A particle of mass 6.0x10^-8kg and charge +7.2x10^-6C is travelling west. The particle enters a magnetic field of magnitude 3.0T, where it completes one-half of a circle before exiting the field moving east. How much time does this charge spend inside the magnetic field?

$\displaystyle B= \frac{uI}{2\pi r}$
$\displaystyle r= \frac {uI}{2\pi B}$
$\displaystyle = (4\pi x 10^-7)(7.2x10^-6C) / (2\pi)(3.0T)$
$\displaystyle =4.8x10^-13m$

$\displaystyle v= \frac {Bqr}{m}$
$\displaystyle v= (3.0T)(1.602x10^-19)(4.8x10^-13m) / (6.0x10^-8kg)$
$\displaystyle =3.84x10^-24m/s$

$\displaystyle t= \frac {d}{v}$
$\displaystyle = \frac {4.8x10^-13m}{2.7683x10^-36m/s}$
$\displaystyle = 1.25x10^11$