Heat and thermodynamics-Part-III

Heat and Thermodynamics-Part-III : Heat and Thermodynamics-Part-III By R.Buvana

Heat and thermodynamics-Part-III : Heat and thermodynamics-Part-III 1.Modes of transfer of heat 2.Conduction ,convection and radiation 3.Application of Conduction ,convection and radiation. 4.Thermal conductivity 5.Newtons law of cooling. 6.Home work questions

Heat Transfer : Heat Transfer Heat energy transfers from a body at higher temperature to a body at lower temperature.The transfer of heat from one body to another body may take place by any one of the following modes. 1.Conduction 2.Convection 3.Radiation

Conduction : Conduction The process by which heat is transferred from hot part to cold part of a body through the transfer of energy from one particle to another particle of the body with out the actual movement of the particles from their equilibrium position s is called conduction. 1.Good conductors-copper, aluminium, mercury etc. 2.Bad conductors-glass felt, wood ,mica etc.

Thermal Conductivity : Thermal Conductivity Consider a small piece of material in the form of a cube .The area of face I is A and the distance between the two faces is X.It is found that the heat flow Q from face I to II is Q=-K A(T/x ) K=Q. Thermal Conductivity of a material is defined as the quantity of heat energy that flow across the opposite faces of a one metre cube in one second, when the opposite faces of the cube are maintained at a temperature difference of 1°C SI unit of K is J/s/m/K

Heat conduction through a composite slab : Heat conduction through a composite slab A composite slab is made of two or more substances of different thermal conductivities and cemented together. Q/t=-K1A (T1-T0)/X1 Q/t=-K2A(T0-T2)/X2 (K2X1+K1X2)T0=K1X2T1+K2X1T2 T0=K1X2T1+K2X1T2/(K2X1+K1X2) Q/t=_K1A/X1(T1-K1X2T1+K2X1T2/(K2X1+K1X2)

Conti… : Conti… Thereforem,the rate of flow of heat per unit area of the composite slab is given by Q/t/A=(T2-T1)/(x1/K1+X2/K2)

Convection and Radiation : Convection and Radiation Convection-The process of heat transfer from one part of a fluid to another part by the actual movement of the particles of the fluid is called convection.Liquid and gases are heated by the process of convectin. Radiation-The process of the transfer of heat from one place to another place without heating the intervening medium is called radiation.Everybody emits energy in the form of waves due to its temperature.These waves are known as thermal radiation or heat radiation.

Newtons law of cooling : Newtons law of cooling According to this law the rate at which a body loses heat in directly proportional to the temperature difference between the body and its surroundings. -dQ/dt=k(T-T0) Loge(T-T0)=-Ct+K Where K is constant of integration

Perfectly Black Body and Black Body Radiation : Perfectly Black Body and Black Body Radiation A pwrfectly black body is one which absorbs all the heat radiation of whatever wavelength,incident on it.Absorption,Reflection and Emission of Radiation Q=Qr+Qa+Qt 1=r+a+t

Kirchhoff’s Law : Kirchhoff’s Law It states that the ratio of the emissive power to the absorptive power for the radiation of given wavelength is the same for all substances at the same temperature and is equal to the emissive power of a perfectly black body for the same wavelength and temperature. E?=e?/a?

Stefan’s law : Stefan’s law According to this law,the amount of heat radiation emitted per unit time from aunit area of abody at absolute temperature T is directly proportional to the fourth power of the temperature.E=sT4 E=esT4 E=AesT4

Wien’s displacement law : Wien’s displacement law The wavelength correswponding to max. intensity of emission of heat radiations decreases with increase in the temperature of the black body. ?mT=b This relation is known as Wien’s displacement law. Where b is constant and is callled Wien’s constant .The value of Wien’s constant in SI unit is 2.898 x10?³mK.

Home work Questions : Home work Questions 1.A composite body consists of two rectangular plates of the same dimensions but different thermal conductivities KA and KB.This body is used to transfer heat between two objects maintained at different temperatures. The composite body can be placed such that the flow of heat takes place either parallel to the interface or perpendicular to it. Calculate the effective thermal conductivities K|| and K| of the composite body for the parallel and perpendicular orientations. Which orientation will have more thermal conductivity?

Conti… : Conti… A double –pane window used for insulating a room thermally from outside consists of two glass sheets each of area 1 m² and thickness 0.0 a1m separated by a 0.05m thick stagnant air space.In the steady state, the room-glass interface and the glass-outdoor interface are at constant temperature of 27°C and 0°C respectively. Calculate the rate of heat flow through the window pane. Also find the temperatures of other interfaces. Given,thermal conductivities of glass and air as 0.8 0.08 W/m/K respectively.