18. Effective magnetic moment of this rotating disk (A) (B) (C)(D) (E) none of above

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