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  Liquids and gases are fluids.The particles in these fluids can move from place to place.
Convection occurs when particles with a lot of heat energy in a liquid or gas move and take the place of particles with less heat energy. Heat energy is transferred from hot places to cooler places by convection.

To understand convection in fluids, let's consider the heat transfer through the water that is being heated in a pot on a stove.
Of course the source of the heat is the stove burner. The metal pot that holds the water is heated by the stove burner. As the metal becomes hot, it begins to conduct heat to the water.
The water at the boundary with the metal pan becomes hot. water (fluids) expand when heated and become less dense.
So as the water at the bottom of the pot becomes hot, its density decreases.
Differences in water density between the bottom of the pot and the top of the pot results in the gradual formation of circulation currents. Hot water begins to rise to the top of the pot displacing the colder water that was originally there. And the colder water that was present at the top of the pot moves towards the bottom of the pot (replace the hot water that raises up) where it is heated and begins to rise.
These circulation currents slowly develop over time, providing the pathway for heated water to transfer energy from the bottom of the pot to the surface.

WE can summarize the convection current as:

1- When the fluid is heated, it will expand.
2- As the fluid expand. its density will decrease.
3- As the density of the hot water decreases it will raises up to the cold surface.
4- the cold water will move down from the surface to  replace the hot water.
And that is we called convection current.

What is convection?

YouTube Video

Convection current in liquids.

YouTube Video

How to calculate the Density of the matter?
 we calculate the density of the matter by this relation 

D = m /V
D is the density 
m is mass
V is volume

Application on convection 

  • In a refrigerator

  • The freezer at the top of the unit contains the evaporator, which cools the air.
  • As cold air is denser than warmer air, it sinks to the bottom of the freezer. 
  • There, the cold air flows through the vents into the refrigerator cabinet where it sinks to the bottom of the fridge.

  • Air which is warmed by the contents of the fridge rises as the cold air displaces it, and the 'warm' air enters into the freezer compartment where the cycle begins again.

        Heat radiator:


    1- hot air expands, so it will has less density
    2- Hot air raises up as it is less dense.
    3- cold air will replace it, and it will be heated and so on.

    Day breeze and Night breeze:

    During the day, land heats faster than water, so the air over the land becomes warmer and less dense. 
    It rises and is replaced by cooler, denser air flowing in from over the water. This causes an onshore wind, 
     called a sea breeze. Conversely, at night land cools faster than water, as does the corresponding air.
    In this case, the warmer air over the water rises and is replaced by the cooler, denser air from the land,
     creating an offshore wind called a land breeze.