Convection: definition, types, land and sea breezes, trade winds and applications (2023)

Convectionis a predominant route by which heat transfer occurs. Have you ever seen a pot of boiling water? The heat is provided right at the bottom of the pot, but we see that after some time the water at the top of the pot starts to boil! How does this happen? How does the heat get from the bottom of the pan to the top? By CONVECTION.

Convection plays a significant role in our daily life. From heating our homes on a hot summer day to keeping our rooms cold on one

On a winter night, convection is responsible for everything.

What is convection?

The process by which heat transfer takes place from one point to another within a medium due to the actual movement of the heated particles of the medium is called convection. It is therefore a heat transfer mode with the actual movement of matter. It is only possible in liquids.

1. Molecules of liquids and gases are much further apart than molecules of solids, which is why molecules of liquids and gases can move about their average position much more freely. Therefore, heat transfer by convection is possible within liquids.

2. Convective heat transfer cannot take place in solids because the molecules in solids are fixed and cannot move freely.

3. Convective heat transfer cannot take place in a vacuum because there are no particles in a vacuum that can move and transfer heat.

How does heat transfer by convection occur?

Convection is the movement of a liquid driven by temperature differences in that liquid. When a liquid is heated, the area closest to the heat source becomes less dense due to the increase in kinetic energy in the particles. The less dense portion of the fluid then rises while the more dense portion of the fluid sinks. The process repeats itself because less dense liquids cool as they move away from their heat source, causing them to sink, while more dense liquids heat up as they approach the heat source, causing them to rise. This creates convection currents.

types of conventions

Based on how liquid flow takes place in the medium, convection can be broadly classified into two types:

a. forced convection
B. Natural convection.

forced convection

As the name suggests, forced convection requires a source to push moving fluid particles. In forced convection, the material is forced to move by a pump or other physical medium, which initiates the heat transfer process of the material. Common examples of forced convection systems are air conditioning, central heating, heating a liquid by rapid stirring are examples of forced convection heat transfer.

In addition, in the human body, the heart acts as a pump that circulates blood to different parts of the body, transferring heat through forced convection and keeping it at an even temperature. In air conditioners, the refrigerant-cooled air is expelled into the room by a pump, and the cooling system of automobiles using the engine as a 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 type of heat transfer. When we heat a liquid, it begins to expand - the volume of the liquid increases and the density of the liquid decreases. The less dense liquid rises and the colder, denser liquid from the upper layers sinks due to liquid buoyancy. This process continues and each time the heated layer rises, a cooler layer of liquid descends.

Thus heat is transferred from bottom to top. The process continues, hence this type of heat transfer is called natural or free convection. Convective heat transfer differs significantly from conduction. In the case of conduction, the particles in the medium do not leave their position, while in the case of convection, there is mass movement of the particles in the medium.

So, free convection results from uneven heating of the liquid, causing warmer liquids and less dense molecules to rise and be replaced by cooler molecules, creating a natural convection flow. Different types of winds around the world are caused by heat transfer due to convection.

Natural convection plays an important role in the natural ecosystem. Determines the wind pattern in an area and thus controls the weather. Below are some common phenomena associated with it:

sea ​​wind and land wind

During the day, the ground warms up much faster than large bodies of water such as rivers or oceans. There are essentially two factors responsible for this:

1. Water has a higher specific heat than air; hence the water takes much longer to heat up to the same volume.
2. The mixing of the currents disperses the heat absorbed by the large body of water.

Now, during the day, the air in contact with the hot ground is heated by conduction. As air heats up, it expands and becomes less dense than the cooler air around it. As a result, warm air rises closer to the ground (representing air currents) and cooler (denser) air above bodies of water moves (including winds) to fill space. This creates a sea breeze near a body of water. A convection cycle occurs between land and water.

At night the cycle is reversed. Earth loses its heat faster than water. Eventually, the air over land becomes cooler, while the air just above the water surface stays warm. Warmer air above the water surface becomes less dense and rises; the coldest air on earth rushes in to take their place. This forms a land breeze.

Passatwinde

These are constant surface winds that blow northeast from the Earth toward the equator. These are caused by uneven heating of the equatorial and polar regions of the earth.

Because the equator is closer to the sun, it is warmer while the poles are colder. Therefore, the air at the Earth's surface near the equator is warmer than the air in the upper atmosphere at the poles. Under ideal conditions, this creates a convection current around the earth. Warmer air near the equatorial surface rises, and this less dense air begins to move poleward, sliding and flowing toward the equator.

However, the direction of this convection current changes due to the Earth's rotation. Because of this motion, the air near the poles is almost at rest, while the air near the equator has an eastward velocity of \({\rm{1600}}\,{\rm{km/h}}.\ ) As has As a result, the air does not sink at the poles, but at latitude \({\rm{30}}{\,^{\rm{o}}}{\rm{N}}\) (North) and returns to the equator. The winds formed in this way are called trade winds.

Monsoon

In summer, the peninsula of Central Asia is much warmer than the surrounding waters of the Indian Ocean. Warm air over the warmer landmass rises and moves towards the Indian Ocean. Cooler, wetter air from over the Indian Ocean sinks towards the warm landmass. This air rises as it crosses the mountains. Humid air cools as it rises to a higher altitude. This causes moisture to condense and settle on the earth's surface.

convection in everyday life

1. hot air balloon– The air near the heater placed in the balloon rises with heating and pushes the balloon up. When the balloon needs to be lowered, the pilot vents some of that hot air, and the cold air from the environment enters in its place, moving the balloon down. Finally, the balloon approaches the ground.
2. chimney effect– This effect, also known as the chimney effect, is generally observed in tall buildings where the inside temperature is significantly higher than the outside temperature. Due to the buoyancy, warmer room air rises and pushes towards the ventilation openings; cooler air from outside flows in to take its place, causing air to flow throughout the building.
3. Gravitational convection– It is the convection of the contents of a liquid, such as mass or heat, produced by currents induced in the liquid due to buoyancy forces. It is affected by gravity acting on density differences within the liquid. The diffusion of dry salt into moist soil is an example of this type of convection.
4. ocean circulation– The circulation of ocean water around the globe is attributed to convection. Colder water from the poles moves toward the equator, while warmer water flows from the equator toward the poles.
5. cloud fluctuation– The accumulation of clouds, which eventually lead to precipitation, also occurs due to the formation of convection currents between land and water.
6. storms– Squall Lines are a type of convection storm. These include a series of storms accompanied by strong winds and rain caused by convective currents.
7. boil water– When we put the pan on the fire, the heat of the flame is transferred from the pan to the water particles closest to the bottom. These particles rise after heating and the cooler water at the top sinks to take their place and eventually all the water is heated enough to reach the boiling state.
8. ventilation: Convection plays an important role in ventilating our homes and offices.
9. cooler– Radiators that are placed in rooms for heating work on the principle of convection.
10. A full cup of tea– We can see steam when we drink a cup of hot tea. This indicates that heat is being transferred from the tea to the air.
11. melting ice– The heat from the air is transferred into the ice and eventually all layers in the ice begin to heat up. This melts ice from solid to liquid.
12. Frozen Food Trick– When working in the kitchen, we regularly use frozen food, and it is much more difficult to extract it with ordinary tap water, but under cold running water, frozen food thaws faster due to convection.
13. operation of a fan– If a fan, pump or suction device is used to facilitate convection, forced convection will result. Fans push cooler air down while warmer air rises to take its place.
14. human body temperature. In the human body, the heart works as a pump that circulates blood to different parts of the body. It is again an example of heat transfer through the process of forced convection. Pumping blood around our body helps maintain a constant body temperature.

summary

The process by which heat transfer takes place from one point to another within a medium due to the actual movement of the heated particles of the medium is called convection. It is therefore a heat transfer mode with the actual movement of matter. Convection can be roughly divided into two types:

1. Natural Convection: In this type of convection, the heated material moves due to the difference in density. Gravity plays a significant role in this type of heat transfer. Each time the heated layer rises, a cooler layer of liquid descends. Thus heat is transferred from bottom to top.
2. Forced Convection: Requires a source to push moving fluid particles. In forced convection, the material is set in motion by a pump or other physical medium that sets up the material's heat transfer process - for example, an air conditioner.

Frequently asked questions about convection

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

F.2. What is convection?
Answer:The process by which heat transfer takes place from one point to another within a medium due to the actual movement of the heated particles of the medium is called convection. Cooling a room with an air conditioner or heating it with a radiator can be attributed to convection.

F.3. What is a sea breeze?
Answer:The earth warms up faster than the water during the day. Because of this, warming makes the air over land less dense, while the air over water is relatively cooler. This warm air from over the land rises while cooler air from over the waters pours in to take its place. This forms the sea breeze.

F.4. What types of convection are there?
Answer:There are two types of convection:
a. natural convection
B. forced convection

F.5. Define forced convection.
Answer:When external sources such as fans and pumps create induced convection, it is called forced convection.