# Radiation | Heat & Mass Transfer | Gate

**Radiation:** Radiation is the third mode of heat transfer the heat transfer takes place due to electromagnetic waves (photons). the transfer of heat known as thermal radiation and it does not require medium to transfer heat.

**Irradiation:** The total incident radiation on the surface from all direction per unit time and per unit of surface. it is denoted by ‘G;.

**Absorptivity:** The fraction of total incident radiations that are absorbed by the material. denoted by α.

**Reflectivity:** The fraction of total incident radiation that is reflected by the material. it is denoted by ‘*ρ’.*

**Transmissivity:**The fraction of total incident radiations that are transmitted by the material. it is denoted by ‘?’.

**Emissive power:** The energy of thermal radiation emitted by per unit area per unit time from the surface of a body at a certain temperature is called emissive power. it is denoted by ‘E’.

**Emissivity:** This is the ability of any object to emit heat energy. the ratio of an emissive power of any body to the emissive power of a black body.

**See also: This Laws And Theories Are Changed our Vision About Radiation**

**Shape Factor:** The fraction of energy emitted by one surface and directly intercepted by the other surface. it depends on the geometry and orientation.

**Radiosity:** Total radiation leaving the surface per unit area, is known as radiosity. it is denoted by ‘J’.

## Some Basic Formula

1. Energy emitted by surface | Q = σAT^{4} |

2. Black Bodies | α = 1, ρ = 0, ? = 0 |

3. Emissive power of black body(Stefan Boltzmann law) | E=σT^{4} |

4. For gas | α + ? = 1 |

5. Intensity of radiation (I) | I = Eb/ π |

6. speed of radiation | wavelength x frequency |

## Shape Factor

1. Reciprocity theorem | A1 F12 = A2F21 |

2. Enclosure theorem | F11+ F12+ F13+…….F1n = 1 |

3. Condition | F11= F22=0 |

4. Rule | F12= F13+F14 |

5. | F21= A1/A2 |

6. | F12 = F21 =1 |

7. Hemispherical shape | F11= 0.5 |

8. Two concentric cylinders | F22= (1-d1/d2) |

9. Equilateral triangle | F12=F13= 0.5, F21=F23=0.5, F31=F32=0.5 |

**Questions****Solutions download**

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