(2014?Suzhou) In the circuit shown in the figure, the supply voltage is constant 12V, the resistance of resistor R1 is 10Ω, the resistance of resistor R2 is 30Ω, the ammeter A

(1) circuit current: I=

R1+R2

12V

10Ω+30Ω

=0.3A;

(2) Power consumed by resistor R1:

P1= I2R1=(0.3A)2×10Ω=0.9W;

(3) the maximum current allowed in the circuit: I max=0.6A,

from I=

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It can be seen that the minimum resistance of the circuit:

R min=

I Max

12V

0.6A

=20Ω,

The minimum resistance of the sliding varistor connected to the circuit:

R2 min=R min-R1=20Ω-10Ω=10Ω,

Then the range of the resistance of the varistor R3 connected to the circuit is: 10Ω～20Ω;

Answer: (1) The current through the circuit is 0.3A.

(2) The power consumed by the resistor R1 is 0.9W.

(3) The range of resistance of the varistor R3 connected to the circuit is: 10Ω～20Ω.

Answer: (3) The range of resistance of the varistor R3 connected to the circuit is: 10Ω～20Ω.