WAEC Physics 2023 Paper

Concept:Magnetic force on a moving charge acts as the centripetal force that keeps the ion moving in a circular path.Explanation:An ion of charge $q$ moving with speed $v$ through a magnetic field of flux density $B$ experiences a magnetic force given by:$$F = qvB$$This force provides the centripetal force required for circular motion of radius $r$:$$F = \frac{mv^2}{r}$$Since these two forces are equal, we equate them:$$qvB = \frac{mv^2}{r}$$Cancel one factor of $v$ from both sides:$$qB = \frac{mv}{r}$$Multiply both sides by $r$:$$qBr = mv$$Finally, solve for $v$:$$v = \frac{qBr}{m}$$Answer:$$v = \frac{qBr}{m}$$This corresponds to option A.