A) 9.6 minute
B) 14.4 minute
C) 16.8 minute
D) 18.2 minute
Correct Answer: A
Solution :
[a] As voltage of appliance remains constant, the amount of heat produced is given by, \[H=\frac{{{V}^{2}}}{R}t\] ...(i) When resistance is reduced by 20%, new resistance is \[R'=R-0.2R=0.8RR'=R-0.2R=0.8R\] \[H'=\frac{{{V}^{2}}}{0.8R}t'\] ?(ii) Equating Eqs. (i) and (ii), we get \[\frac{{{V}^{2}}}{R}t=\frac{{{V}^{2}}}{0.8R}t'\] \[\Rightarrow t'=0.58t=0.8\times 12=9.6\text{ min}\]
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