Xiao Ming's family has an electric kettle. The name plate is shown in the picture on the right. He uses this electric kettle to boil water under the rated voltage and pass hot water Xiao Ming's family has an electric kettle. The name plate is shown in the right figure. Rated voltage: 220 v. rated power: 840 W. frequency: 50 Hz. Capacity: 4 L. how much current does he use this electric kettle to boil water at rated voltage? If all electric energy is converted into internal energy, how much electric energy does it consume to boil a kettle of water at 20 ℃ (one standard atmospheric pressure)? (excluding the heat absorbed by the kettle itself)

Xiao Ming's family has an electric kettle. The name plate is shown in the picture on the right. He uses this electric kettle to boil water under the rated voltage and pass hot water Xiao Ming's family has an electric kettle. The name plate is shown in the right figure. Rated voltage: 220 v. rated power: 840 W. frequency: 50 Hz. Capacity: 4 L. how much current does he use this electric kettle to boil water at rated voltage? If all electric energy is converted into internal energy, how much electric energy does it consume to boil a kettle of water at 20 ℃ (one standard atmospheric pressure)? (excluding the heat absorbed by the kettle itself)

1. Hot water kettle works under rated voltage, and its current is equal to P / u = 840 / 220 = 3.82a
2.Q=c*m*(T2-T1),
The specific heat capacity of water is 4.2 kJ / (kg ·℃);
M is the mass of the object, the density of water is 1, that is, 1 liter of water is 1kg;
(t2-t1) is the change of temperature after heating
Therefore, the heat required for 4 liters of water from 20 ℃ to 100 ℃ is
Q = 4.2 * 4 * 80 = 1344 kJ = 1344 kwh
If all the electric energy is converted into the internal energy of water, the electric energy consumed by boiling 4 liters of water is
1344 / 3600 = 0.373 kwh = 0.373 degree

Xiaoming's family bought an electric kettle, and its name plate is as shown in the table. ⊙⊙⊙⊙ rated voltage ⊙ 220 V ⊙ frequency ⊙ 50 Hz ⊙ heating power ⊙ 1000 W ⊙ insulation power ⊙ 40 W ⊙ maximum capacity ⊙ 2 L ⊙ ambient humidity ⊙ ≤ 90% (1) the meaning of "50 Hz" in the name plate is___ (2) Xiaoming and Xiaohua want to measure the actual power of the electric kettle. When only the electric kettle works, Xiaoming finds that the indicator light of the pulse electric energy meter flashes 90 times in 3 minutes (the dial of the pulse electric energy meter is marked with "2000imp / kW · H", which means that the power consumption in the electric circuit is 1kW · h for every 2000 flashes of the indicator light). What is the actual power of the electric kettle? (3) During the above test, Xiaohua filled 2L water at 20 ℃ in the hot water pot, and the water temperature in the pot rose to 35 ℃ within 3 min. the efficiency of the electric kettle was calculated. [C water = 4.2 × l03j / (kg ·℃)]

(1) The number of times that the current changes periodically in each second is 50 Hz; (2) the electric energy consumed by the indicator light of the electric energy meter flashing 90 times: w = 902 000 kW · H = 0.045 kW · H = 1.62 × 105 J, then the actual power of the electric kettle: P = wt1.62 × 105 J, 3 × 60 s = 900 W; (2) from ρ = MV, the mass of water: M = ρ v = 1 × 103 kg / m3 × 2 × 10-3 m3 = 2 kg, the heat absorbed by water: q = cm △ t = 4.2 × 103 J / (kg ·℃) × 2 K G × (35 ℃ - 20 ℃) = 1.26 × 105J. Efficiency of electric kettle: η = QW × 100% = 1.26 × 105J 1.62 × 102J × 100% = 77.8%