Transmission of Heat
The process of transmission of heat from higher temperature to lower temperature is known as transmission of heat.
• Conduction: Conduction of heat is that mode of heat transfer in which molecules of the body transfer heat from a place at higher temperature to a place at a lower temperature without actually moving the body.
• Convection: Convection is the mode of heat transfer by actual motion of the matter. Convection is not possible in solids and can take place in fluids (liquid and gas) only.
• Radiation: The mode of transfer of heat from one body to another without any actual movement the particles involved and without heating the intervening medium is called radiation.
• Thermal Conductivity: Thermal conductivity of a solid is a measure of ability of solid to conduct heat through it.
Heat, \(Q = KA \frac{(\theta _1 - \theta_2)}{l}t\)
where, \(\theta_1\) and \(\theta_2\) = temperatures of ends of rod,
A = cross-section area of rod,
l = length of rod,
t = time
and K = thermal coefficient.
• Newton's Law of Cooling: According to this law, the rate of cooling of body is directly proportional to the temperature difference between the body and the surroundings.
• Black Body: The black body is the one which absorbs the radiations of all the wavelengths that are incident on it. The absorbing power of perfectly black body is 1.
• Kirchhoff's Law of Radiation: The ratio of emissive power and absorptive power of a body is same for all surfaces at same temperature and is equal to the emissive power of a perfectly black body at that temperature.
\(\frac{e_\lambda}{a_\lambda} = E_{\lambda}\) (constant)
• Stefan's Law: The emissive power of a perfectly black body directly proportional to the fourth power of its absolute temperature.
\(E \propto T^4\)
⇒ \(E = \sigma T^4\)
where, σ = Stefan's coefficient = 5.67 x 10-8 Wm-2 K-4
• Wien's Displacement Law: At a particular absolute temperature of perfectly black body, the product of maximum wavelength (\(\lambda_m\)) and absolute temperature T is constant.
\(\lambda_mT=b\)
where, b is constant, called Wien's constant.
