
The difference between natural convection and forced convection. The force that causes the convection currents.

Explanation of Solution
Convection:
It is the mode of heat transfer takes place between the solid materials and the fluids that is in motion. The kinetic energy of the molecules or atoms of a substance is measured in a scale known as temperature. In liquids and gases, the molecules can move freely. Every molecules possess different kinetic energies. When the molecules collide, the kinetic energy is transferred from high kinetic energy molecule to low kinetic energy molecule. This phenomenon is referred as kinetic energy theory. The heat transfer due to this phenomenon is known as convection.
Forced convection:
In convection heat transfer, when the motion of the fluid is caused by the external equipment such as fan and pump, or the wind is known as forced convection.
Natural convection:
In convection heat transfer, when the motion of the fluid is caused only by buoyancy forces is known as natural convection. It is also called as free convection. The buoyancy forces are resultant of density difference due to the temperature variation in the fluid.
Thus, the force that causes the convection currents is buoyancy force.
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