Question

A current carrying coil is bent sharply so as to convert it into a double loop both carrying current in the same direction. If B be the initial magnetic field at the centre, then what will be the final concentric magnetic field?

• A. 2 B
• B. 4 B
• C. 8 B
• D. Zero

Hint:

When a loop is folded, radius reduces and field increases.

Answer 1

The Correct Option is B

Step 1: For a circular loop, $B = \frac\mu₀ I2r$.

Step 2: When bent into double loop, each loop has radius $r/2$. Field due to each: $B' = \frac\mu₀ I2(r/2) = \frac\mu₀ Ir$. Two loops: $Btotal = 2 × \frac\mu₀ Ir = 4B$.