Practise charged-particle questions by linking field and motion directions to force, velocity change and magnetic-force strength.
The diagram shows a machine that can be used to measure the speed of fast-moving protons.
The proton passes through a hole in the negatively charged plate and enters an area where there is a magnetic field.
The magnetic field exerts a force on the proton, as shown in the diagram.
This force causes the proton to follow a circular path without changing speed.
Give the direction of the magnetic field.