Circular Motion of Charged Particle in Magnetic Field
A negatively charged particle moves in the plane of the page in a region where the magnetic field is perpendicular into the page (represented by the small circles with x's—like the tails of arrows). The magnetic force is perpendicular to the velocity, and so velocity changes in direction but not magnitude. Uniform circular motion results.
Source
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"OpenStax College, College Physics. January 16, 2015."
http://cnx.org/content/m42375/latest/?collection=col11406/1.7
OpenStax CNX
CC BY 3.0.