A student was trying to construct the circuit shown in the figure below marked (a), but ended up constructing the circuit marked (b). Realizing her mistake, she corrected the circuit, but to her surprise, the output voltage (across R ) did not change. The value of resistance R is :-

Step-by-step solution

For circuit (a), i R = 10 300 R 300 + R ↑ Current   through   cell + 300 × 300 300 + R [Note : 300   Ω   &   R are in parallel which is in series with 100   &   200   Ω ] ∴   V R a = 10 × 300 R 300 R + 300 2 + 300 R [ V R a is potential difference across resistance R ] For circuit (b), i R = 10 ( 200 + R ) ( 300 ) 200 + R + 300 ↑ Current   through   cell + 100 × 300 300 + 200 + R [Note: R   &   20