Transmission Flashcards
Rate of heat flow
/\Q/ /\t = KA/\T /l
Unit of thermodynamics constant K
Wm-1K-1
Convection
Is the transfer of heat by actual motion of matter
Carriers of radiant energy
Electromagnetic waves
Absorptive power (a)
a = energy absorbed/ energy incident
For a perfect black body a =
1
For a perfect black body integral of specific absorptive power
=a
Emissive power (e)
e= (/\Q/ /\t)/A
Emissivity
It’s the ratio of emissive power of a body to the emissive power of a perfectly black body
Amount of heat incident =
Qabsorbed + Qrelflected + Qtransmitted
absorptivity
a= Qa/Qi
Reflectivity
r = Qr/Qi
Transitivity
t= Qt/Qi
No material body is
Perfectly black
Nearly perfect black body
Platinum and lamp black
Kirchhoff’s Law (heat)
Ratio of a to e is always constant and is equal to emitivity of black body
a/e = E=constant
Stefan-Boltzmann Law
Energy radiated ~ T^4
E = sigma X AT^4 (for black body)
Else : E = sigma X eAT^4
Where sigma-stefan’s constant = 5.67 X 10^-8 Wm-2K-4
Wien’s Displacement Law
Wavelength ~ 1/T
Wavelength X T = constant = b
Where b is Wien’s constant = 2.89 X 10^-3 mK
Newtons Law of cooling
Rate of cooling of a body ~ temp. Difference between the body and its surroundings
Newtons law of cooling is a special case of
stefan’s law provided temp. Difference is small enough
Mathematical form of Newtons law of cooling
dT/dt = k(T1T2/2 -To)
Wiedmann and Franz Law
K/conductivity = constant
Lorentz extension
K/conductivity xT = constant
