12. Capacitance( C ):(A)2πε0a (B) πε0a (C) 4πε0a (D)8πε0a (E)πε0a/2

20. Please calculate the rate of ice formation (in centimeters per hour) on the bottom of the ice slab. Take the thermal conductivity of icc to be 0.004 cal/sㆍ cm ㆍ℃, the latent heat of fusion of water to be 80 cal/g, and density of ice to be 0.92 g/cm3； here we assume no heat transfer through the tank walls or bottom. (A) 0.55 cm/r (B) 0.39 cm/hr(C)0.62 cm/hr(D)0.24cm/hr (E) 0.12 cm/hr