study material-physics-thermodynamics

# HEAT TRANSFER - 7

__ HARD__:

H1. The rate at which the radient energy reaches the surface from the sun is about 1.4 kw/m

^{2}. The distance from the earth to the sun is about 1.5 x 10

^{11}and the radius of sun is about 0.7 x 10

^{9}m

(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 10

^{3}

P = 3.96 x 10

^{26}

= 6.43 x 10

^{7}wm

^{-2}

(b) = 1

E = T

^{4}

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

^{0}c see.

Solution: H =

H = -

Q

_{1}- Q

_{2}= 2

^{0}c

H3: Consider a spherical black body for the body to cool down from 1000k to 300 K ? Density of Cu = a x 10

^{3}kg/m

^{3}and specific heat = 4 KJ/kg.

Solution: = AT

^{4}

= mc

t = 127 x 10^{3} sec |

**Keywords**(1) Conduction

(2) Convection

(3) Radiation

(4) Absorptive Power(a)

(5) Emmissive Power(e)

(6) Emmisitivity

(7) Black Body