Convection is a prevalent way by which the transfer of heat takes place. Have you ever seen a pot of boiling water? Heat is supplied directly at the bottom of the pot, yet we see that water at the top of the pot starts boiling after some time! How does that happen? How does the heat travel from the bottom of the pot to its top? By CONVECTION.

Convection plays a significant role in our day-to-day life. From warming up our homes on a hot summer day to the cold sensation in our rooms on a

winter night, convection is responsible for it all.

What is Convection?

The process by which the transfer of heat takes place from one point to another within a medium due to the actual motion of the heated particles of the medium is known as convection. Thus, it is a mode of heat transfer with the actual motion of matter. It is possible only in fluids.

1. The molecules of liquids and gases are much farther apart than the molecules of solids, and that is why the molecules of liquids and gases are muchr to move about their mean position. Thus is why heat transfer due to convection is possible within fluids.

2. Transfer of heat by convection cannot occur in solids because molecules in solids are fixed and cannot movely.

3. Transfer of heat by convection cannot occur in a vacuum because there are no particles of any kind in a vacuum that can move and transfer heat.

How Does Heat Transfer by Convection Take Place?

Convection is the motion of a fluid-driven by the differences in temperature across that fluid. When a fluid is heated, the region closest to the heat source becomes less dense due to increased kinetic energy in the particles. The portion of the less dense fluid then rises, while the denser portion of fluid sinks. The process repeats itself because the less dense fluids cool down as they move away from their heat source, making them sink, while the denser fluids heat up as they near the heat source, making them rise. This creates convection currents.

Types of Convention

Based on how the flow of fluid is taking place in the medium, convection can broadly be categorised  into two types:

a. Forced Convection
b. Natural Convection.

Forced Convection

As the name suggests, forced convection requires a source to push the particles of fluid into motion. In forced convection, the material is forced to move by a pump or other physical means, which sets up the material’s heat transfer process. The common examples of forced convection systems are air-conditioning, central heating system, heating a liquid by brisk stirring are examples of forced convection heat transfer.

Also, in the human body, the heart acts as the pump that circulates blood through different parts of the body, transferring heat by forced convection and maintaining it at a uniform temperature. In air conditioning systems, the cooled air from the refrigerant is blown out by a pump into the room, and the cooling system of automobiles with the engine as its pump follows a similar process.

Natural Convection

In natural convection, the heated material moves due to the difference in density. Gravity plays a significant role in this mode of heat transfer. When we heat a fluid, it starts to expand—the volume of fluid increases, and the density of fluid decreases. The less-dense fluid rises, and the colder, denser liquid from the upper layers rushes down due to the buoyancy of the fluids. This process goes on, and each time the heated layer shifts up, a colder layer of fluid rushes down.

Thus the heat is transferred from the bottom to the top. The process goes on then this mode of heat transfer is called natural or convection. Heat transfer due to convection is significantly different from conduction. In conduction, the medium particles do not leave their position, while in the case of convection, there is a bulk motion of particles of the medium.

Thus, convection results from the unequal heating of the fluid, which causes the heated and less dense molecules fluids to rise and are replaced by the colder molecules, thus setting up a natural convection current. Different kinds of winds all across the globe are caused by the transfer of heat due to convection.

Natural convection plays a major role in the natural ecosystem. It decided the wind pattern in an area, and hence it controls the weather. Following are some common phenomena associated with it:

Sea Breeze and Land Breeze

During the day, the ground gets heated up more much faster than the large bodies of water like rivers or oceans. There two main factors responsible for this:

1. The water has a greater specific heat than air; thus, water takes a much longer time to heat up for the same volume.
2. The mixing of currents disperses the absorbed heat throughout the great volume of water.

Now, in the daytime, the air in contact with the warm ground is heated by conduction. As the air heats up, it expands, becoming less dense than the cooler air in its surroundings. As a result, the warm air closer to the ground rises (which constitutes the air currents), and the cooler (denser) air above the water bodies moves (which includes the winds) to fill the space. This results in the formation of a sea breeze near a water body. A convection cycle is set up between land and water.

At night, the cycle is reversed. Land loses its heat faster than water does. Thus, eventually, the air above land becomes cooler, while the air just above the water surface remains warm. The warmer air from above the water surface becomes less dense and rises; the cooler air from the land rushes in to take its place. This forms a land breeze.

Trade Winds

These are the steady surface wind that blows in the north-east direction on the earth, towards the equator. These are formed due to uneven heating of the equatorial and polar regions of the earth.

Since the equator is closer to the sun’s location, it is warmer while poles stay cooler. So the air at the earth’s surface near the equator is hotter than the air in the upper atmosphere of the poles. Thus, in ideal conditions, a convection current will be set up around the earth. The warmer air near the equatorial surface rises, and this less dense air starts moving towards the poles, sliding and streaming in towards the equator.

However, the direction of this convection current gets modified due to the rotation of the earth. Due to this motion, the air close to the poles is almost at rest while the air near the equator has an eastward speed of \({\rm{1600}}\,{\rm{km/h}}.\) As a result, the air descends not at the poles but \({\rm{30}}{\,^{\rm{o}}}{\rm{N}}\) (North) latitude and returns to the equator. The winds so formed are called trade winds.

Monsoons

During the summertime, the peninsular mass of central Asia gets much more heated up than the surrounding waters of the Indian ocean. Hot air from above the warmer landmass rises and moves towards the Indian ocean. The cooler and moisture-filled air from above the Indian ocean descend towards the warm landmass. This air, when it comes across mountains, it rises up. The moist air, on reaching greater height, gets cooled. Thus, the moisture gets condensed and precipitates over the surface of the earth.

Convection in Everyday Life

1. Hot air balloon – The air near the heater placed inside the balloon rises upon heating and forces the balloon upwards. When it is required to lower the balloon, the pilot releases a part of this hot air, and cool air from the surroundings comes in to take its place and shifts the balloon downwards. Eventually, the balloon lowers itself closer to the ground.
2. Stack effect–  Also known as the chimney effect, this effect is generally seen in tall buildings where the indoor temperature is significantly higher than the outside temperature. The warmer indoor air rises due to buoyancy and pushes towards the exits; the cooler air from outside rushes in to take its place, thus causing the air to move across the building.
3. Gravitational convection – This is the convection of contents of a fluid, such as mass or heat, produced by the currents induced in the fluid due to the buoyant forces. It is affected by gravity that acts on the density differences within the fluid. Diffusion of dry salt into wet soil is an example of this type of convection.
4. Oceanic circulation – The circulation of ocean waters all over the globe is attributed to convection. The cooler water from the poles shifts towards the equator, while the warm water from the equator moves towards the poles.
5. Floating of clouds – The build-up of clouds that eventually lead to rainfall also occurs due to the formation of convective currents between land and water.
6. Thunderstorms– Squall lines are a type of convective thunderstorm. These comprise of a line of thunderstorms accompanied by high wind and blowing rain caused by the convective currents.
7. Boiling of water – As we put the pot on a flame, the heat from the flame gets transferred from the pot to the water particles closest to the bottom. These particles rise after heating, and the cooler water from the top shifts down to take its place, and eventually, the entire water gets heated enough to reach the boiling state.
8. Ventilation: Convection plays a major role in the ventilation at our homes and offices.
9. Radiator – Radiators put up in the rooms to provide heating works on the principle of convection.
10. A cup full of tea – We can see the steam when drinking a cup of hot tea. This indicates that heat is being transferred from tea into the air.
11. Melting of ice– The heat from the air gets transferred within the ice, and eventually, all the layers within the ice start to get heated. As a result, the ice melts from a solid to a liquid.
12. Frozen food hack– While working in the kitchen, we use frozen food items regularly, and it is much harder to extract them using normal tap water, but when placed under cold running water, frozen food thaws more quickly due to convection.
13. Working of a fan– When a fan, pump, or suction device is used to facilitate convection, the result is forced convection. Fans push the cooler air downwards as the hotter air rises to take its place.
14. Human body temperature. In the human body, the heart acts as a pump that circulates blood through different parts of the body. It is again an example of heat transfer by the process of forced convection. The pumping of blood across our body helps to maintain constant body temperature.

Summary

The process by which the transfer of heat takes place from one point to another within a medium due to the actual motion of the heated particles of the medium is known as convection. Thus, it is a mode of heat transfer with the actual motion of matter. Convection can broadly be categorised into two types:

1. Natural Convection: In this convection type, the heated material moves due to the difference in density. Gravity plays a significant role in this mode of heat transfer. Each time the heated layer shifts up, a colder layer of fluid rushes down. Thus the heat is transferred from the bottom to the top.
2. Forced Convection: It requires a source to push the particles of fluid into motion. In forced convection, the material is forced to move by a pump or other physical means, which sets up the material’s heat transfer process—for example, air conditioning system.

Frequently Asked Questions on Convection

Q.1. Can we boil water inside an earth satellite?
Ans: No, we can not boil water inside a satellite. This is because the water, when heated, becomes less dense and it rises. Within a satellite, due to lack of gravity, this is not possible.

Q.2. What is convection?
Ans: The process by which the transfer of heat occurs from one point to another within a medium due to the actual motion of the heated particles of the medium is known as convection. Cooling of a room due to AC or warming up due to a radiator can be attributed to convection.

Q.3. What is a sea breeze?
Ans: The land during the day gets heated up faster than water. Due to this, the air above land becomes less dense due to the warming, while the air above the water is relatively cooler. This warm air from above the land rises while the cooler air from above the water bodies rushes in to take its place. This forms the sea breeze.

Q.4. What are the types of convection?
Ans: There are two types of convection:
a. Natural Convection
b. Forced Convection

Q.5. Define Forced convection.
Ans: When external sources such as fans and pumps create induced convection, it is known as forced convection.