In the circuit above, the ammeter reads a current of 5.0 A when R = 8Ω and reads 7.0A when R = 5Ω. The value of the unknown resistance X is
Attached Figure / Diagram
A
10.0Ω
B
7.5Ω
C
5.0Ω
D
2.5Ω
Verified Correct Answer: D.
Physical Principles & Numerical Solution
From Ohms Law, E = I(R +X)
=> E = I1(R1 + X)
again, E = I2(R2 + X)
=> I1(R1 + X) = I2(R2 + X)
∴ 5(8 + X) = 7(5 + X)
40 + 5X = 35 + 7X
7X - 5X = 40 - 35
2X = 5
X = 5/2
= 2.5Ω
Source: Myschool Past Questions Archive (Record #11271)