First slide

Radiation

Question

Four spheres P, Q, R and ,S are of same radius but made of different metals. Their densities are in the ratio 3 : 4 :6 : 5 and specific heats are in the ratio 2 : 5 : 3 : 6. These are
initially kept at same temperature and placed in the same surroundings. The sphere which has the slowest rate of cooling is:

Difficult

A

P
B

Q
C

R
D

S

Solution

Rate of cooling $= \frac{σA (T^{4} - T_{0}^{4})}{mS}$
$\frac{d Q}{d t} = \frac{m c d θ}{d t} α A T^{4} m α r^{3} ρ \frac{d θ}{d t} α \frac{1}{r c ρ} R a t i o o f r a t e s = 30 : 9 : 10 : 6$

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