Question
Question: Two resistances, a 60 ohm and an unknown one are connected to a power source in a series arrangement...
Two resistances, a 60 ohm and an unknown one are connected to a power source in a series arrangement. Power of the unknown resistance is 60 watt. What is the voltage of the power source at the least?
A
60 volt
B
120 volt
C
140 volt
D
180 volt
Answer
120 volt
Explanation
Solution
= 
...(1)
V1 + V2 = V ...(2)
from (1) & (2)
Ž V2 = 
...(3)
= 60 watt ...(4)
From (3) & (4)
V2R = 60 (60 + R)2
assuming 60 + R = x \ R = x – 60
60 x2 – V2x + 60 V2 = 0 ...(5)
For real x
V4 – 4 × 60 × 60 × V2 ³ 0
V2 [V2 – 4 × 60 × 60] ³ 0
\ V ³ 2 × 60
V ³ 120 volt