Thermodynamics - Heat Transfer Laws

Heat Transfer Laws

Kirchhoff’s law according to this aw, the ratio of emissive power to adsorptive power is same for all surfaces at the same temperature and is equal to the emissive power of a perfectly black body at that temperature. Thus for a body having e and a as emissive and absorptive power

e/a = E/ A = E/1 = E

Where E = emissive power of a black body

It implies a good absorber and is also a good emitter (radiator)

Fraunhofer lines they are the dark lines in the spectrum of the sun and are explained on the basis of kirchoff’s law. White light emitted from core (photosphere) of the sun when passes through its atmosphere (chromospheres) radiations of those wavelengths will be absorbed by the gases present there, which they usually emit (in emission spectrum) resulting in dark lines in the spectrum of sun.

Stefan’s law radiant energy emitted per unit area per second (or emissive power or intensity) of a black body is directly proportional to the fourth power of temperature.

ER ∝ T⁴ or ER = σT⁴

If the body is not perfectly black then 

ER = eσT⁴ where σ = 5.67 x10^-8 Wm^-2 K^-4

Energy radiated per second or radiant power

PR = e_AσT⁴ or ∫Rλdλ αT⁴

Cooling by radiation if a body is at temperature T in an environment of temperature T₀ (< T), then body loses energy by emitting radiations at a rate

P₁ = e_AσT⁴

And it receives energy by absorbing radiation at a rate 

P2 = e_AσT⁴₀

So the net rate of loss is p = p₁ – p₂

Or p = e_A σ(T⁴ – T⁴₀)

When a body cools by radiation, that rate of cooling depends upon the following factors:

Nature of the radiating surface, that is, emissivity

Breather the emissivity faster will be the cooling

Area of the radiating surface more the surface area of the radiating surface, faster will be the cooling.

Mass of the radiating surface greater the mass of the radiating body slower will be the cooling.

Specific heat of the radiating body more the specific heat slower will be the cooling.

Temperature of radiating body higher the temperature faster will be the cooling. 

Temperature of the surrounding lesser the temperature of the surrounding faster is the cooling

Newton’s law of cooling the rate of cooling is proportional to the temperature difference between body and the surroundings provided the temperature difference is not very large from the surroundings. That is, 

Dθ/dt = - K (θ – θ₀)

Or ∫_^θ₂^_θ2¦?(dθ? / θ )– θ₀ = ∫t₀- K_dt

Log_e ^{(θ₂ – θ₂)} / (θ₁ – θ₀) = -K_dt

Show the temperature vs time curve.

If time intervals are equal and successive then

(θ₂ – θ₀) /( θ₁ – θ₀) = (θ₃ – θ₀) /( θ₁ – θ₀)

Assuming temperature at t =0 is θ₁, after first interval it is θ₂ and after second equal interval of time it isθ₃.

Solar constant the sun is a perfectly black body as it emits all possible radiations (e = 1) solar consent s is defined as the intensity of solar radiation at the surface of the earth. That is, 

S = pR / 4πr² = 4πr² σT⁴ / 4πr²

R = 1.5 x 10¹¹ km

(Distance between the sun and the earth R = 7 x 10⁸ km (radius of the sun)

S = 2cal / cm² min = 138 W/m²

Wien’s displacement law when a body is heated it emits all possible radiations. However, intensity of different wavelengths is different. According to wine’s law the product of wavelength (corresponding to maximum intensity of radiations) and temperature of the body in Kelvin id constant

λ_mT = b (constant)

b = 2.89 x 10^-3 m ^{– K}

Planck’s law Planck assumed that electromagnetic radiations are not emitted or absorbed continuously but in energy associated with each photon is 

E = hv

Where h is Planck’s constant (= 6.625 x 10^-34 j ^{– s}). On the basis of quantum theory Planck showed that 

E_R(λ) = 2πhc² / λ⁵ /1 [e_hc / λ_KT – 1] 


ExpertsMind.com - Heat Transfer Laws Assignment Help, Heat Transfer Laws Homework Help, Heat Transfer Laws Assignment Tutors, Heat Transfer Laws Solutions, Heat Transfer Laws Answers, Thermodynamics Assignment Tutors, Physics Help