**Convection**

It is the process in which heat is transferred through a substance from one point to another with the actual motion of heated particles. It is observed that the rate of transfer of heat is directly proportional to the temperature difference between two parts of the convective fluid and to the surface area of the fluid exposed. So,

Q/t α A. Δθ

or, Q/t = h. A. Δθ

**Applications of convention**

**i. Ventilations**: The rooms are provided near the ceiling. Warmer air in the room due to the respiration of people in the room has less density and moves upwards. To refill this air, cool and fresh air from outside the room enters the room and keep the room cool inside

**Radiation: **It is the process in which heat is transmitted from one place to another directly without heating or intervening the medium.

**Inverse-square law in Heat radiation**

Consider a point source S which is emitting heat in all directions uniformly. Let's draw two spheres of radius r

_{1}and r_{2}as shown in the figure. Then the intensity for area 4πr_{1}^{2}and 4πr_{2}^{2}. I

_{1}= Q/ 4πr_{1}^{2 }…… (i) I

_{2}= 4πr_{2}^{2}………. (ii)Dividing Equation (i) by equation (ii), we get

I

_{1}/I_{2}= r_{2}^{2}/ r_{1}^{2}In general,

I α 1/ r

^{2}Thus, the intensity of radiation at a point around is inversely proportional to the square of distance from the source.

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