Question

The strength of the magnetic field at a point r near a long straight current carrying wire is B. The field at a distance $\frac{r}{2}$ will be

[MP PMT 1990]

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

Answer 1

The correct option is C

2B

$B\infty \frac{1}{r}\Rightarrow \frac{B_1}{B_2}=\frac{r_2}{r_1}\Rightarrow \frac{B}{B_2}=\frac{r/2}{r}\Rightarrow B_2=2B$