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Conduction, convection and radiation are the processes of heat transfer between two bodies. The three processes differ from each other in terms of how the process of heat transfer occurs in each one of the them. Heat transfer refers to the physical transmission of thermal energy between two bodies in the form of heat. The basic principles of heat transfer are temperature and heat flow. Temperature represents the total amount of thermal energy and heat flow describes the movement of thermal energy.
Heat can be transferred from one object to the other through conduction, convection or radiation. Though these processes are different from each other, however, in some cases they can occur simultaneously as well.
Read More: Thermal Properties of Matter
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Key Terms: Conduction, Convection, Radiation, Thermal Energy, Heat Transfer, Temperature, Buoyant Force, Density
What is Conduction?
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Conduction is the process in which heat transfer occurs by direct contact. In this process, transfer of heat takes place via direct molecular collisions. Thermal energy is transferred from the area of higher mechanical energy to the area of lower mechanical energy. The increase in mechanical energy of the area takes place as the high-speed particles collide with the low-speed particles. Physical contact is must for the heat conduction process.
Factors Affecting Conduction
The following are the factors affecting the conduction process:
- Temperature Gradient – The amount of heat transferred during the conduction process is directly proportional to the temperature gradient. Higher is the temperature difference, more will the heat transfer.
- Cross-Sectional Area – Cross-sectional area is one of the most significant factors that affect heat transfer. Objects with larger cross-sectional areas require more energy to heat up. A larger cross-sectional area also means that the heat will be lost at a fast rate.
- Length of the Path – The length of the path refers to the height of the object. Objects having greater height are supposed to take more time for the transfer of heat.
- Physical Material Properties – Objects which allow heat to transfer through their molecules are called heat conductors. Metals are considered very good conductors of heat.
Thermal Conductivity of Solids, Liquids & GasesThe thermal conductivity of solids is more than liquids. This is because the molecules in the solid material are very closely packed. In this arrangement, it becomes easier for the molecules to pass the thermal energy from themselves to their adjacent molecules and so on. In liquids and gases, the molecules are loosely packed and are far from each other, hence it takes more time for heat to transfer from one molecule to another. |
What is Convection?
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Convection is the process of heat transfer by the movement of molecules within a fluid (air or liquid). In this process, the initial transfer of heat between the source and the fluid occurs by conduction but the heat transfer within the fluid happens due to movement of the molecules.
- The molecules of the fluid expands because of the thermal energy.
- The molecules of the fluid at the bottom becomes hot first due to which their density decreases.
- The molecules in the upper layers are comparatively cold and thus has high density.
- The warm molecules with lower density rises up and the cold molecules with high density are pushed downwards.
- This process gets repeated again and again replacing the cold molecules with the hot ones leading to the transfer of heat to the entire fluid.
Convection cannot take place in solids as they do not let the bulk current flow. The molecules are so tightly packed that they do not leave the surface after being heated up.
Types of Convection
There are mainly two types of convection:
- Natural Convection – This type of convection is seen in oceanic winds. It occurs due to the difference in densities caused by the temperature difference. As a result of this, buoyant force acts leading to the convection.
- Forced Convection – Forced convections are carried out with the help of external sources like fans and pumps. Example of forced convection include geysers used for heating water.
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What is Radiation?
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The transfer of heat needs a medium for both the process of convection and conduction. In contrast to this, radiation is the process of heat transfer in which there is no contact between the two bodies. A common example of radiation is the heat coming from the sun, however there is no direct contact with the sun. Thermal radiation results from the random motion of molecules in the matter.
All objects around us absorb and emit radiation. In order to keep the temperature constant of the object, the absorption and emission of the energy should be balanced. When the absorption and emission is imbalanced then the transfer of energy takes place. If the absorption is more than the emission, then the temperature of the object rises. On the other hand, if the absorption is smaller than the emission, the temperature of an object falls down.
Check Also: NCERT Solutions for Thermal Properties of Matter
Difference Between Conduction, Convection and Radiation
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The key differences between conduction, convection and radiation are tabulated below:
| Parameters | Conduction | Convection | Radiation |
|---|---|---|---|
| Definition | It is the process in which heat transfer occurs between objects having direct contact of their surfaces. | It is a process in which heat transfer occurs due to bulk movement of molecules inside fluid. | In this process, heat transfer occurs between objects without any physical contact. |
| Example | Transfer of heat through direct contact from the source like a metal pan heated for cooking. | Boiling water is an example of convection. | Heat passing through a vacuum like the sun rays travelling in space and coming to the Earth. |
| Cause | The tightly packed molecules easily transfers heat energy to one another. | The hot molecules of water becomes less dense than the cold molecules when heat is supplied. This causes hot molecules to move up and cold molecules to settle down. | All objects radiate energy if they are at a temperature greater than 0 K. |
| Mechanism | It occurs due to molecular collisions in solid substances. | The bulk movement of molecules in liquid or air. | It occurs through electromagnetic waves. |
| Speed | It occurs at a slow rate. | It occurs faster than conduction. | It is the fastest among the three processes. |
Things to Remember
- Heat transfer is the process of movement of molecules from the region of high temperature to low temperature.
- Conduction, convection and radiation are the three ways of heat transfer.
- Conduction is the process in which thermal energy is transferred from one body to the other by direct contact.
- Convection occurs in fluids. Heat causes thermal expansion of the molecules due to which molecules flow in bulk leading to heat transfer.
- Radiation is the process of heat transfer without any contact between two bodies.
Sample Questions
Ques. What is Emissivity? (1 Mark)
Ans. Emissivity corresponds to the ratio of the radiated energy from the surface of a material to the radiated energy from a perfect emitter. The ratio varies from 0 to 11.
Ques. What is the effect of the medium in radiation? (1 Mark)
Ans. Medium do not affect the process of radiation. It heats the area it falls upon.
Ques. State the astronomical level of convection. (1 Mark)
Ans. On astronomical levels, it has been found that convection of gas and dust occurs in the accretion disks of black holes, at a rate that may closely approach that of light.
Ques. What are the factors on which conduction depends? (2 Marks)
Ans. Conduction relies upon the following factors:
- Temperature gradient
- Cross-sectional area
- Length of the path
- Physical material properties
Ques. Give an example of radiation. (2 Marks)
Ans. Heat coming from the sun is an example of radiation. We can feel the heat but we do not touch the sun. Thermal radiation is the reason that heat can be transmitted through an empty area like our space. These radiations correspond to infrared radiation.
Ques. Explain convection in solid. (2 Marks)
Ans. Convection cannot take place in solids as they do not let the movement of molecules in bulk. The molecules in solids are tightly packed with each other. They do not leave the surface even after being heated up. The diffusion of matter does not happen in solids but diffusion of thermal energy is possible in solids and that is one of the reasons for heat conductivity in solids.
Ques. Explain radiation by electrons. (2 Marks)
Ans. An object can emit radiation when an electron with very high energy present in an exceedingly higher atomic level jumps to lower energy levels. The energy loss because of the jumping of electrons from higher to lower atomic levels results in the emitting of non-particulate radiation.
Ques. Explain conductivity in liquid. (2 Marks)
Ans. Conductivity in liquids is less than the solids. This is because the molecules in liquids are far apart from each other. Due to this, the transfer of thermal energy from one molecule to the other takes place slowly. In liquids, heat transfer occurs mainly through convection.
Ques. What is thermal contact conductance? (3 Marks)
Ans. Thermal contact conductance is the study of thermal energy conduction between solid bodies which are physically in contact with each other. A temperature drop takes place in between the two surfaces in contact. This happens as a consequence of thermal contact resistance existing between the contacting surfaces. Contact resistance affects the thermal resistance because even the atomically perfect interfaces show thermal resistance.
Ques. Explain the effect of temperature, wavelength and frequency over radiation. (3 Marks)
Ans. The radiation of thermal energy is dependent on the temperature of the object. Hotter objects tend to radiate more energy. At the traditional space temperatures, objects radiate energy as infrared waves. The wavelength and frequency of the radiated waves are dependent on the temperature of the object. When the temperature increases, the wavelength decreases and the frequency of the emitted radiation increases.
Ques. At 27ºC certain gas occupied a volume of 4 litres. If the volume of this is to be increased to 12 litres at the same pressure, to what temperature should the gas be heated? (5 Marks)
Ans. PV = nRT
n and R are constant, so
PV/T = constant
Therefore, P1V1 / T1 = P2V2 / T2
Here, P2 = P1
V1 = 4L
T1 = 300 K
V2 = 12L
T2 = ?
Putting all the values in the above equation gives,
T2 = 900 K
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