PHY1: CONVECTION

This unit is about Convection as a form of heat transfer.

CONVECTION

Is the kind of heat transfer through a fluid which involves the bulk movement of materials?

An experiment to demonstrate convection in liquids

CONVECTION

– A crystal of potassium permanganate is a place at the bottom of a beaker full of water.

– When the beaker is gently heated from the bottom, potassium permanganate is seemed to rise.

– At the top of the water, the potassium permanganate spreads out and descends to the bottom in an organized manner.

– In the course of heating, the water on the top becomes hot. This shows that heat has been transferred from the bottom upward.

Explanation

– When the liquid is heated, it expands and becomes less dense. The less dense liquid rises and carries the heat absorbing it thus transferring heat upwards.

– The cold liquid sinks to the bottom in order to replace the warm water.

At the bottom of the water, the cold liquid becomes less dense as it absorbs the heat and this cycles / process is repeated until liquid boils.

– The movement of liquid from the bottom up ward then down wards from top is called convectional transfer/ currents.

THIS VIDEO EXPLAINS IT IN DETAIL

CONVECTION IN GASES 

CONVECTION 1

 

  The candle is lit and placed inside a box near a chimney B when a smoldering smoke is observed to flow downwards through A and then upwards through B.

This is because air around the candle rises on receiving the heat and the cold air moving downwards through A.

A takes up the place of this warm air and as a result of the two movements convection current is set up.

 

Application of fluid convection

– Ventilation

– Cooking together

– Rainfall formation

– Land and sea breezes

– Hot water and domestic supply system

Explanation

– When the liquid is heated, it expands and becomes less dense. The less dense liquid rises and carries the heat absorbing it thus transferring heat upwards.

– The cold liquid sinks to the bottom in order replace the warm water.

At the bottom of the water, the cold liquid becomes less dense as it absorbs the heat and this cycles / process is repeated until liquid boils.

– The movement of liquid from the bottom up ward then down wards from top is called convectional transfer/ currents.

HOT WATER SUPPLY

CONVECTION 2

The domestic hot water supply system consists of a boiler, hot water storage tank, a cold water supply tank, interconnected pipes.

When the system is working a convection current of water from the boiler rises up the pipe A while cold water descends to the boiler through to pipe B, where it becomes heated in turns.

In this way, a circulation is set up with the result that  hot water storage tank gradually becomes filled with hot water from the top down wards.

The flow pipe A leaves the boiler at the top of the water tank while a pipe B is connected to the bottom of the boiler.

Hot water for use is taken from the pipe leading from the top of the lost water storage tank.

Where water is run off enters the hot water through pipe C thus system is kept full of water and no air can enter.

The water level in the cold tank is maintained by supply from the main which enters there is a ball/cork.

An expansion pipes rises from the top of the hot tank and is bent twice at right angles so that its end is over the cold tank.

This is a safety precaution. If the heater, over heats the water, and the water boils, steam and hot water are discharged homelessly into the cold water tank and no damage results.

The expansion pipe also permits the escape of dissolved air which comes out of the water when it is heated as otherwise this may cause trouble as some air block in the pipe.

 

SEA AND LAND BREEZE

 

Sea breeze [day]

CONVECTION 3

During the day, the temperature of the land rises faster than the temperature of the sea water and the air over land becomes warmer than that over sea.

The warm air of less density rises from land allowing the cold dense air over sea to blow to the land; this creates a sea breeze during day.

LAND BREEZE

CONVECTION 4

 

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ASSIGNMENT : CONVECTION assignment MARKS : 10  DURATION : 1 week, 3 days

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