Heat Transfer and Thermal Radiation: Conduction Convection

Heat and work are two different forms of energy. Heat energy can transfer from one body to another, or from one body location to another.The study of the methods and techniques adopted to transfer heat energy is called 'Heat Transfer.' In this lesson we will discuss heat transfer in detail and their different mechanisms. First let us examine the basics of heat transfer in three different modes of heat transfer: conduction, convection and radiation in detail with examples. Thermal radiation is the part of radiation heat transfer which we shall study in brief with the equation of Stefan Boltzmann law.

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Difference Between Heat and Temperature

Key words: Heat, work, conduction, convection, radiation, thermal radiation, Stefan Boltzmann law.

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Basic of heat transfer

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In thermodynamics, flow of heat through a system is called heat transfer. Heat is a form of energy that is transferred between a system and its surroundings by virtue of a temperature difference. The heat transfer only takes place because of temperature difference and it cannot take place if the temperature of two systems is same. At the same time, it is very important to understand that heat only flows from a body at higher temperature to a body at a lower temperature. Several terminologies are in common use today related to heat transfer such as heat flow, heat gain, heat rejection, heat absorption, heat loss, heat generation, heat of reaction, specific heat, heat sink, heat source etc. Thus, heat is energy in transition and heat transfer is recognized only when it crosses the boundary of the system.

A good example of heat transfer is a hot boiled egg.The hot boiled egg contains energy but this heat energy transfers only as it passes through the skin of the egg (the system boundary) to reach the air. After heat is transferred in the surrounding it becomes part of the internal energy of the surroundings.

In kinetic theory, heat is defined as the energy derived from the random motion of atoms and molecules. Heat is also the realization of motion at molecular level.

A process in which there is no heat transfer is called an adiabatic process. The word adiabatic means not to be passed. There are two ways in which the system is called adiabatic: one is well insulated and the other implies both system and surroundings having the same temperature.

There are three mechanisms or modes of heat transfer.

1. Conduction: It is defined as the transfer of heat between two solids.
2. Convection: It is defined as the transfer of heat between solid and liquid.
3. Radiation: It is defined as the transfer of heat when there is no media involved.

Read more : Joule Thomson Effect

Conduction 

Conduction is defined as the ‘Transfer of energy by direct interaction or direct contact between higher energetic values to adjacent lower energetic values’. Conduction is also known as thermal conduction or heat conduction. Suppose we take two blocks: the temperature of one block A is 100ËšC and another block B at 10ËšC, in heat transfer of conduction type, the temperature flows from block A to block B only when these blocks are connected to each other. The two bodies through which the heat transfer takes place must be in contact with each other which means Direct contact occurs in heat transfer of conduction type. Eventually, the temperature of block B will rise and the temperature of block A will drop. In solid objects, thermal conduction is an important mechanism of heat transfer.

In conduction, heat transfer occurs due to the collision of particles. There is no movement of particles from one place to another. Based on the Thermal conductivity, substances can be distinguished as conductors and insulators. Substances that conduct heat quickly are called conductors while the one which does not conduct heat are called insulators.

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Convection

In convection type of heat transfer, the transfer of energy takes place between solid surface and adjacent fluid that is in motion. Thermal convection involves the effect of conduction and fluid motion together. Convection heat transfer occurs partly because of either the actual movement of molecules or due to the mass transfer. In convection, the significant part is motion of fluid occurring either naturally or by external means. In such a process, particles move from one place to another to another to transfer the heat.

The two types of convection heat transfer

1. Natural convection- It means any fluid motion which is caused by natural means. For example, fall of the cooler fluid or rise of warm fluid.
2. Forced convection- It implies that the fluid is forced to flow over a surface by external source such as fan, pump, suction devices etc

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Radiation 

Radiation differs from heat transfer occurring through conduction and convection. In radiation, heat transfer takes place by electromagnetic waves. It is also defined as heat waves emitted that are absorbed, or transmitted through a colder body. One such example is that the sun heats the earth by electromagnetic waves. Hot bodies emit hot waves and cold bodies emit cold waves.

The video below explains this:

Heat Transfer Formula Detailed Video Explanation:

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Thermal Radiation

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Thermal radiation is the transmission of thermal energy without any physical contact between the bodies involved. Such type of heat transfer does not depend on the medium.

Unlike heat transfer by conduction and convection, transport of thermal radiation does not necessarily occur through the material medium between heat source and receiver. Energy released by a radiating surface is not continuous but it is in the form of successive and separate packets, also quanta of energy called photons.

The heat transferred from sun to earth without any direct physical contact, is a best example of thermal radiation.

Solid and liquid surfaces at all temperatures emit thermal radiation. Thermal radiation can be measured by Stefan-Boltzmann law. The expression below explains Stefan Boltzmann law,

Rate of emission per unit area=  QA∝T4

Q/A- w/m2 and T- Absolute temperature in Kelvin

Thermal radiation is emitted in the form of electromagnetic waves and continuous spectrum of all wavelengths.

In thermal radiation, the space between the surfaces is a vacuum or occupied by gas which does not participate in a radiative exchange in any way.

Perfect black body- It is a body which absorbs all the radiation falling on it regardless of its wavelength or direction. A black body is a perfect thermal radiator which absorbs all energy that reaches it and reflects nothing. For a given temperature and wavelength, a black body emits maximum amount of energy.

Total power of black body is denoted by eb.

The emissivity for a black body in a thermal equilibrium is 1.

ε=1

Wien’s displacement law states that, black body radiation curves at different temperatures and will peak at different wavelengths which are inversely proportional to temperature.

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Things to Remember 

• Heat flow is called heat transfer
• Three modes of heat transfer are
• Conduction
• Convection
• Radiation
• There are two types of convection
• Natural convection
• Forced convection
• The emissivity for a black body in a thermal equilibrium is 1.

ε=1

• Rate of emission per unit area QA∝T4

Q/A- w/m2 and T- Absolute temperature in Kelvin

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