Convection Currents

Introduction

Tea is one of the most famous beverages in India and almost in the world. But have you ever made a cup of tea for yourself or any other? If not, then definitely you have seen at least. When the tea starts boiling then you must observe the tea leaves which were at the bottom start coming at the top of the pot.

Science is all about questioning, then start questioning how it is happening?

It is happening due to Convection current. But what is convection?

Convection Currents

Convection is the mode of heat and mass transfer in the fluids by the movement of fluids. Also we can say that the convection is the addition of Conduction plus Advection. Now you may ask what is Conduction and Advection? So conduction is the mode of heat transfer due to the temperature gradient and the advection is the bulk movement of the fluids. Now the movement of fluids due to the convection process is called Convection current.

Till now we have learnt that the convection is the mode of heat transfer by the movement of fluids, but what if I say that it happens also due to the density difference in the fluids. Yes, the movement of fluids also occurs due to the density difference. This phenomenon can be seen in the case of sea breeze. In this case, the air near the hot sand heats up and its density decreases, as the density decreases it starts rising up, at the same time the air over the sea is colder and having a higher density. This colder air having high density will occupy the place of low density hot air. Now due to this phenomenon a current will form between sea and land. And this is also called Convection current.

Convection current is of two types, namely -

• Natural convection: In natural convection, the movement of fluid is due to the density difference which arises due to the temperature difference. Since the less dense fluid moves upward and denser fluid moves down due to buoyancy.

• Forced convection: In forced convection, there is a need for external force like fan or pump.

Convection Current in Real Life

There are many examples in our day to day life which show convection current. Let us see a few of them one by one.

• Making of tea or coffee: During the boiling process of tea or coffee, the fluid which is in immediate contact with the bottom of the pot gets hotter first than which is away from the bottom. This will create a temperature difference and thus the density difference. Due to this the hotter fluid starts moving up and the colder fluid starts moving down and sets up a convection current.

• Thunderstorm: It is a natural phenomenon in which there is formation of clouds due to the convection. In this the warm water from the top of the ocean evaporates to form saturated droplets of water which forms clouds. Then these smaller clouds join to form the larger one.

• Refrigerator: We have seen at our home that the freezer portion of the refrigerator is always at the top of the Refrigerator. But why? The reason is that the colder air from the freezer is having more density and will go down.

  Similarly, the hot air from the bottom will come into the freezer to get cold and the circulation will keep the refrigerator at the desired temperature.

• Bonfire: We all must do the bonfire party, especially in the winters. But do you think that the heat from the bonfire will reach us? It is due to the convection current of the hot wire which is moving from the bonfire to you.

Factors Affecting the Convection Current

The convection depends on many factors and so does convection current. In this tutorial, we will study all those factors.

• Temperature difference: Temperature difference is very important for the convection current. It will cause the change in the density of the fluids so that the hotter or lighter fluid starts moving toward upward and the denser fluid starts moving toward downward.

• Presence of acceleration: Acceleration is very essential for the natural convection. The downward movement of fluid of high density is possible only due to the presence of acceleration due to gravity. In the absence of acceleration due to gravity, the Centrifugal acceleration or Coriolis acceleration will cause the natural convection.

• Geometry of the object The convection current depends on the geometry of the body.

Images Coming soon

Figure - 1

From the figure - 1, we can see that the nature of convection current for the different shapes of objects is different.

Convection Current on Earth

On the Earth, whether it is air or ocean or even the molten metals inside the Earth, all are circulating by the phenomena of convection current.

• Convection current in air: As all we know that the hottest part of our earth is near the equator and the coldest part is near the poles. So the hot air which is lighter will rise up and the cold air which is denser will shift down. The cold air will come toward the equator to fill the vacuum created by the hot air and hot air will flow toward the poles. This convection is a continuous process and helps in maintaining the temperature of our earth.

• Convection current inside the Earth surface: The molten rocks or minerals inside the earth are in the bottom part of the mantle by getting the heat from the core. So the density of these molten rocks decrease and they will rise up and the rocks present at the top of the mantle are cooler than it so they start shifting down. This process of convection current continues and some geologists think that this is one of the prominent reasons for the volcanoes, earthquakes etc.

• Convection current in the ocean: In the ocean, the water from the polar reason which is colder and the ocean water near the gulf which is hotter exchanges the water by the convection. It will help in maintaining the temperature of the oceans all over the earth.

So in this tutorial we have learned all the basics of Convection current. Now it's time to see some FAQs.

FAQs

Q1. What do you mean by Convection?

Ans - It is a mode of heat and mass transfer which occurs due to the density difference of fluids and the density difference arises due to temperature difference.

Q2. What do you understand with conduction?

Ans - Conduction is the mode of heat transfer by the virtue of temperature gradient.

Q3. What are the two types of convection?

Ans - Convection is of two types -

• Natural convection - In this, the movement of fluid is due to the difference in density which arises due to the temperature difference.

• Forced convection - In forced convection, external source is required for the movement of fluid.

Q4. Why is the freezer of the refrigerator kept at the top of the refrigerator?

Ans - The freezer is the coldest area of the refrigerator, so air in the freezer is very cold and dense. So it starts moving downward and absorbs the heat from the remaining part of the refrigerator. Also the warm air from the remaining part will move to the freezer and reject heat. That is why the freezer is kept at the top.

Q5. Name the three factors which are affecting the convection current.

Ans - The three factors are -

• Temperature difference of the fluid

• Presence of acceleration

• Geometrical shape of the body