A) \[5\,\Omega \]
B) \[1\,\Omega \]
C) \[7\,\Omega \]
D) \[3\,\Omega \]
Correct Answer: B
Solution :
(b) \[1\,\Omega \] As \[l=\frac{\varepsilon }{R+r}\] In first case, \[l=0.5\,\,A;R=12\,\Omega \] \[0.5=\frac{\varepsilon }{12+r}\Rightarrow \varepsilon =6.0+0.5\,r\] (1) In second case, \[l=0.25\,\,A;R=25\,\Omega \] \[\varepsilon =6.25+0.25r\] (2) From eqs. (1) and (2), \[r=1\,\Omega \]
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