(2) The rated power of electric water heater is P=2KW, and the rated voltage is U=220V. From P=U2R, the resistance of the water heater in normal operation is R = U2p = (220v) 22000w = 24.2ω.
(3) The volume of the water heater is V= 12L=0.0 12m3, and the mass of water in the water heater is M = ρ V =1×103kg/m3× 0.012m3 =/kloc-0.
When the water temperature rises from 20℃ to 60℃, the heat absorbed by water is Q = MC△ T =12kg× 4.2x0103? J/(Kg？ ℃)×(60℃-20℃)= 2.0 16× 106j。
(4) The total work done by the electric water heater is W = Q η = 2.016× 06j80% = 2.52×106j, and the time required for the heating process is T = W=Pt = 2.52×106j200W = 60.
(5) Advantages of solar water heater: safety, economy, energy saving and inexhaustible.
Answer: (1) For safety reasons, the socket used in this electric water heater should be as shown in Figure B. (2) When this electric water heater works normally, the resistance of the heating wire is r = 24.2.
(3) Water needs to absorb 2.0 16× 106J of heat. (4) The time required for heat treatment is 1260s. ..
(5) Safe, economical, energy-saving and inexhaustible.