A long, solid, copper rod has a circular cross$-$section of diameter $10\mathrm{.}0$ cm$.$ The rod has a current of $5\mathrm{.}0$ A$,$ uniformly distributed across its cross$-$section.

Calculate the magnetic field strength

$($a$)$ at the centre of the rod

$($b$)2\mathrm{.}5$ cm from the centre

$($c$)5\mathrm{.}0$ cm from the centre

$($d$)7\mathrm{.}5$ cm from the centre.