a$.$ Derive an equation for magnetic force on a moving charge considering $\text{'}B\text{'}$ as magnetic flux density, velocity' and charge $\text{'}Q\text{'}.$

$(7$ Marks$)$

b$.$ Consider a rectangular coil as shown in figure $1$ is kept in magnetic field of $\frac{?}{\text{b}\text{a}\text{r}}(B)=-\frac{2}{b}ar(a_x)\frac{+}{b}ar(a_y)-\frac{?}{\text{b}\text{a}\text{r}}(a_2)T,$

Find the torque about the $z$ axis when the coil is in the position shown and carries a current of $6$

A$.$

$(8$ Marks$)$