glide from one thermal to another and thus travel through large distances and hardly need to flap their wings.