In a 10-metre-deep lake, the bottom is at a constant temperature of 4°C. The air temperature is constant at -4°C. The thermal conductivity of ice is 3 times that of water. Neglecting the expansion of water on freezing, the maximum thickness of ice will be
(a) 7.5 m
(b) 6 m
(c) 5 m
(d) 2.5 m
Correct Answer is: (a) 7.5 m
The rate of heat flow is the same through water and ice in the steady state.
I = KA 4-0/10-x = 3KA 0-(-4)/x
x = (10 - x) .3
or x = 7.5 m.
