A proton moving to the right enters a uniform 0.25 T magnetic field region. The proton moves in a clockwise circle in the plane of the page with a tangential speed of 2.8 x
105 m/s.
What is the direction of the magnetic field?

A) Right
B) Left
C) Into the page
D) Out of the page
E) Upwards

According to the right-hand rule, the magnetic field is into the page.

Explanation:

According to the right-hand rule, the magnetic field is into the page. According to this rule, if the middle finger points in the direction of initial motion of the charge as it enters the magnetic field to the right, and point the thumb and index finer perpendicular to each other and the middle finer perpendicular to the index finer. We then point the middle finger to the right in the direction of motion of the proton. If we are to have a clockwise force on the proton in the plane of the page, the thumb must point parallel to the page, downwards towards the south. The direction of the index finger is thus the direction of the magnetic field, and this is into the page.