Which of the following is the constant of proportionality in the Biot-Savart law?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → \( \mu_0/4\pi \)
NCERT (2026-27):
Magnetic Field Due to a Current Element, Biot-Savart Law – “… According to Biot-Savart’s law, the magnitude of the magnetic field dB is proportional to the current I, the element length |dl|, and inversely proportional to the square of the distance r. Its direction is perpendicular to the plane containing dl and r. Thus, in vector notation,
\( \begin{aligned} d\vec{B} &\propto \frac{I\,d\vec{l}\times\vec{r}}{r^{3}} \\ &= \frac{\mu_0}{4\pi}\,\frac{I\,d\vec{l}\times\vec{r}}{r^{3}} \qquad [4.7(a)] \end{aligned} \)
where μ0/4π is a constant of proportionality. The above expression holds when the medium is vacuum.”