Ever wondered the reason behind the change in seasons all around the year? Well the answer has to do with the convection currents that flow all over the country and thus is responsible for different seasons. To know more about this we need to have an insight in what is convection current or more important what is convection?
Ever wondered what makes people feel cold sitting under a fan on a hot summer day or what makes the flowing air providing more comfort compared to still air? To know the reason for such questions one needs to have knowledge about a mode of heat transfer known as convection.
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Convection is heat transfer by the bulk movement of molecules within fluids such as liquids and gases.
Initially heat transfer between the fluid and object takes place through conduction, but the after bulk heat transfer takes place due to the motion of the fluid. This mode of heat transfer by actual movement of medium is called convection.
The figure above shows one of the consequences of convection. When the fluid in contact with the beaker gets heated its density decreases and thus moves upwards whereas cold fluids move downwards and take heat from the beaker and thus again move upwards.
There are two types of convection namely natural and forced convection.
The rate of heat transfer by convection is given by,
where,
h = Convection coefficient
A = Surface area of the body
T = Temperature of the body
T0 = Temperature of the surrounding
Thermal resistance
Here the value of heat-transfer coefficient depends on:
Q. A spherical body having radius is transferring heat to its surrounding through convection. Find the temperature difference between surrounding and the body surface, if the rate of heat transfer is
(Convection coefficient, )
A.
Given,
Radius of sphere,
Surface area,
Rate of heat transfer,
Convection coefficient,
We know,
The temperature difference between surrounding and the body is
Q. If the temperature of the surrounding is increased keeping all other parameters constant, then what will happen to the rate of heat transfer from a comparatively hotter body due to convection?
A We know that, rate of heat transfer in convection,
As the surrounding temperature increases, will decrease.
Thus, rate of heat transfer will decrease.
Q. Draw the curve showing the relation between convective resistance and radius of hot sphere being placed in a stream of cold air?
A. We know that, convective resistance,
[]
Relation is identical to the relation of the hyperbola, . Thus, the curve between convective resistance and radius of sphere is a hyperbola.
(Recreated the image)
Q. A new computer chip with a surface area of generates of heat. Determine the convective heat transfer coefficient needed to keep the temperature of the chip less than above the environmental temperature, if the chip is to be cooled with liquid coolant.
A.
Surface area,
Rate of heat transfer,
Convection coefficient
Temperature difference,
We know,
This value of corresponds to forced convection range of liquid and cannot be achieved using air.
Q. Why is it hotter for the same distance over the top of the fire than it is by the side?
A: At a point in front of fire, heat is received significantly due to the process of radiation, while at a point above the fire, heat reaches both due to radiation and convection.
Q. Why does convection occur when fluids are heated from the bottom?
(C) The heated fluid becomes denser than the cold fluid above it.
(D) The heated fluid becomes less dense than the cold fluid above it.
A:
The fluid in contact with the bottom becomes less dense due to heating and thus rises upwards whereas the relatively colder fluid which is more dense descends lower. After receiving heat from the bottom its density decreases and thus again rises upwards. In this way the convection occurs when fluids are heated.
The density of a fluid decreases as the molecules of the fluid move away from each other. It happens to the molecules that are at the bottom. This leads to convection current. So, the heated fluid at the bottom becomes less dense than the cold fluid above it.
Thus, option (D) is the correct answer.
Q. Is the sea breeze an example of forced or natural convection?
A: Natural Convection
Q. In which of the following processes, convection does not take place primarily?
A: Heating of glass bulbs due to filament is primarily due to radiation not by convection.