Question

A wire of fixed length is turned to form a coil of one turn. It is again turned to form a coil of three turns. If in both cases same amount of current is passed, then the ratio of the intensities of magnetic field produced at the centre of a coil will be

• A. 9 times of first case
• B. $\frac { 1 } { 9 }$ times of first case
• C. 3 times of first case
• D. $\frac { 1 } { 3 }$ times of first case

Answer 1

Answer: (C)

3 times of first case

Answer 2

Since dl and r for each element of the straight segments are parallel therefore

$\overrightarrow { \mathrm { dl } } \times \overrightarrow { \mathrm { r } } = 0$

Hence B is also zero.