Modes of Heat Transfer

In modes of heat transfer, we will learn about the movement of heat from one thing to another by means of conduction, convection or radiation. All forms of heat transfer may occur in some systems at the same time.

Heat is the amount of thermal energy that is transferred between two objects due to temprature difference. Heat transfer takes place in three modes –
1. Conduction
2. Convection 
3. Radiation

1. Conduction

The process, in which heat energy is transferred from one system to another system due to the movement of particles through large distances, is called convection.
When heat transfer takes place due to the vibration of molecules, it is known as conduction. These vibrations travel in the form of heat waves of very high frequency.
It takes place in all three states of matter

2. Convection

The process, in which heat energy is transferred from one system to another system due to movement of particles through large distances, is called convection.
It occurs due to bulk motion of molecules. It does not occur in solids beacause solids can not diffuse into each other.

3. Radiation

The process in which heat energy travels from one system to another in the form of electromagnetic waves with no need of material medium is called Radiation.
In radiation, heat travels from one system to another in the form of electromagnetic radiations (photons). Thus no medium is required for this mode of transfer of heat. Heat from the sun reaches earth by radiation

Mechanism of Heat Transfer by conduction

The process of heat conduction has been defined as the transfer of heat energy through the substances without any significant motion of the molecules from high temprature region to lower temprature region.
This mode of heat transfer by conduction is attained by following two mechanism
1. Due to lattice vibrations
2. Due to transport of free electrons

1. Due to Lattice Vibrations

The molecules of a substance continuously vibrate about the same mean position. These vibrations are known as lattice vibrations
We know that the kinetic energy of the molecules in case of liquids and gases is due to their random translation or rotation and vibrational motions. While the solids only vibrate on their lattice. The temperature of the substance corresponds to its kinetic energy i.e. higher is the average kinetic energy of molecules, higher will be the temperature of the substance.
The molecules of solid materials while vibrating they collide each other and the molecules with higher kinetic energy, transfer some part of its energy by impacting adjacent molecules with lower kinetic energy. This type of energy transfer will continously take place through substance as long as there exists a temperature gradient. Therefore the rate of heat transfer due to lattice vibration depends upon the rate of collision between the molecules.

2. Due to the transport of free electrons

The mechanism of heat conduction and the mechanism of transport of electric current are both dependent upon the flow of electrons.
The valence electrons in the outermost shell of an atom get excited on the availability of energy. They overcome the binding force to become free and move within the boundaries of the solid. Such electrons are called free electrons. These free electrons impart their energy by moving from higher level to lower level.
Good electric conductors are good heat conductors. Because good electric conductors have a large number of free electron e.g. Aluminium, copper, gold, etc.
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