試題詳解

10. A resistor of unknown resistance and a 15 Ω resistor are connected across a 20 V emf in such a way that a 2 A current is observed. What is the value of the unknown resistance?
(A) 5 Ω
(B) 7.5 Ω
(C) 12 Ω
(D) 30 Ω
(E) 75 Ω