K A( )dK A( )tQ = = ;22111 2KdKd( )AtQ+ =where is common temperature or interface temperature.Transmission of Heat2Interface temperature=221122 211 1dKdKdKdK++15. If two slabs of thicknesses d1 and d2 and of thermal conductivities K1 and K2 are placed in contact, thenthe combination behaves as a single material of thermal conductivity K and is given byK=1 2 2 11 2 1 2K d K d(d d )K K++or K=22111 2KdKdd d++.16. If three slabs have the same length and have cross-sectional areas A1, A2, A3 respectively and whenheat flows from the upper surface to the lower without loss of heat, then the equivalent conductivity K isgiven byK=1 2 31 1 2 2 3 3A A AK A K A K A+ ++ +17. The thermometric conductivity or diffusivity is defined as the ratio of the coefficient of thermalconductivity to the thermal capacity per unit volume of the material.Thermal capacity per unit volume= sVm =s where is density of substance.sDiffusivity D K =18. Heat flow through aconducting rodCurrent flow through aresistanceHeat current H =dtdQ= rate of heat flowH =RT.D.RT =R =KAlK=thermal conductivityElectric current i =dtdq =rate of charge flowi =RP.DRV =R =Al=electrical conductivity19. Thermal resistance of metal rod R=K.AL where L=length of the rod; A=area of cross-section,K=coefficient of thermal conductivity.20. Unit of thermal resistance is KW1.21. If different rods are connected in series, then heat flowing per second is same, i.e., H1=H2=H3 and thenet thermal resistance R=R1+R2+R3+22. If different rods are connected in parallel, then the net resistance R is given by ....R31R21R11R1 = + + +23. Thermal resistivity is the reciprocal of thermal conductivity.24. Ingen-Haux experiment is used to compare thermal conductivities of different materials. If L1 and L2 arethe lengths of wax melted on rods of thermal conductivities K1 and K2, then222121LLKK =25. When the temperature falls below 0C say to C, the time taken for the thickness of ice growing fromx1 cm to x2 cm on a lake is given by t= (x x )2KL 2122 where density of ice, K=coefficient of thermalconductivity of ice, L=latent heat of fusion of ice.26. Convection : The transmission of heat from one part to another by the actual transfer of particles ofmatter is known as convection.27. Although conduction does occur in liquids and gases also, heat is transported in these media mostly byconvection.Transmission of Heatf