 ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ. ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ ಈ First of all, we will try to understand what is the meaning of heat transfer. As we know, the thermal energy can be in two forms. One form is in storage, which we call it as internal energy. And whenever the thermal energy is in transit, it is called as heat. The definition of heat transfer can be given as it is a transmission of heat from one region to another due to temperature difference. The heat is transferred with the help of three various modes. Conduction, convection and radiation. These three modes of heat transfer have two common things. One is the temperature difference must be present in all the modes without temperature difference or temperature gradient. The heat will not be transferred. Second common thing in all modes of heat transfer is that heat always flows from high temperature to lower temperature. That is the direction of heat transfer is always fixed. Basic laws of heat transfer. First of all, we will try to understand Fourier law of conduction. Fourier law is an empirical law stated by the scientist Fourier, which governs the conduction mode of heat transfer. Let us see according to Fourier, this conduction rate of heat flow in case of conduction q is directly proportional to the area normal to the direction of heat transfer. It is also directly proportional to the temperature difference and it is inversely proportional to the thickness or length in the direction of heat flow. Therefore, if we consider a plain wall through which the heat is conducted assuming that this plain wall is made up of homogeneous material, then we can see that suppose the heat is conducted in x direction, the heat flow rate is q and this particular plain wall is having the uniform thermal conductivity k and the thickness of this plain wall is dx, then we can write the Fourier law in equation form as q is directly proportional to a, where area is the area, capital area a is the area normal to the direction of heat flow. If there is a temperature difference present is dt across the two faces of the wall, then q is directly proportional to dt and it is inversely proportional to the thickness in the direction of heat flow. So, by combining all these terms, we get that q is directly proportional to a into dt by dx. If we remove this proportionality sign, then we get that q is equal to minus kA dt by dx, this is called as Fourier law of heat conduction, where k is known as a constant of proportionality which is also called as thermal conductivity of material. When Fourier stated this law, he was not aware that it is a property of material, he introduced it as a constant of proportionality, but later on it was found that k is the physical property of the material which indicates ability of material to conduct the heat, the mode of heat transfer in which the heat is transferred by combined action of conduction and convection. The heat transfer by convection occurs from solid surface to the adjacent fluid. Initially the heat is conducted through the thin layer of fluid which is adjacent to the solid surface and the heat transfer then further occurs due to the motion of the molecules. The hotter molecules being lighter, they will move upward and the colder or more denser molecules they will flow downwards. Thus convection we can say that it is combined mode of conduction and convection. The Newton has stated empirically the equation for which governs the convection heat transfer mode and it is called as Newton's law of cooling. Newton's law of cooling states that q that is the heat flow rate is equal to HA into TS minus TF, where q is the heat flow rate in Watt. H is called as heat transfer coefficient which is having unit of Watt per meter square Kelvin and A is the surface area or the area of contact between solid and fluid which will be given in meter square. T is the temperature of surface and TF is temperature of fluid. The major difference between conduction and convection is that in case of solid the heat is conducted due to two mechanisms mainly due to the molecular vibration or lattice vibration and secondly due to the flow of free electrons. In case of metal because of free electrons are present the heat transfer due to flow of electrons is much dominant compared to the molecular vibration. Whereas in case of non-metal the heat is conducted purely because of the molecular or atomic vibration. The molecules vibrate and they give part of their energy to the adjacent molecule by impact or collision. Whereas in convection the major heat transfer is occurring due to the mixing motion of the molecules. The hotter molecules will move downward due to gravity whereas colder or denser molecules will move downward and hotter molecules will move upward. Like this the convection currents are set up in the body and depending upon the cause behind the mixing motion of the molecules or mixing motion of the fluid. The convection heat transfer is classified further into two types. One is called as a natural convection. In case of natural convection the fluid is circulated because of the density difference which is the result of temperature difference. Whereas in case of force convection if we induce the motion of the fluid mixing motion of the fluid due to some external agency like pump blower. Whereas in case of natural convection these natural convection currents are set up due to the density difference. So most of the problems associated in engineering are related to force convection. Now we will come to the last mode of heat transfer that is radiation heat transfer. Radiation heat transfer is the third mode of heat transfer in which the heat is transferred because of the properties of the substances. All substances solid liquid or gases have property to emit and absorb the radiations at all temperatures. These radiations are in the form of electromagnetic waves. There are two theories associated with radiation heat transfer. One is called as Maxwell's wave theory. According to Maxwell's wave theory the radiations travel through a hypothetical medium called as ether through the space. And they travel all radiations travel with the speed of equal to that of the speed of light. These radiations when they come obstructed are obstructed by a certain surface then they give their part of energy. It is always seen that the hotter body will emit more energy than it receives. And therefore it will transfer the heat energy in the form of electromagnetic radiation to the colder body. Because colder body receives more energy in the form of radiation compared to the hotter than it emits. Therefore in case of radiation heat transfer the heat transfer between the two bodies or two surfaces can occur whenever they are separated from each other in the form of electromagnetic waves. For radiation heat transfer though according to Maxwell's wave theory the ether medium was required. According to the second theory that is called as the Planck's quantum theory these radiations are emitted in the form of the energy packet discrete quanta which can be called as energy packets known as quanta in terms of photons. And these photons possess a certain energy given by E is equal to H into F where E is the energy possess by each photon. H is the Planck's constant and F is the frequency of radiation. These energy radiation heat transfer is governed by the basic law which is called as Stefan Boltzmann law. The Stefan Boltzmann law can be stated as the heat radiated by a black surface per unit area per unit time is directly proportional to the fourth power of its absolute temperature. That by removing the proportionality sign we can write that QB is equal to sigma A into T raised to fourth. Where sigma is called as Stefan Boltzmann constant which is having the value of 5.67 into 10 to the power minus 8 Watt per meter square degree Kelvin raised to 4. Thus the Stefan Boltzmann constant is the proportionality constant which relates the heat energy radiated from the black surface. A is the surface area of radiating body and T is the absolute temperature. From above law we can see that the all bodies will emit radiations at all temperature except at absolute zero. The body will see emitting radiation at absolute zero temperature. The references are heat transfer by P.K. Nag, Tata McGrawill publishing company limited New Delhi, heat transfer by J.P. Holman, McGrawill book company New York. Thank you.