Paper 2, Section I, 6C\mathbf{6 C}

Electromagnetism
Part IB, 2018

Derive the Biot-Savart law

B(r)=μ04πVj(r)×(rr)rr3 dV\mathbf{B}(\mathbf{r})=\frac{\mu_{0}}{4 \pi} \int_{V} \frac{\mathbf{j}\left(\mathbf{r}^{\prime}\right) \times\left(\mathbf{r}-\mathbf{r}^{\prime}\right)}{\left|\mathbf{r}-\mathbf{r}^{\prime}\right|^{3}} \mathrm{~d} V

from Maxwell's equations, where the time-independent current j(r)\mathbf{j}(\mathbf{r}) vanishes outside VV. [You may assume that the vector potential can be chosen to be divergence-free.]