A cell of internal resistance r supplies current to a 6.0Ω resistor and its efficiency is 75%. Find the value of r.

A. 4.5Ω
B. 1.0Ω
C. 8.0Ω
D. 2.0Ω

Correct Answer:

Option D = 2.0Ω

Explanation

Efficiency = power out put/power input x 100/1
= power out put/ power + that due x 100/1
to internal resistance
=> 75/1 = 1²R x 100/1
1²(R+r)
i.e 75/100 = 6/6+r => 75 (6+r) = 100/6
i.e 450 + 75r = 600
75r =600 – 450
i.e r = 150/75 = 2Ω
i.e r = 2Ω