HEAT TRANSFER - 7

HARD:

H1. The rate at which the radient energy reaches the surface from the sun is about 1.4 kw/m². The distance from the earth to the sun is about 1.5 x 10¹¹ and the radius of sun is about 0.7 x 10⁹m
(a) What is the rate radiation of energy, per unit area from the sun’s surface?
(b) What is the temperature of sun’s surface assuming it to be a black body?

Solution: (a) The energy crossing through a unit area per second
   =    D: Distance between sun and earth.
   = 1.4 x 10³
P = 3.96 x 10²⁶
= 6.43 x 10⁷ wm^-2

(b) = 1
E = T⁴
T = = 5803 K


H2: A 2m long wire of resistance 4 ohm and diameter 0.64 is coated with plastic insulation of thickness 0.06mm. when a current of 5a flows through the wire, find the temperature difference across the insulation in steady state if K = 0.16 x 10^-2 cal/cm ⁰c see.

Solution: H =
H = -
Q₁ - Q₂ = 2⁰ c


H3: Consider a spherical black body for the body to cool down from 1000k to 300 K ? Density of Cu = a x 10³ kg/m³ and specific heat = 4 KJ/kg.

Solution: = AT⁴
= mc

t = 127 x 103 sec

Keywords

(1) Conduction
(2) Convection
(3) Radiation
(4) Absorptive Power(a)
(5) Emmissive Power(e)
(6) Emmisitivity
(7) Black Body