Problem $2$:$$ An unknown wire in a circuit has a circular cross$-$section with diameter $1\mathrm{.}1$ mm$.$ It carries a current of $6\mathrm{.}8$ mA$.$

Part $($a$)$ What is the current density, in milliamperes per square centimeter, in this wire?

Part $($b$)$ If you assume the drift velocity in this wire is $110-2$ cm$/$s$,$ what is the density of charge carriers per cubic centimeter?