Question

The electric current in a circular coil of 2 turns produces a magnetic induction $B _1$ at its centre The coil is unwound and is rewound into a circular coil of 5 turns and the same current produces a magnetic induction $B _2$ at its centre The ratio of $\frac{ B _2}{ B _1}$ is :

• A. $\frac{5}{2}$
• B. $\frac{25}{4}$
• C. $\frac{5}{4}$
• D. $\frac{25}{2}$

Answer 1

Answer: (B)

$\frac{25}{4}$

Answer 2

B=2RNμ0​i​
B1​=2R1​N1​μ0​i​
For N2​=5
Radius of coil =R2​=N2​N1​×R1​​
B2​=R2​N2​μ0​i​
B1​B2​​=N1​N2​​⋅R2​R1​​=N1​N2​​×N1​N2​​;B1​B2​​=425​