如图所示,电源电压恒为3V,知R1=12Ω,R2=6Ω.求: 1)当开关S断开时电流表A的读数; 2)当开关S闭合时电流表A的读数.

问题描述:

如图所示,电源电压恒为3V,知R1=12Ω,R2=6Ω.求:
1)当开关S断开时电流表A的读数;
2)当开关S闭合时电流表A的读数.

(1)当开关S断开时;电路中只有R1连入电路,电流表测量R1的电流,
∴由欧姆定律可得:通过R1的电流I1=

U
R1
=
3V
12Ω
=0.25A.
(2)当开关S闭合时,两电阻并联在电源两端,电流表干路的电流,
∵R2的电流I2=
U
R2
=
3V
=0.5A.
∴I=I1+I2=0.25A+0.5A=0.75A.
即电流表A的示数为0.75A.
答:(1)当开关S断开时电流表A的读数为0.25A;
(2)当开关S闭合时电流表A的读数为0.75A.