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Chapter 1: Heat and Temperature-6 (in Hindi)
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In this lesson discuss the Thermal Conductivity and thermal Resistance.

Vishal Garg
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  4. THERMAL CONDUCTIVITY Heat current, H- It has been found experimentally that in steady state, heat current will be (a) proportional to the area of the cross-section A of the rod. (b) proportional to temperature difference (91-92). (c) inversely proportional to the distance between ends of the rod l. 2 Thus we find that eoc_A(01-02 ) where K is a constant of proportionality called the thermal conductivity of the substance. Coefficient of thermal conductivity (K) : The coefficient ofthermal conductivity, K, of a material is defined as the amount of heat that flowing per second through a rod of that material having

  5. unit length and unit area of cross-section in the steady state, when the difference of temperature between two ends of the rod is 1 "C and flow of heat is perpendicular to the end faces of the rod Unit of coefficient of thermal conductivity in SI system is watt/m-K Dimensions: [M L T"3 -1] For a perfect conductor thermal conductivity K is infinite and for a perfect insulator K is zero Thermal Resistance : The thermal resistance of a body is a measure of its obstruction to the flow of heat through it It is defined as the ratio of temperature difference to the heat current (rate of flow of heat) Now, iftemperature difference e-length of rod (01-02) Heat current, H- A-area of cross section of rod 2 (Q/t) KA(0 02)/ KA "Th H KA or Unit in SI system: Kelvin sec/ kilojoule

  6. Combination of Metallic Conductors Equivalent thermal conductivity in series combination of conductors 1 +22 Ki K 2Ki K2 1 +K 2 If we have more than two slabs in the compound slab. The total thermal resistance of the slabs connected in series slabs connected +A, Reg. = R1+R2+ Ri + R2 + R3 + Equivalent thermal conductivity in parallel combination of conductors 15 R RIR2

  7. Equivalent thermal conductivity in parallel combination of conductors R R1 R2 For more than two slabs, we have R R1 R2 R3 K1A1 + K2 A2 A1 A2 eq Ki + K- 2 lf, Al=A2, then Keg Convection Convection like conduction requires a material medium. It is the process in which heat is transferred from one place to other by actual movement of heated material particles.It is possible only in fluids. The type of convection which results from difference in densities is called natural convection. For example, a fluid in a container heated through its bottom. However, ifa heated fluid is forced to move by a blower, fan or pump, the process is called forced convection. The rate of heat convection from an object is

  8. proportional to the temperature difference (A0) between the object and convective fluid and the area of contact A, dQ dt Le., where, h represents a constant of proportionality called convection viscosity, specific heat and thermal conductivity. convection coefficient and depends on the properties of fluid such as density,

  9. Radiation When a body is heated and placed in vacuum, i loses heat even when there is no medium surrounding it. The process by which heat is lost in this case is called radiation. This does not require the presence of any material medium Radiation has the following properties: (a) Radiant energytravels in straight line and when some object is placed in the path, it's shadow is formed at the detector. It is reflected and refracted or can be made to interfere. The (b) (c) It can travel through vacuum. (d) Intensity of radiation follows the law of inverse square. (e) Thermal radiation can be polarised in the same way as light eleadiole1012 reflection or refraction are exactly as in case of light. by transmission through a nicol prims. Radiation Y-rays X-rays Ultraviolet rays Visible rays Infra red rays Microwaves Radiowaves Wavelength range 10-9 m to 6x 10-12 m 3.8 x 10-7m to 6 x 10-10 m 7.8 x 10-7m to 3.8 x 10-7 m 10-3 m to 7.8 x 10-7 m 0.3 m to 10-3 m Few Km to 0.3 m m. 10 Absorption coefficient or absorptance:

  10. Absorption coefficient or absorptance: Qa Quantity of radiant energy absorbe L Luantity of radiant energy incident on the surface of the body Reflection coefficient or reflectance: Quantity of radiant energy reflected 2 Quantity of radiant energy incident on the surface of the body