(20 10? Xiaoxing designed a multi-stage electric heater for the laboratory, and its working principle diagram is shown in the figure, where R 1, R2 and R3 are

The current of (1) 1 during normal operation is I1= p1u = 200W220v ≈ 0.91a;

The heat generated by normal operation of the second gear for 5 hours is q = w = P2T = 300 w× 5× 3600s = 5.4×106j.

(2)∵ The total power is equal to the sum of the power of all electrical appliances,

∴ When the maximum power of electric heater P7=900W, switches S 1, S2 and S3 should be closed;

The power of R3 is P3 = p7-p1-p2 = 900w-200w-300w = 400w.

The resistance of R3 is R3 = u2P3 = (220v) 2400w =121ω.

(3) According to the meaning of the question, P3=400W, P4=P 1+P2=200W+300W=500W, P5=P 1+P3=200W+400W=600W, P6 = P2+P3 = 300w.

Answer: (1) When the electric heater works normally in 1 gear, the current is about 0.9 1A, and the heat generated when it works normally in gear 2 for 5 hours is 5.4× 106J. j .

(2) When the power is 900W, the switches S 1, S2 and S3 are all closed; The resistance of R3 is 12 1ω.

(3) The power values of the other four power gears are 400W, 500W, 600W and 700W respectively. They are.