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Chapter 1: Heat and Temperature-7 (in Hindi)
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In this lesson discuss the Convection and Radiation.

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

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sir tgt ke liye bhi slides daldo computer ki preparation ke liye
Lalit Kumar
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  4. 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

  5. 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

  6. 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,

  7. 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:

  8. 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

  9. Transmission coefficient or transmittance: QQuantity of radiant energy transmitted by the bo Quantity of radiant energy incident on the surface of the body And Qa+,+2,-O and also, atrtt-1 Diathermanous substances : The substances which can transmit the radiant heat incident upon their surfaces are called dathermanous substances. (t 0). Athermanous substances: The substances which cannot transmit the radiant heat incident upon their surfaces are called athermanou are athermanous s substances. (t-0). Water vapour and carbon dioxide

  10. Emissive power (e) : For a given surface it is defined as the radia energy emitted per sec per unit area of the surface. Spectral emissive power (e): If we consider emissive power ofa surface for a particular wavelength instead of all wavelengths, it is called spectral emissive power and is represented by e Total emissive power, e- e d Emissive power ofa surface depends on its nature and temperature It is maximum for a perfectly black bodyand minimum for a smoot shining body. Unit in SI system W/m2 Absorptive power (a): Absorptive power ofa surface is defined as the ratio of the radiant energy absorbed by it in a given timeto the total radiant energy incident on it in the same time. IfQ calorie ofheat fall on the surface and Q' calorie out of it are absorbed, the absorptive power of the surface is given by, a- For a perfectly black body, absorptive power is maximum and unity. It has no unit and dimensions.