A particle of unit mass and unit charge is moving in a magnetic field, which varies as \(\vec{B}(\vec{r})=b_0 \vec{r} / r^3\) (b0 is a constant) over a region far away from the origin. If \(\vec{L}\) is the instantaneous angular momentum of the particle within that region, then \(d \vec{L} / d t\) is
1
\(2 b_0 \frac{d}{d t}\left(\frac{\vec{r}}{r}\right)\)
2
\(-b_0 \frac{d}{d t}\left(\frac{\vec{r}}{r}\right)\)
3
\(b_0 \frac{d}{d t}\left(\frac{\vec{r}}{r}\right)\)
4
0