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Convection- Definition, types, examples, convection current and equation, practice problems, FAQs

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.

Table of Contents

  • What is Convection?
  • Types of convection
  • Convection current
  • Convection equation
  • Practice problems
  • FAQs

What is Convection?

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. 

Natural convection

  • This convection takes place due to the density difference that arises because of the variation in temperature. The velocity of the fluid is negligible.
  • It takes place under the influence of gravity and the heat is transferred from the bottom to the top.
  • Example: The formation of land and sea breeze

Forced convection

  • Forced convection is a special type of heat transfer in which fluids are forced to move, in order to increase the heat transfer. This forcing can be done with a ceiling fan, a pump, suction device, or other.
  • The forced convection happens due to external factors. 
  • The velocity of fluid particles and the direction of heat flow completely depends on the external factor.
  • Example: Using a fan or cooler on a summer day to get relief from heat.

Convection example

  • Convection heat transfer is happening all around us. Wind is among the best examples of natural convection. During the day time, land heats up faster when compared to seawater. The air that comes in contact with the warm ground gets heated up by conduction. Therefore, its volume increases and it becomes less dense. However, the air which is present above the surface is cooler than air at the surface. This air is much more dense than hot air. The hot air lifts up and cold air fills its space. So during the day time wind flows from sea towards the land. However at night, the surface of the water is warmer than the ground. The wind flow reverses.

  • The cold air from the air conditioner comes down as it is heavy. The air closer to the surface is warm and lighter than cold air. So, it rises and the convection current sets up in this way. This makes the entire room cool. That's why air conditioner should be placed at some height. 


Convection current

  • The molecules that come in contact with the hot surface gain its heat energy and move away from each other. This results in a decrease in the density of the fluid. 
  • The air molecules that have less energy take the place of these hot molecules (lighter than cold molecules) which have already moved up. 
  • The molecules will get energy from the hot plate. This process continues till the entire fluid becomes hot. 
  • In simple words, this process of heat transfer is due to the heat current and it is known as convection current. Overall, in this mode of heat transfer, there is an actual motion of molecules.

Convection equation

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:

  • Density
  • Viscosity
  • Thermal conductivity
  • Specific heat capacity

Practice problems

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?

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.

FAQs

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?

  1. The molecular motion of fluids gets aligned.
  2. Molecular collisions within the fluid increase.

(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?

  1. Sea and Land breeze
  2. Boiling of water
  3. Heating air around a furnace
  4. Warming of glass of bulb due to filament

A: Heating of glass bulbs due to filament is primarily due to radiation not by convection.

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