(2) According to c=λf, the wavelength of microwave in vacuum is λ = CF = 3×108m/S2500×106Hz = 0.12m;
(3) Because P=UI, the current I = Pu =1100W220v = 5a,
Normal operation of microwave oven t=6min= 1 10h Power consumption:
w = Pt = 1. 1kW× 1 10h = 0. 1 1kW？ H=0. 1 1 degree;
(4) According to the density formula: ρ=mV, the quality of milk:
m =ρV = 1× 103kg/m3×250× 10-6 m3 = 0.25kg，
Calories absorbed by milk:
Q=cm△t=4× 103J/(Kg？ ℃)×0.25kg×(45℃-20℃)= 2.5× 104j，
Power consumption of microwave oven: w ′ = pt ′ =1100 w× 60s = 6.6×104j,
Efficiency of microwave oven: η = qw×100% = 2.5×104j6.6×104j ≈ 37.88%;
Answer: (1) The microwave oven must be well grounded to prevent electric shock accidents;
(2) The wavelength of microwave emitted by microwave oven in vacuum is 0.12m; ;
(3) The watt-hour meter of Xiaoling's classmate's house passes through a current of 5A, and it consumes 0. 1 1 kwh after working for 6 minutes;
(4) The absorbed heat of milk heated by microwave is 2.5× 104J, and the efficiency of microwave oven is 37.88%.