A) \[\frac{{{B}_{0}}}{9}\]
B) \[9\,{{B}_{0}}\]
C) \[\frac{{{B}_{0}}}{3}\]
D) \[3\,{{B}_{0}}\]
Correct Answer: B
Solution :
The magnetic field produced at the centre of the circular coil carrying current is given by \[B=\frac{{{\mu }_{0}}NI}{2\,r}\] For one turn \[N=1\] \[{{B}_{0}}=\frac{{{\mu }_{0}}I}{2\,r}\] As the coil is rewound \[r'=\frac{r}{3},\,N'=3\] \[\therefore \] \[B'=\frac{{{\mu }_{0}}I\times 3}{2\times \left( \frac{r}{3} \right)}\] \[=\frac{9\,{{\mu }_{0}}\,I}{2\,r}=9\,{{B}_{0}}\]
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