WAEC Physics 2020 Paper

Concept:When a charged particle moves perpendicularly to a uniform magnetic field, the magnetic force provides the centripetal force required for circular motion.Explanation:For an electron of mass $m$ and charge $e$ moving with speed $v$ in a magnetic field $B$, the magnetic force is $F = evB$.This force acts as the centripetal force, giving $evB = \frac{mv^2}{r}$.Simplifying, the radius of the circular path is $r = \frac{mv}{eB}$.The time for one complete orbit, called the period $T$, equals the circumference divided by the speed: $T = \frac{2\pi r}{v}$.Substituting $r = \frac{mv}{eB}$: $T = \frac{2\pi}{v} \times \frac{mv}{eB}$.Cancelling $v$, we get $T = \frac{2\pi m}{eB}$.Answer:$T = \frac{2 \pi m}{B e}$, so the correct option is C.