(20 13? The schematic diagram of the electronic balance is shown in the figure. A is the tray, R is the sliding rheostat, and the slider P is fixed on the spring through the connecting rod.

(1) When measuring the maximum mass, the slider P is at the lowest end of R, and the circuit is a simple circuit of R0.

According to ohm's law, the current in the circuit:

I = UR0 = 3V5Ω = 0.6A

(2) When the mass is not measured, the slider P is at the top of R, and R is connected in series with R0.

The total resistance in the series circuit is equal to the sum of the individual resistances,

∴ current in the circuit:

I string = ur+r0 = 3v 25ω+5ω= 0. 1a，

Power consumption of rheostat:

W=I string 2rt = (0.1a) 2× 25ω× 5× 60s = 75j.

Answer: (1) When the measured mass reaches the maximum, the current in the circuit is 0.6A 。

(2) When the mass is not measured, the power consumption of sliding rheostat R is 75 J when it is electrified for 5 minutes.