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Chapter 1: Heat and Temperature-5 (in Hindi)
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In this lesson discuss the Heat Transfer.

Vishal Garg
Mechanical Engineer | Unacademy Plus Educator | Railway Category Expert | Follow me to learn Reasoning |

U
thank you very much sir..
You're most welcome Sekendar. ☺️
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4. Heat Transfer Conduction Conduction is that mode of transmission ofheat in which heat is transferred from a region ofhigher temperature to a region of lower temperature by the aid of particles of the body without their actual migration. Conduction requires material medium Variable state and steady state: When one end ofa rod is heated, then initially the temperature of various points of the rod changes continuously and the rod is said to exist in a variable state After some time, a state is reached, temperature of each cross- section becomes steady. This state is known as steady state If rod is in contact with atmosphere or surroundings and it is heated at one en then distance temperature curve is as shown in fig. alongside. when the versus Distance rom hot end

5. And ifrod is insulated with surroundings then graph is straight line. Temperature gradient: The rate of change of temperature with distance between two isothermal surfaces is called temperature gradient. Ifthe temperature ofthe isothermal surfaces be and - and perpendicular distance between them is then. Temperature gradient between them Change in temperature Distance between two surfaces Ax The negative sign show that temperature decreases as the distance x increases in the direction of heat flow. Unit in SI System C/metre

6. 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

7. 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

8. 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

9. 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

10. 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,