根据题意,电压表的内阻可以忽略.
电源电压是:E = 6.0V
则当开关闭合以后,电路总电流:
I = 5.6V/7Ω = 0.8A
所以,电源内阻:r = (E - U)/I = 0.4V/0.8A = 0.5Ω