A) \[{{300}^{o}}C\]
B) \[{{400}^{o}}C\]
C) \[{{500}^{o}}C\]
D) \[{{200}^{o}}C\]
Correct Answer: B
Solution :
Let resistance of bulb filament is \[{{R}_{0}}\]at \[{{0}^{o}}C\]then from expression \[R={{R}_{0}}\,[1+\alpha \,\Delta \theta ]\] we have, \[100={{R}_{0}}[1+0.005\times 100]\] and \[200={{R}_{0}}[1+0.005\times x]\] where \[x\] is temperature in \[^{o}C\] at which resistance becomes \[200\,\,\Omega \]. Dividing the above two equations, we have \[\frac{200}{100}=\frac{1+0.005x}{1+0.005\times 100}\] \[\Rightarrow \] \[x={{400}^{o}}C\]
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