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How can a magnetic field make a charged particle travel in a circular path?

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A magnetic field makes a charged particle travel in a circular path through the action of the Lorentz force. When a particle with charge q moves with velocity v through a magnetic field B, it experiences a magnetic force that is always perpendicular to both its velocity and the magnetic field.12

When the particle's velocity is exactly perpendicular to the magnetic field (an angle of 90°), this force acts as a centripetal force, constantly pulling the particle toward the center of its motion without changing its speed. This results in a circular trajectory with a radius defined by the equation:

$r = \frac{m v}{q B}$

In this formula, m represents the mass of the particle, v its velocity, q its charge, and B the strength of the magnetic field. If the velocity is not perpendicular to the field, the particle instead follows a helical path, while motion parallel to the field lines results in no magnetic force, allowing the particle to move undeflected.123