(1) As can be seen from Figure B, when the current representation number is 5A, I2=5A, and the voltage U2 across R2 =10V.

According to the formula I=UR,

R2 access circuit resistance: R2 = U2I2 = 10V5A = 2ω.

Answer: The resistance of the variable resistor R2 connected to the circuit is 2 Ω.

(2) When the current is 2.5A, the voltage across R2 is 12.5V,

According to the formula I=UR,

Then: r' 2 = U2' I2 = 12.5V2.5A = 5ω,

When the current is 15A, the voltage across R2 is 0.

Then: R2 "= U2" I2 = 0V 15A = 0,

∴ The resistance value of R2 access circuit varies from 0 to 5 Ω.

Answer: The resistance of R2 access circuit ranges from 0 to 5 Ω.

(3) When the current is 2.5A, there are:

u = u 1+U2′= 2.5r 1+ 12.5v，①

When the current is 15A, there are:

u = u 1′+U2 ″= 15r 1+0，②

① ② Two equations can be solved at the same time:

r 1 = 1ω，U= 15V。

Answer: The power supply voltage is 15V, and the resistance value of the resistor R 1 is 1ω.