Question

The electric current in a circular coil of two turns produced a magnetic induction of $0.2T$ at its centre. The coil is unwound and is rewound into a circular coil of four turns. The magnetic induction at the centre of the coil now is, in $T$ :
(if same current flows in the coil)

• A. $0.6$
• B. $0.8$
• C. $0.2$
• D. $0.4$

Answer 1

Answer: (B)

Coil is unwound and is rewound into a circular coil of 4 turns,
$\therefore$ $2\pi R=4\times 2\pi r$
$r=\frac{R}{4}$
$\left|B\right|=\frac{{\mu }_{0}I}{\frac{2R}{4}}$ $=0.2\times$ $4=0.8T$