[Physics Class Notes] on Convection Currents Pdf for Exam

What is Convection?

Convection is a process of heat transfer by the movement of fluids (gas or liquid) between regions of various temperatures. If convection occurs naturally it is called natural convection or free convection. If convection is forced to occur, say, if fluid is circulated using a pump or a fan, it is called forced convection.

What are Convection Currents?

Convection currents are generated by the differences in densities of the fluid that occur due to temperature gradients. The activity that results from the continuous replacement of the heated fluid in the area of the heat source by the nearby present cooler fluid is called a natural convection current. The heat and the mass transfer which is enhanced due to this natural convection current are called natural convection heat and mass transfer. 

In natural convection, the heat and matter are said to be moved from one location to the other one. The convection currents are linked with natural convection in which the fluid motion occurs naturally such as buoyancy (Optimism).

The convection current cannot take place in solids as the particles within the solids cannot flow easily and most of the free movement is mainly because of the difference in the density that is caused due to a huge transfer of heat between the plates.

Examples of Convection Current:

1. Refrigerator:

In the case of a refrigerator, the freezer unit is present at the top of it. The main reason is that the warm air present inside the refrigerator will rise upwards and the cold air in the freezer point will move downwards and it will keep the lower portion of the refrigerator in a warm condition.

2. Thunderstorm:

A thunderstorm can be the best example of convection currents. The warm water in the air rises upwards and turns into saturated water drops which form the clouds. In this process, the smaller clouds run into each other and hence the bigger clouds are formed. The thunderstorms or cumulonimbus clouds are formed on reaching the final growth stage.

3. Steaming Beverage:

Steaming beverages are a simple example of convection. It is usual that steam comes out from a cup of hot coffee or tea. The warm air that is in the steam rises upwards which is due to the heat of the fluid.

4. Campfires:

The reason for the hotness above the campfire than the heat next to it is due to the convection currents. If you place your hands in front of the campfire (of course, at a safe distance; in any case, do not place the hand above the fire), you can feel the heat which is due to the presence of a few convection currents that rises up towards you.

Why do Convection Currents Form?

The difference in temperature level causes the particles to move to result in the creation of the current. In plasma and gases, the difference in temperature leads to regions of low and high density, where molecules and atoms move to fill in the low-pressure areas. Unless energy sources like sunlight, heat, etc are present, convection currents will continue until a uniform temperature is reached.

How are Convection Currents Created?

Based on three physical assumptions, convection currents are created. They are as follows:

Heat Source:

The presence of a heat source is important because the convection currents are generated by the differences in density of the fluid that occurs due to temperature gradients. In the case of natural convection, the fluid surrounding the heat source receives heat. Due to thermal expansion, it becomes less dense and rises above. The fluid’s thermal expansion plays an important role in the creation of the convection currents. In simple words, the components that are denser or heavier will move downwards while the less dense or lighter components will move upwards which leads to bulk fluid movement.

Presence of Proper Acceleration:

Natural convection occurs only in a gravitational field or when there is a presence of proper acceleration like Centrifugal force, Coriolis force, etc. It essentially does not operate in the earth’s orbit. For instance, other heat transfer mechanisms are required to prevent electronic components from overheating in the orbiting International Space Station.

Proper Geometry:

The magnitude and the presence of natural convection will also depend on the geometry of the problem. In the gravitational field, the presence of a fluid density gradient does not ensure the existence of natural convection currents. 

This problem can be demonstrated from the following figures, where the fluid is enclosed by two large horizontal plates with different temperatures. 

Case A: 

In this case, the temperature of the lower plate is higher than that of the upper plate. Here, there is a decrease in the density in the direction of the gravitational force. This geometry induces fluid circulation and through natural circulation, heat transfer occurs. Being warm in the process, the heavy fluid will move down while the lighter fluid will move upwards, cooling as it moves. 

Case B: 

In this case, the temperature of the lower plate is lower than that of the upper plate. Here, the density increases according to the direction of the gravitational force. This geometry leads to a stable temperature gradient, stable conditions and does not induce the circulation of the fluid. Also, the heat transfer occurs only through thermal conduction. 

Convection is different from conduction, which is the transfer of the heat between the substances in direct contact with each other. Convection currents transfer the heat through the mass movement of fluids such as water, molten rock, or air from one place to the other.

Convection in Ocean

In the oceans, the convection drives ocean currents like the Gulf Stream and the other currents which turn over and mix up the waters. From the higher latitudes, the cold polar water is drawn downwards and sinks into the bottom of the ocean. It is pulled downwards to the equator as the light and warm water rises upwards to the surface of the ocean. In order to replace the cold water that is pulled towards the south direction, the warm water is pulled towards northward. The soluble nutrients and heat are distributed around the world due to this process.

Convection in Air

The circulation of air in the earth’s atmosphere is driven by convection. Near the equator of the earth, the sun heats the air which becomes less dense and rises upwards. It cools down as it rises and becomes less dense than the air that is around, spreading out and descending again towards the equator. The constantly moving cells of cold and warm air are known as the Hadley Cells. It drives the circulation of air continuously at the surface of the earth is what we call wind. The atmospheric convection currents are also the reason for the clouds to be upwards.

Convection in Earth

There is a belief among geologists that the molten rock deep within the earth is circulated by the convection currents. Being in a semi-liquid state, the rock should behave like any other fluids, rising up from the bottom of the mantle after getting hotter and less dense from the heat of the core of the earth. The rock becomes relatively denser and cooler, sinking back down to the core as it loses heat into the earth’s crust. The constantly circulating cells of the cool and hot molten rock are considered to help in
heating the surface. Also, some Geologists believe that the convection currents within the earth are the contributing cause for earthquakes, volcano eruptions, and continental drift.

Convection Currents – Atmospheric Circulation

The atmospheric circulation is the most important phenomena in the terrestrial climate. It is the movement of air in the large scale and is a means by which the thermal energy together with ocean circulation is distributed on the surface of the earth. Each year, the atmospheric circulation of the earth varies but the large-scale structure of the circulation remains quite constant.

The atmospheric circulation is a consequence of the illumination of the earth by the sun and the laws of thermodynamics. It can be viewed as a heat engine driven by the energy of the sun and whose energy sinks ultimately in the blackness in the space and also the wind turbines are powered by the sun.

What Will Happen if Convection Currents on Earth are Stopped? 

Suppose, if all the convection currents on earth are stopped, it would affect us the worst. The amount of heat that is radiated from the sun sets the earth’s surface temperature. If there is no convection, then the equator will get hotter and hotter and the north and south poles will become cooler and cooler. 

The oceanic currents from the tropical regions will bring the warm water more towards the north and the currents from the cooler regions will bring the cool water towards the equator. Hence, if the convection is stopped completely, ocean currents will occur and the very low and very high temperatures will force the living beings on the earth to move away from the equator and poles.

Most of the rocks present in the earth convect on a large scale. The rocks can drift very slowly even though they are solids. Convection helped the formation of large islands. There won’t be any new volcanoes on islands if the rocks stop flowing inside the earth.

Impact of Convection on the Earth’s Climate

The convection that happens in the deep surface of the Earth’s mantle also will impact the climate and surface of the earth. Through the movements of the Ocean and Continental plates, convection influences the atmosphere. A massive amount of air is circulated by the atmosphere and the position of the basins and continents in the ocean changes as to how weather and air movement around the globe. The air and ocean current fluctuations allow the precipitation to move towards the various areas of the globe.

Also, it is supposed that the convection that happens in the earth’s mantle is responsible for the creation of the Earth’s magnetic field. Due to the flow of liquid iron through the mantle, the earth’s magnetic field arises and creates electric currents.

Facts About Convection

Since olden times, convection currents have been utilised for various daily life purposes like heating, ventilation, etc.

Convection currents are also helpful in mining sites. This is because, at times, these currents are able to assist ventilation in mines.

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