Question

An electron and a proton each travel with equal speeds around circular orbits in the same uniform magnetic field as indicated (not to scale) in fig. The field is into the page on the diagram. The electron travels .... around the .... circle and the proton travels .... around the .... circle

• A. clockwise, smaller, counterclockwise, larger
• B. counterclockwise, larger, counterclockwise, smaller
• C. clockwise, larger, counterclockwise, smaller
• D. counterclockwise, larger, clockwise, smaller

Answer 1

The correct option is A clockwise, smaller, counterclockwise, larger

$r=\frac{mv}{qB}$
Since speed $v$ is equal for both and $q$ is numerically equal for both,
$r\alpha m$, the smaller circle will be for the electron.
Since electron and proton have equal and opposite charges, both will have motion in opposite directions.
From $\overrightarrow{F}=q(\overrightarrow{v}\times \overrightarrow{B})$, electron will move counter clockwise and proton clockwise.