211) Use newtons law of cooling;
d(theta) / dt = -bA (theta - theta (surr))
so u see ,time is inversely proportional to AREA
so u get 25 sec as the answer ;)
Ques 1) A solid Copper cube of edge 1 cm is suspended in an evacuated enclosure. It's temperature is found to fall from 100 ° C to 99 ° C in 100 sec. Another solid copper cube of edge 2 cm with similar manner. The time required for this cube to cool from 100 ° C to 99 ° C will be approximately : -
(a) 400 sec (b) 200 sec (c) 50 sec (d) 25 sec
Ques 2) A solid Copper sphere (density \rho and specific heat c ) of radius r at an initial temp 200 K is suspended inside a chamber whose walls are at almost 0 K. Find the time required for the temperature of the sphere to drop to 100 K .
Ques 3) The earth receives at it's surface radiation from the sun at the rate of 1400 W/m2 . The distance of the centre of the sun from the surface of the earth is 1.5 X1011 m and the radius of the sun is 7 X 10 8 m. Treating the sun as the black body ,find the surface temperature of the earth.
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5 Answers
21213) I think i m right
U(energy per unit time) = stefan constn * A * T(sun)4 where A = area of sun
Now U / 4 pi R^2 = (Radiation we get per unit time per unit area), because radious of earth <<<<<that of sun
( where R = distance b2n earth and sun )
from here u can calculate T, which is the temp of sun
21yes i was careless about the mass, thanks for pointing out the mistake ;(:::::)
