A galvanometer connected with an unknown resistor and two identical cells in series each of emf 2 V shows a current of 1 A. If the cells are connected in parallel, it shows 0.8 A. Then the internal resistance of the cell is

Options

A. `1 Omega`
B. `0.5 Omega`
C. `0.25 Omega`
D. `0.33 Omega`

Answer

A

Explanation

Series,
`I = (2E)/(2r + R)`
`rArr 2r + R = 4`…….. (i)
Parallel,
`I = 2/(R + r//2)`

`rArr 0.8 = 2/(R + r//2)`
`rArr 0.8R + 0.4R = 2`
`R + r/2 = 2/0.8 = 5/2`…… (ii)
Subtract (i) from (ii)
`-(3r)/2 =-3/2 rArr r = 1 Omega`