Heat Transfer – Conduction, Convection and Radiation

Heat Transfer: Heat is a form of energy that transfers from a hot object to a cold object. When we touch a cold object, heat from our body flows to the cold object. The flow of heat occurs between objects having different temperatures. Once the two objects attain the same temperature, the flow of heat stops. Let us learn more about Heat Transfer.

What is Meant by Heat Transfer?

Heat transfer is the phenomenon in which energy from a body at a higher temperature flows to a body at a lower temperature. The energy in transit is called heat. When we hold an ice cube in our palm, our palm feels cold. This happens because heat from our body gets transferred to the ice cube. This loss of heat from our body reduces the temperature of our palms and makes us feel cold. The higher the temperature difference between two objects, the faster heat transfer occurs between them.

What are the 3 Types of Heat Transfer?

Heat transfer takes place from a hot body to a cold body by any of the following three modes:
1. Conduction
2. Convection
3. Radiation

Conduction

When heat transfer takes place in a medium from one point to the other only due to the vibrations of the medium particles, without their net motion, it is known as conduction. When a solid is heated to one end, the energy of the atoms or molecules present on that part increases, and they start vibrating faster at their mean position. These high energy atoms and molecules collide with their neighbouring particles and transfer their energy to them. This process continues until the whole solid is heated uniformly. In the process of thermal conduction, the particles of a solid do not move from one place to another.

They can only execute vibrational motion about the fixed position. The conduction method of heat transfer occurs within a solid or when a hot body is in direct contact with a cold body. Heat Transfer via conduction mainly occurs in solids. It does take place in liquids but is very insignificant.

All metals and their alloys are good thermal conductors. Mercury exists in the liquid state, but it transfers heat by the process of conduction. This property of mercury makes it a suitable heat-sensitive material because of which it is used in thermometers to measure the temperature of a body.

Thermal Conductors and Insulators

The objects that allow heat to pass through them are known as thermal conductors. For example, iron, copper, steel, aluminium and silver are good conductors of heat.
The objects that do not conduct heat are known as thermal insulators or poor conductors of heat. For example, plastic, wood, air, bakelite and glass are thermal insulators.
Cooking utensils are made up of metals and alloys like steel and aluminium because they conduct heat and help in fast cooking. But handles of cooking utensils are made of wood and bakelite because they do not conduct heat and protect us from getting burnt while handling a hot utensil.

Convection

Liquids and gases are poor conductors of heat. The transfer of heat in fluids (liquids and gases) occurs mainly by the process of convection because particles of fluids are loosely packed and are to move from one place to another place. In the convection method of heat transfer, the particles of a fluid (liquid and gas), after gaining heat energy, move from the hotter region to the colder region of the fluid.

For example, when a liquid is heated in a tea kettle, as shown below, the particles near the source of heat gain energy, due to which they spread farther away in the liquid. This results in a decrease in the density of the liquid near the heat source. The colder region of the liquid is still dense. The hot liquid starts rising up because of the reduced density, and the cooler liquid starts sinking and come near the source of the heat. Now, these particles gain heat and rise up. In this way, a convection current sets up in the liquid.

The convection method of heat transfer does not occur in a solid because particles of a solid are tightly packed and cannot move from one place to another.
Let us take the example of a room heater to understand the convection in air. We all know that a room heater is always placed on the floor of the room because it heats up the air present near the room’s floor. Since warm air is lighter than cold air, it rises, and the cold air sinks down and occupies the low-pressure region created by the warm air. Now, this cold air is heated by the heater and rises. This causes more cold air to sink down near the heater setting up a convection current. Thus, all the air in the room is heated.

Sea Breeze and Land Breeze

The sea breeze and land breeze are natural phenomena that are based on the convection method of heat transfer in the air. During the daytime, the land gets heated faster than the sea water by absorbing the heat energy from the sun. The warm air above the land is less dense as compared to the air above the surface of the sea, so it rises up, creating a low-pressure area. The cold air from the sea rushes towards the land to occupy the low-pressure area created by the warm air.

This movement of cold air from the sea towards the land is called sea breeze. The land loses heat faster than seawater at night, and seawater remains warmer for a long time. So, the warm air over the sea rises upwards, and the cold air from the land rushes towards the sea to occupy the low-pressure region created by the warm air above the surface of the sea. This movement of cold air from the land towards the sea is called land breeze.

Radiation

When we sit near a bonfire, we feel warm because heat from the bonfire reaches us by radiation. Thermal radiation or simply radiation is a mode of heat transfer that does not require any material medium to transfer heat. It can occur in a vacuum too, and it is the fastest mode of heat transfer.

The radiation mode of heat transfer happens through photons. All hot bodies emit heat radiations in all directions. Objects on which these heat radiations fall experience a sense of warmth. Our earth receives the heat from the sun by the method of thermal radiation.

The amount of heat that a body can emit or absorb depends on the colour of the body. Dark objects are good absorbers as well as good emitters of heat. On the other hand, light-coloured objects are good reflectors of heat. This is the reason because of which we prefer to wear white or light-coloured cloth in summer because it reflects most of the heat radiations falling on it and makes us feel cool. The inside of a solar cooker is painted black to increases the absorption of heat from the sun. The car radiators are painted black to increase the emission of heat by radiation.

Thermos Flask

Thermos flask is a storage container that does not allow heat transfer between the liquid which is kept inside it and the outside environment. It is also known as the vacuum flask. It keeps the hot liquid kept in it hot and the cold liquid kept in it cold. It is a double-walled bottle, the space between which has a vacuum. This space does not allow the transfer of heat from the liquid kept in it to its surroundings by the process of conduction and convection. The walls of the thermos flask are coated with silvery paint, which prevents absorption and loss of heat by the process of radiation.

Summary

From this article, we conclude that heat always travels from a body at a higher temperature to a body at a lower temperature. The conduction of mode of heat transfer occurs within a solid by the process of collision between its atoms or molecules. The convection mode of heat transfer occurs in liquids and gases where atoms and molecules are to move. The radiation mode of heat transfer can occur in the presence of a material medium or even in a vacuum.

FAQs Based on Heat Transfer

Q.1. Which mode of heat transfer is the fastest?
Ans: Radiation is the fastest mode of heat transfer.

Q.2. Why do we prefer to wear woollen clothes in winter?
Ans: We prefer to wear woollen clothes in winter because woollen clothes trap the air in their fibres. Air is an insulator of heat. It prevents the transfer of heat from our body to the surroundings.

Q.3. What is meant by heat transfer?
Ans: Heat transfer is a phenomenon of transferring energy from a body at a higher temperature to a body at a lower temperature.

Q.4. What are the three types of heat transfer?
Ans: Conduction, convection and radiation are the three types of heat transfer.

Q.5. What is the convection mode of heat transfer?
Ans: The convection mode of heat transfer occurs in liquids and gases. In the convection mode of heat transfer, the particles of a liquid or gas move from one place to another place to transfer heat.

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