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Chapter 1: Heat and Temperature-8 (in Hindi)
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In this lesson discuss the Heat and mass transfer.

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

U
Unacademy user
Very nice technique to remember UT and state capitals. Some funny ones according to me are: Goa and Panaji can be learned as: Goa pan masala, Gangtok and Sikkim as: Gang ka sikka chalta hai and MP main jai bhole (Bhopal and MP).
  1. Vishal Garg (B.Tech ME Er.) Star and Verified Educator unacademy SSC and Railway Category Expert


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  3. Basic Science & Engineering INDIA RRB ALP & Technicians Recruitment 2018 for Railway ALP CBT2 Basic Science Heat & Temperature


  4. 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:


  5. 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


  6. 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


  7. 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.


  8. Spectral absorptive power (a^) : Spectral absorptive power i defined as the ratio of the radiant energy of a given wavelength absorbed by a given surface in a given time to the total radiant energy of that wavelength incident in the same time on the same surface within a unit wavelength range. Total absorptive power, a - andn Absolute emissivity : The emissivity e of a surface is defined as the quantity of heat radiated per second by one unit area of its surface when its temperature is 1 C above the surroundings. Relative emissivity (e) : It is defined as the ratio of radiant energy emitted per unit time per unit surface area of a body at a given temperature (E) to the radiant energy emitted per unit time per unit surface area of a perfectly black body (Eb) at the sami temperature.


  9. Different bodies (i) If a = t = 0 and r-1 body is perfect reflector (i) If r = t = 0 and a = 1 body is perfectly black body (ii) Ifa and t -1-> body is perfect transmitter (iv) If t :-0 r + a l or -1-r, i.e., good reflectors are bad absorbers. Perfectly Black Body A black body is defined as one that will completely absorb all the radiations of whatever wavelength which falls on it. For perfectly black body, a-1 Properties ofperfectly black body: (i) A perfectly black body absorbs all the radiant heat incident (ii) A perfectly black body does not reflect or transmit the radiant (iii) The coefficient ofemission of a perfectly black body is l. It (iv) A perfectly black body does not exist in nature. upon it. (i.e. a-1) heat incident upon it. (i.e. t 0, r is very good emitter of heat. O)